Observation of Carrier Distribution in Compound
Semiconductors Using Off-axis Electron Holography
by Hirokazu Sasaki *, Shinya Ootomo *, Takeyoshi Matsuda * and Hirotatsu Ishii *It is well known that carrier distribution in semiconductors can be observed byusing electron holography, and many such studies on Si semiconductors havebeen reported. In this paper, sample preparation and observation techniques are optimized withthe aim of improving the observation technology of carrier distribution in compound
semiconductors. The authors have been successful in making clear observation of the p-n+junction in GaAs semiconductor, as well as in clearly distinguishing between the n layer and nlayer. The techniques developed here are applicable to various semiconductor products such as
lasers, holding promise for contributing to performance and reliability improvements.

ABSTRACT1.

INTRODUCTION

In the development of various semiconductor devices, it isessential to confirm whether the structure is fabricated as
designed or not, as well as to identify their electrical performance. Typical means of structural observation are
SEM (Scanning Electron Microscopy) for low-power observation and TEM (Transmission Electron Microscopy) for
high-power observation, and these means play an important role not only in research and development but also in
product management. Ordinary TEM methods --capableof obtaining transmission images, bright-field images,
dark-field images, and high-resolution images, etc.-- allow
for real space observation of such information as crystal
orientation, distribution of dislocations, and crystallinity,
making themselves an indispensable evaluation techniquein the development of semiconductor materials. However,
it is impossible for these TEM methods to obtain magnetic
and/or electric field information on minute parts.
Information on these micro structures is needed in thedevelopment of devices using semiconductors and magnetic materials in some cases, and electron holography is
one of the techniques that meet this requirement.
Electron holography is the application of holography
techniques invented by Gabor 1) to electron waves.
Historically, Tonomura et al. implemented the in-line electron holography 2), and the off-axis electron holography
was realized using the biprism invented by Möllenstedt 3).
Since then, with the improvement in measuring equipment, numbers of remarkable fruits of research in magnetic field observation have been reported 4), including
magnetic field distribution in magnetic materials andsuperconductive single flux quantum.
Frabboni et al. succeeded in the observation of semi* Yokohama Research Lab., R&D Div.

conductors using electron holography for the first time in1985 5). In their experiment, the electric field around thep-n junction of Si caused by application of an electric voltage was observed. The experimentation is pioneering inthe sense that this is the first application of electron
holography to semiconductors. McCartney et al. was successful in the direct observation of the p-n junction insemiconductors, which was reported in 1994 6). In 1999,
Rau et al. first applied the technique to a practical device.
They were successful in the observation of Si MOSFET,
report of which triggered the observation of semiconductors based on electron holography by many research
institutes. Then Wang et al. succeeded in observing Si
MOSFET prepared as a TEM sample using FIB (Focused
Ion Beam) 8), and Twitchett et al. reported on their study
on amplified electrostatic potential at a p-n junction generated by the application of an electric voltage 9).
All the observation examples thus far were made on Si
semiconductors, and no observations ever were made on
the p-n junction of compound semiconductors. Since
Furukawa Electric’s semiconductor products belong to
the compound semiconductor groups, we embarked on
the study of electron holography observation techniques
for compound semiconductors such as GaAs and InP. In
this paper, the outline and techniques for electron holography observation of compound semiconductors are presented, along with quantitative discussions on the experimental results.

2.

OTHER TECHNIQUES FOR CARRIER
DISTRIBUTION OBSERVATIONOther evaluation techniques for dopant distribution andcarrier distribution in semiconductors include those by
SIMS (Secondary Ion Mass Spectrometry), SEM 10), andSCM (Scanning Capacitance Microscopy) 11). In thisFurukawa Review, No. 34

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Observation of Carrier Distribution in Compound Semiconductors Using Off-axis Electron Holography

3.

