We have studied molecular beam epitaxy grown GaN films of both polarities using electric force microscopy to detect sub 1 µm regions of charge density variations associated with GaN extended defects. The large piezoelectric coefficients of GaN together with strain introduced by crystalline imperfections produce variations in piezoelectrically induced electric fields around these defects. The consequent spatial rearrangement of charges can be detected by electrostatic force microscopy and was found to be on the order of the characteristic Debye length for GaN at our dopant concentration. The electric force microscope signal was also found to be a linear function of the contact potential between the metal coating on the tip and GaN. Electrostatic analysis yielded a surface state density of 9.4 ± 0.5 × 10^10 cm – 2 at an energy of 30 mV above the valence band indicating that the GaN surface is unpinned in this case

Bandić, Z. Z.

Bridger, P. M.

McGill, T. C.

Piquette, E. C.

English

Caltech Authors - Main

APPLIED PHYSICS LETTERS VOLUME 74, NUMBER 23 7 JUNE 1999Measurement of induced surface charges, contact potentials,
and surface states in GaN by electric force microscopy
P. M. Bridger, Z. Z. Bandic´, E. C. Piquette, and T. C. McGilla)
Thomas J. Watson, Sr. Laboratory of Applied Physics, California Institute of Technology,
Pasadena, California 91125
~Received 4 March 1999; accepted for publication 12 April 1999!
We have studied molecular beam epitaxy grown GaN films of both polarities using electric force
microscopy to detect sub 1 mm regions of charge density variations associated with GaN extended
defects. The large piezoelectric coefficients of GaN together with strain introduced by crystalline
imperfections produce variations in piezoelectrically induced electric fields around these defects.
The consequent spatial rearrangement of charges can be detected by electrostatic force microscopy
and was found to be on the order of the characteristic Debye length for GaN at our dopant
concentration. The electric force microscope signal was also found to be a linear function of the
contact potential between the metal coating on the tip and GaN. Electrostatic analysis yielded a
surface state density of 9.460.531010 cm22 at an energy of 30 mV above the valence band
indicating that the GaN surface is unpinned in this case. © 1999 American Institute of Physics.
@S0003-6951~99!01223-1#Nitride based devices have been of great interest in the
last few years, notably due to their success in optoelectron-
ics, where lasers and diodes have been demonstrated and
successfully commercialized.1 Further applications of ni-
trides are expected in the arena of high power and high tem-
perature devices,2–4 as well as solar blind ultraviolet
detectors.5 It has been recently demonstrated that the large
intrinsic piezoelectric coefficients of GaN and AlN are re-
sponsible for a high concentration two-dimensional electron
gas at the AlGaN/GaN interface in heterojunction field effect
transistors ~HFET!.6,7 Other possibilities exist for the en-
hancement of electric properties of contacts to nitrides by
piezoelectric engineering as recently demonstrated in the
case of Schottky contacts.8 While most of the recent research
has emphasized electronic device aspects of the piezoelectric
effect,6–8 comparatively little work has concentrated on the
investigation of fundamental properties and nanoscale char-
acterization of piezoelectrically induced phenomena. One
consequence of the piezoelectric effect is that it allows elec-
trostatic force imaging of charge redistribution around de-
fects due to local variations in strain caused by crystalline
imperfections. Although the magnitude of the charge density
is nonquantitative, electric force microscopy ~EFM! can still
provide interesting insight into the nature of defects, the pi-
ezoelectric effect in nitrides, as well as measurement of the
surface state density and energy.9,10
The gallium nitride layers studied here were grown on
c-plane sapphire substrates by radio frequency plasma as-
sisted molecular beam epitaxy. Ga-polar GaN films were
nucleated using AlN buffer layers whereas N-polar films
were nucleated using a GaN buffer layer. Polarity was deter-
mined by reflection high-energy electron diffraction
~RHEED! reconstruction at low temperature,11 and by
~KOH! etching.12,13 Other details of the growth conditions
are presented elsewhere.14,15
a!Electronic mail: tcm@ssdp.caltech.edu3520003-6951/99/74(23)/3522/3/$15.00
Downloaded 03 Apr 2006 to 131.215.225.171. Redistribution subject tA variety of different metals were used for coating
atomic force microscope ~AFM! tips to vary the metal work
function for electric force microscopy. Cobalt coated AFM