Bohm effect 12). Historically this technique has been broadly used in the fundamental physics, achieving very
remarkable results. But it has been less widely used in thelaboratories of private companies for observation of practical materials.
Figure 1 illustrates how two electron waves emitted
from an electron source with coherence reaches anobserver after passing through two trajectories. In orderto understand the physical phenomenon of electron
waves observed by this observer, the Schrödinger equation shown below has to be solved.
1
2-・ ＋eA
｛−ih
（r）
｝
−V
（r） ψ
（r）
＝Eψ
（r）
2m

(1)

∆

Section, these techniques are described for their characteristics, and are compared with electron holography.
SIMS is a surface analysis technique in which a primary
ion beam is irradiated on the sample surface, and thesecondary ions emitted through sputtering phenomenon
are measured to evaluate the chemical composition of thesurface. SIMS is capable of evaluating trace amounts of
dopants, and its sensitivity is extremely high. Its high sensitivity, while depending on the combination of the sample
matrix and dopant, can evaluate low-concentration dop-3
13
16
ants in the range of 1.0×10 ~1.0×10 cm , making it
indispensable equipment for material and device development. Being a depth profile analysis technique, however,
it is basically used for one-dimensional analysis.
SEM is an analysis technique in which a focused electron beam is used to irradiate the object, and secondary
electrons and backscattered electrons emitted from theobject are detected. In case SEM is used for semiconductor surface observation, it is possible to observe the p-ncontrast using a cleaved surface. Being an easy-to-use
evaluation technique, it is very useful in the manufacturing
and development fields. In terms of SEM contrast, information resulting from surface irregularities and different
compositions are often utilized, but in the case of semiconductors, it is known that contrast is also generated by
the kind and quantity of dopants 10). Since the SEM technique is significantly influenced by oxygen and the likeadsorbed on semiconductor surfaces, it is somewhat difficult to make quantitative evaluation. However, it is now
extensively used for its ease of observation using simpleto-prepare cleaved samples.
SCM is a kind of scanning probe microscope in which aprobe is used to scan the surface of a semiconductor
sample, whereby it detects the p-n contrast by measuring
the electric capacitance on the surface. Since SCM gathers information with a depth of several tens nanometersfrom the surface rather than several nanometers, itenables quantitative evaluation without being influenced
by the conditions of extremely shallow surfaces. The spatial resolution of SCM is, while depending on the qualityof the metal probe tip, around several tens nanometers.
Electron holography is the method for measuring theinner potential of a sample. In comparison to the various
methods mentioned above, this method has its own
advantages and disadvantages, but its most remarkableadvantage is that it has high spatial resolution. Therefore,
as the semiconductor devices decrease in size, it is
becoming an indispensable technique in the development
sectors for Si devices. Since it is a technique for evaluating electrostatic potential distribution, it may be defined
as an indirect evaluation technique for carrier distribution.

where e, m, and ψ are the charge, rest mass, and wave
- is Dirac’s confunction of electron, respectively; and hstant, E is the energy of electron, A is the vector potential,
and V is the scalar potential. In electron holography, theequation can be solved by using a phase integral technique of the WKB method to obtain the phase change
which is expressed as follows:∆φ＝∫ k＋
c

V

eA
k− 2E
h

(2)

ds

where k is the wavenumber. The first term on the righthand side represents the phase change due to the optical
path difference. This term can be ignored in the experimental system of electron holography, since the electron
trajectories 1 and 2 have the same optical path. The second term represents the phase change due to electrostatic potential. The third term represents the contribution ofvector potential A, reflecting the conditions of the magnet-

Electron source

Trajectory 1

Trajectory 2

Specimen：
inner potential V
Path C
Magnetic ﬂux density B

PRINCIPLE OF SEMICONDUCTOROBSERVATION USING ELECTRONHOLOGRAPHY

3.1 Electron Holography TechniqueElectron holography, a kind of TEM techniques, became
widely known for its successful validation of the Aharonov-

ObserverFigure 1

Phase measurement of electron wave.