tips were obtained commercially from Digital Instruments.
Titanium, Al, Pt, Pd, Ce, W, and Au coated tips were fabri-
cated in the following manner. Commercial silicon tapping
mode AFM tips were plasma cleaned in a 30 W Ar plasma at
a pressure of 831024 Torr. A 15 nm metal layer was then
sputter deposited in sputter chamber at a base pressure of 1
31027 Torr to provide the contact necessary for applying a
bias to the tip.
The EFM data was collected using a Digital Instruments
Nanoscope IIIa controller and a Bioscope scanning probe
microscope operating in tapping mode. Electric force mi-
croscopy was performed in two ways: by detecting electro-
static forces and by detecting the surface potential. Surface
potential measurements will be presented elsewhere. To de-
tect the electrostatic forces, a voltage is applied to AFM tips
coated with the above metals which are scanned across the
surface at a constant tip-sample separation. Phase differences
induced by electrostatic forces on the oscillating tip during
scanning are detected and give a qualitative measurement of
the local charge density.
When the metal tip is modeled as a small capacitive
element, in similar fashion to Refs. 10, 16, 17, the force it
feels under an applied dc bias will be due to the charge-
charge interaction, and changes in the capacitive energy,
Force5
qsqt
4pe0z2
1
1
2
dC
dz ~Vapplied2Vcontact!
2
, ~1!
where dC/dz is the derivative of the sample-tip capacitance,
qs is surface charge, qt is charge induced on the tip, and
Vcontact5fm2xGaN2DE f n2Df is the contact potential be-
tween the tip metallization and GaN semiconductor. In this
formula, fm is metal work function, xGaN54.2 eV is the
electron affinity of GaN,18 Df is band bending caused by2 © 1999 American Institute of Physics
o AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
3523Appl. Phys. Lett., Vol. 74, No. 23, 7 June 1999 Bridger et al.surface states, and DE f n is the Fermi level position in the
GaN referenced to the bottom of the conduction band.
For the experiments considered here, the tip-sample
separation, z, was set at either 30 or 50 nm.
The electrostatic forces were measured as a function of
tip voltage to rule out topographical artifacts. Variation in
the induced surface charges result in a force differential be-
tween the tip and the surface that increases with tip voltage
which can be observed in the series of EFM images in Fig. 1.
It was found that the electrostatic force was a function of the
magnitude of the tip voltage and not the sign, consistent with
the theoretical V2 dependence.10,19 From Eq. ~1!, we can also
observe a force minimum when the applied bias is equal to
the contact potential between the GaN and the tip metalliza-
tion. This was experimentally observed, and Fig. 1 illustrates
FIG. 1. Electrostatic force image of the surface of molecular beam epitaxy
~MBE! grown GaN as a function of tip applied voltage for a Ga-polar
sample. For reference, the top image is the AFM scan of the same area
imaged with EFM. Scans A-D are the EFM data with the tip bias increasing
from A-D. Notice the signal null at a tip bias of 0.5 V. The tip sample
separation was 50 nm in all the EFM images.Downloaded 03 Apr 2006 to 131.215.225.171. Redistribution subject tthis effect with a signal ‘‘null’’ at a tip voltage of 0.5 V in
case of Co coated tips. The inset of Fig. 2 is a plot of the
signal root-mean-square ~rms! roughness ~in arbitrary units!
as a measure of contrast against the tip voltage to illustrate
the minimum force condition. Typically, complete cancella-
tion of the signal is not expected due to the first term in Eq.
~1! leaving some residual image due to electrostatic forces.
Figure 2 is a plot of the measured tip null voltage Vnull as
a function of the difference between metal work function and
electron affinity of GaN. The observed dependence between
Vnull and fmetal2xGaN is linear. Aluminum and cerium were
found to be anomalous where the opposite sign of the volt-
age was required to null the EFM signal. This is attributed to
different work functions of oxides which formed when the
tips were exposed to air. A least squares linear fit of this
experimentally observed dependence gives slope of 0.66
60.03, and an intercept of 20.0460.03. The slope is less
than 1 and indicates the presence of surface states.
The situation will be considered as an electrostatic
analysis of the following: ~1! An ideal metal with with work
function fm . ~2! A dielectric interface region with a thick-
ness of the tip-sample separation. ~3! A semiconductor sur-
face with surface states up to a particular energy. An analysis
of the barrier energy as a function of the metal work function
and semiconductor electron affinity20 yields the following
expression:
fBn5a~fm2x!1~12a !S Egq 2f0D2Df
5a~fm2x!1b , ~2!
where fBn is the barrier energy between a metal and an
n-type semiconductor, a and b are slope and intercept, fm is
the metal work function, x is the electron affinity, Eg is the
band gap, Df is the barrier lowering, and f0 is the energy of
the surface states relative to the valence band. The density of
surface states can now be expressed in terms of the regres-
sion coefficients, a and b, as well as the dielectric constant,
e i , and thickness, d, of the metal-semiconductor interface:
f05
Eg
q 2
b1Df
12a , ~3!
FIG. 2. Plot of the tip voltage for a minimum force condition vs the work
function difference between the tip metallization and GaN. Inset: Plot of the
rms contrast against the tip bias in arbitrary units illustrating the null con-
dition for the cobalt coated tips.o AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
3524 Appl. Phys. Lett., Vol. 74, No. 23, 7 June 1999 Bridger et al.Ds5
~12a !e i
adq . ~4!
Substituting for the slope and intercept, the maximum sur-
face state energy, f0 , lies 30 mV690 mV above the valence
band. This is consistent with an observation of surface states
at or below the valence band maximum using photoemission
spectroscopy.21,22 Using an air gap as the dielectric for the
interface, the calculated density of surface states is 9.460.5
31010 cm22. If water, e i580e0 , is used as the dielectric to
try to account for a typical surface contaminant, the surface
state density becomes 7.560.431012 cm22. Since the den-
sity of chargeable defects required to pin the Fermi level is
on the order of 1014 cm22,23 we conclude that the GaN sur-
face is unpinned in this case.
Figure 3 shows that the nature of the charge rearrange-
ment is due to screening of the piezoelectrically induced
charges caused by strain relaxation at numerous defects. The
N-polar @Figs. 3~a! and 3~b!# and Ga-polar @Figs. 3~c! and
3~d!# films have different defect structure and hence different
surface morphology. Figures 3~a! and 3~b! show EFM and
AFM image and the associated profile for N-polar film. We
can observe steps ~approximately 5 nm in height! on the Fig.
3~b!, and associated charge accumulated on these steps @Fig.
3~a!#. In case of Ga-polar films, charge accumulation @Fig.
3~c!# is observed at the edges of the hexagonal pits. How-
ever, in both cases the strain relaxation and consequent
charge rearrangement has a spatial extent of 60 nm. A cal-
culation of the Debye length gives LD5AeskT/q2N
<100 nm where both films have N>1015 cm23, es is the
dielectric constant of GaN, q is the elementary charge, and
T5293 K. The 60 nm spatial extent of the measured charge
FIG. 3. ~a! EFM image of the surface of N-polar GaN and its associated line
profile. The arrows indicate the 60 nm spatial extent of the screening charge
associated with strain relaxation at the steps indicated in the following im-
age. ~b! The AFM and associated line profile for the same area in ~a!.
Arrows indicate the steps of interest. ~c! EFM of Ga-polar GaN and its
associated line profile. Again, arrows indicate the spatial extent of the
screening charge associated with the defect structure indicated by the AFM
image in ~d!. In all cases the tip-sample separation was 30 nm.Downloaded 03 Apr 2006 to 131.215.225.171. Redistribution subject tis within the experimental error in the determination of the
doping density. Therefore, since the spatial extent of the
charge density surrounding the defects is approximately
equal to the Debye length within experimental error, it is
believed that the observed charge is a screening charge rather
than the bare surface charge density that would be induced
on the surface due to the termination of the polarization of
the film.
In summary, we have successfully demonstrated that
EFM techniques can be used to detect local variations in
piezoelectrically induced charge and potential on the sub 1
mm scale. These charges are believed to be screening charge
on the surface since their spatial extent was comparable to
the Debye length. We have also demonstrated that the EFM
signal could be minimized by applying a voltage roughly
equal to the contact potential between the tip metal and the
GaN. An analysis of this contact potential variation gave a
surface state density of 9.460.531010 cm22 at an energy of
30 mV above the valence band.
The authors would like to thank E. T. Yu and D. L.
Smith for their useful comments and suggestions. This work
was supported by DARPA/EPRI and monitored under Grant
No. MDA972-98-1-0005.
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o AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp


Measurement of induced surface charges, contact potentials, and surface states in GaN by electric force microscopy

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