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Observation of Carrier Distribution in Compound Semiconductors Using Off-axis Electron Holography

ic flux. Thus as shown in Figure 1, both the inner potentialchange in a sample placed in the electron trajectory andthe flux that penetrates the area enclosed by the electron
trajectory can be observed as the phase change of awave function. The observation of carrier distribution insemiconductors described in this paper takes advantageof the information on the inner potential represented inthis second term.
Relationship between Electrostatic Potential andPhase Difference
It is widely known that electrostatic potential distribution
in semiconductors such as the p-n junction can beobserved by using electron holography. This is because
differing electrostatic potentials can be expressed as aphase difference of electron waves as shown in the following equation:

have a great influence raising a substantial problem. The
damaged layer consists of fine crystals and amorphous
substances, with a thickness up to 30 nm in some cases,
and causes noise in the phase information. For these reasons, the TEM samples for the measurement presented
here were prepared by cleaving. The cleaved TEM sample permits quantitative discussions, because, in spite of
the fact that its thickness changes, the rate of changeremains constant.

3.2

∆φ＝

π
λE

Vt

Observation of p-n Junction in Compound
Semiconductors Using Cleaved Samples

4.2.1 Observation Conditions
As the electron holography observation equipment, weused a Hitachi HF-2000 equipped with an FE (Field
Emission) electron gun. The electron beam acceleration
voltage was 200 kV. The phase image was reconstructedusing Fourier transformation.

(3)

where λ is the wavelength of electron wave and E isthe energy of electron beam, and hence π/λE is a constant. Furthermore, t is the sample thickness, V is thepotential. From Equation (3), the electrostatic potential
distribution can be observed, if the thickness of TEM
sample is uniform, by detecting the phase distribution.
At the p-n junction, an electrostatic potential distribution
is formed such that the Fermi levels come into coincidence. Thus while the n-type and p-type semiconductors
from the same kind of semiconductor have almost thesame mean inner potential, the resulting mean innerpotential has the same distribution as for the electrostatic
potential, due to the Fermi level shift. More specifically,using electron holography, it becomes possible to measure this inner potential that underwent an apparentchange.

4.

4.2

OBSERVATION OF COMPOUNDSEMICONDUCTORS BY ELECTRONHOLOGRAPHY: IN CASE OF SAMPLESPREPARED BY CLEAVING

4.1 Advantages of Cleaved Samples
As can be seen from Equation (3) where the thickness
information as t is included in the right-hand side, theinformation on sample thickness appears in the phase
image if the TEM sample has thickness nonuniformity. It isnecessary, therefore, to prepare a TEM sample with a uniform thickness. However, since this requirement for sample thickness uniformity is higher in electron holography
than in standard TEM, it is difficult to fabricate a sample
with amply uniform thickness by means of TEM sample
preparation techniques relying on mechanical polishingand Ar ion milling alone. Even when the FIB technique isused for sample preparation, the damaged layer on the
TEM sample surface formed during the FIB process may

4.2.2 Observation Samples
The p-n junctions of GaAs were observed. The p-type
GaAs was doped with silicon with a dopant concentration-318
of 1.0×10 cm ; while the n-type GaAs was doped with-3
18
carbon with a dopant concentration of 2.0×10 cm . The
samples were prepared by cleaving. Figure 2 shows theprocedure of sample preparation. As shown, two cleaved
surfaces were exposed by two cleaving operations, andsubsequently the cleaved sample was mounted with a tilt
angle of 45° on to the TEM holder.
4.2.3 Observation Results
Figure 3 shows the hologram and phase image obtained.
The downside of the image corresponds to the vacuum
region. In the phase image shown in Figure 3 (b), theposition of the p-n junction is clearly visible. Also it can beseen that the phase jump has larger deflections in the
thicker layers.
4.2.4 Analysis of Phase Image
An attempt was made to quantitatively obtain, from thephase image shown in Figure 3 (b), the potential difference between the p-type and n-type semiconductors. The
phase change due to the inner potential and the sample
thickness can be expressed, as has been describedabove, by the following equation:cleavedpncleavedepn

cleavedsurfaces

Figure 2

cleaved surfaces

Preparation of cleaved samples for electron holography.

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Observation of Carrier Distribution in Compound Semiconductors Using Off-axis Electron Holography

∆φ＝

π
λE

where CE has replaced the constant term.
In this experiment, the edge has a 90° angle since thesample is cleaved, and when the sample is tilted to 45°
from the horizontal, the following relation is detemined:
(5)

t＝2 x

where x is the distance from the sample edge, t is the
sample thickness. Hence the following relationship isderived from Equations (4) and (5).

V＝

1 dφ
CE

1 dφ
1
α
＝
≡
dt 2CE dx 2CE

（a）

p

n

specimen
edge

50 nm

50 nm

p-n junction(a)Hologram and (b) phase image of p-n junction in
GaAs cleaved sample.

（a）

n

（b）

（c）

20
15
10
5
0
0

Figure 4

50

100

nm

n-GaAs3025phase（rad）

phase（rad）

50 nm

p-GaAs

30

150

200

（αn−αp）
2CE(7)

To perform numerical analysis, as shown in Figure 4, anaveraged phase profile over the p-type and n-type regions
is created from the phase image, and α n and α p are
obtained from the slope of the profile. Here, Vn-Vp was
calculated from the slope α n -α p, and the result was
1.2 eV.
On the other hand, the difference between the Fermi
levels of p-GaAs(2e18) and n-GaAs(1e18) was calculated
to be approximately 1.35 eV, showing moderate agreement between the measured and calculated values. The
disagreement may be attributable to the influence of surface depletion layer to be described later and the inaccuracy in the inclination angle, and so on 12).

5.

（b）

p

1

(6)

where dφ/dx is replaced by α. From the above, thefollowing equation is established:

Figure 3

V n−Vp＝(4)

Vt≡CEVt

25
20
15
10
5
0

0

50

100

nm

150

200

Hologram and phase profiles of p-type region andn-type region.

OBSERVATION OF COMPOUNDSEMICONDUCTORS BY ELECTRONHOLOGRAPHY: IN CASE OF SAMPLESPREPARED BY FIB

5.1 Necessity of Removing FIB Damaged Layer
Preparation of TEM samples becomes very important in
the application of electron holography to compound
semiconductors. Use of FIB, although its capability of processing any specific part on the device is advantageous,
may pose the problem of forming a damaged layer on thesample. This problem is particularly significant with semiconductors such as GaAs, GaN, and InP, such that use of
a Ga ion beam with an acceleration voltage of 30~40 kV
can form a damaged layer of several tens nm in thickness
on the processed surface. It is the characteristic of thesedamaged layers that, unlike those in Si, fine crystals coexist in an amorphous substance layer 13). These fine crystals
is very problematic in that, when making electron holographic observations, the electron waves transmitting
through these fine crystals are influenced by diffraction
contrast, resulting in noise generation. In their observation
experiments on the GaAs p-n junctions using electron
holography, Cooper et al. were unable to reconstructclear phase images from TEM samples prepared by FIB
alone 14). They resorted to crystal recovery by annealing
the TEM sample in situ, thereby succeeding in obtaining aclear image. Although this in situ annealing is one of thepossible solutions, it is not necessarily applicable to any
materials, and in addition, a more rapid sample preparation technique is desirous from the standpoint of observing actual devices.
Accordingly, we have applied in this work another technique to the preparation of TEM samples for electron
holographic observations, in which as shown in Figure 5,
the damaged layer is removed by Ar ion milling after FIB
processing. This technique enables reducing the FIB
damaged layer down to several nanometers inthickness 13).

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Observation of Carrier Distribution in Compound Semiconductors Using Off-axis Electron Holography

5.2

Observation of Compound Semiconductor
Samples by Electron Holography

5.2.1 Observation Samples
Figure 6 shows a schematic diagram of the sample used
for observation. It is a model sample in which thin layersof p-GaAs and n-GaAs are deposited on a GaAs substrateusing MOCVD. In the n-GaAs layer, a high-concentration+n region and a low-concentration n region have been
fabricated. The dopants for this sample are silicon for then-GaAs and carbon for the p-GaAs. The dopant concen+
trations determined by SIMS for the n layer, n layer, and
-3
-3
-3
16
18
19
p layer are 1.3×10 cm , 3.0×10 cm , and 1.0×10 cm ,respectively.
5.2.2 TEM Sample Preparation Conditions
FIB was used for sample preparation. The FIB processing
equipment is a Hitachi FB-2100. A Ga ion beam of 40 kV
in acceleration voltage was used. The TEM sample wasprocessed to a thickness of 300 nm while monitoring the
thickness by SIM (Scanning Ion Microscope) image
observation. After the FIB processing, Ar milling was performed at room temperature. The Ar milling equipment is
a GATAN dual ion milling; the incident angle of the Ar ion
beam is 10°; and the sample was Ar ion milled for fiveminutes without being rotated.

Ga+

（a）

5.2.3

Measurement Conditions for Phase Shifting
Electron Holography
Observations were made using a JEOL-3000F equippedwith an FE electron gun. The acceleration voltage for theelectron gun was 300 kV. The holograms were imaged
using a GATAN 794 CCD camera with 1024×1024 pixels.
Under the observation conditions adopted, one pixel of
CCD camera corresponds to 2 nm in size. The voltage
applied on the biprism was 13 V. The phase shifting method 15) was employed for reconstructing the phase images,
and 13 holograms were taken.
5.2.4 Observation Results
Figure 7 shows a transmission electron micrograph. A
tungsten protection layer against FIB processing is located on the left-side of the sample. As has been revealed bythis transmission electron micrograph, it is impossible toobserve contrast between the regions with different dopants. Figure 8 (a) shows one of the holograms and Figure

n＋
4e18

p
4e19

Tungstenprotection
layer

n−
2e16

n−
2e16

p
4e19

W

Semiconductor

300 nmAl thin foil（b）

GaAs semiconductor

Figure 7

Transmission electron micrograph of FIB-prepared+
GaAs sample with p, n , and n regions.

（a）Tungsten

GaAs

Ar+

300 nmAr+（b）
200 nmFigure 5

Preparation of TEM samples using FIB and Ar milling.

（c）
200 nm

100 nm

700 nm

300 nm

700 nm

500 nm（1.3 e16）n−n＋（3.0 e18）n−（1.3 e16）p（1.0 e19）
Semi-insulatingp（1.0 e19）n−（1.3e16）

n＋（3.0e18）n−（1.3e16）p（1.0e19） Semi-insulating
GaAs substratep（1.0e19）

Figure 6

Schematic diagram of the GaAs p-n-p test sample.

Figure 8(a) Electron hologram obtained from GaAs sample.
(b) Corrected hologram without Fresnel fringes
obtained from the region enclosed by broken lines
shown in (a). (c) Reconstructed phase image
obtained from 13 holograms.

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Observation of Carrier Distribution in Compound Semiconductors Using Off-axis Electron Holography

8 (b) shows interference fringes obtained from the region
enclosed by broken lines shown in Figure 8 (a), after the
Fresnel fringes are removed and contrast adjusted. It canbe confirmed that, whereas in Figure 8 (a) the interference
fringes are invisible under the influence of Fresnel fringes,
they become clearly visible by the removal of Fresnel
fringes, showing deflections at the interface between theregions with different dopants. It may be noted that the
spacing between the interference fringes is about 60 nm.
Figure 8 (c) shows a phase image reconstructed from 13
holograms. This phase image shows that the p region isclearly distinguishable from the n region and that the n+region and n region can be clearly distinguished aswell 16).
5.2.5 Discussions
Figure 9 shows the averaged phase profile generated forthe purpose of interpreting the phase image shown in
Figure 8 (c). The phase difference across the p and n+regions is about 1.1 rad, and the one across the n and nregions is about 0.8 rad. Figure 10 shows the electrostatic
potential profile generated using the dopant concentration obtained from SIMS measurements. The profile was

p

2

n−

1.5
Phase（rad）

n−

n＋

p

0.8 rad1where t 0 is the thickness of a damaged layer. It should
be noted that Equation (8) is valid when the electrostatic
potential stays constant along the direction of the electron
beam in the TEM sample. Whereas Equation (8) indicatesthat the electrostatic potential difference and the phasedifference are in a linear relationship, the results of thisexperiment shows that the phase difference and the electrostatic potential difference for the n and p regions are
1.1 rad and 1.4 eV, respectively, and that the phase differ+ence and the electrostatic potential difference for the n
and n regions are 0.8 rad and 0.2 eV, respectively, thus
contradicting a linear relationship. This contradiction may
be attributed to invalidity of Equation (8), or more specifically, to the fact that the electrostatic potential fluctuates
along the transmission direction of the electron beam. In
such a case, the following equation should be used toexpress the relationship between the electrostatic potential and phase:t−2t 0(9)V（z）dz

1.1 rad

0

0

500

1000

1500

2000

2500

Distance（nm）-

Averaged phase profile across p-n junction and n+/ninterface obtained from phase image.

Figure 92p1.5

Potential energy（eV）(8)

φ＝CEV（t−2t 0）

φ＝CE∫00.5−0.5

generated by Band Calculator. In this profile, the electrostatic potential across the n and p regions is about 1.4+
eV, and the one across the n and n regions is about 0.2
eV. The relationship between the electrostatic potential
difference and the phase difference can be expressed by
the following equation, which is a modification of Equation(4) that takes into account the effects of the layer damaged at the time of sample preparation:

p

1
0.5
0

n−

n＋

500

1000

where z is the component along the transmission direction of the electron wave. To determine the changes in theelectrostatic potential and carrier distribution along the z
direction, a simulation was carried out taking into consideration the potential changes in the z direction. Figure 11
shows the directions along which the simulation was run.
The simulation was carried out under several assumptions such that the Fermi levels on the sample surface
from the bottom end of conductor are 0.90 eV for p-GaAs
and 0.65 eV for n-GaAs, respectively 17), 18), and that all the
dopants are activated. The TEM sample thickness wasdetermined by CBED (Convergent Beam Electron
Diffraction) measurement to be 280 nm.
Figure 12 shows the simulation results for the band dia-

n−

−0.5
-1
−1.5
−2

0

1500

2000

2500

p

n−

n＋

n−

p

substrate

Distance（nm）
（eV）
c

（eV）
v

（eV）
f

Figure 10 Band diagram of sample obtained by solving
Poisson’s equations self-consistently.

Line ALine BLine C

Figure 11 Schematic diagram of the directions along which
simulations were run.

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Observation of Carrier Distribution in Compound Semiconductors Using Off-axis Electron Holography

Potential Energy (eV)

1.5

（a）

1
0.5
0
−0.5
−1
−1.5
−2

0

100

200

region.
Based on these results and using Equation (9), we
recalculated the phase difference. The result shows thatthe phase differences across the n and p regions (Δφn-p)+
and the n and n regions (Δφn+n-) are 1.62 rad and 1.11
rad, respectively. These results are in better agreementwith the experimental results than before in terms of linear
relationship, but the agreement is still insufficient. This is
attributable to the influence of an electric inactive layerthat has been formed by the Ga ion beam. Assuming thata 40-nm thick electric inactive layer exists on the TEM
sample surface, we have a new set of Δφn-p and Δφn+nvalues of 1.08 rad and 0.84 rad, respectively, resulting ingood agreement with the experimental results.
Figure 13 shows a cross-sectional structure of TEM
sample that is deduced from the experimental results
above described. A damaged layer due to FIB, consistingof amorphous substances and fine crystals existed on thesurface but this was removed by Ar milling, so that a dam-

Carrier concentration（cm−3）

gram and the carrier concentration, in which Figure 12 (a)+and (b) show those for the n region, Figure (c) and (d)for the n region, and Figure (e) and (f) for the p region. It+
can be seen that, in the n and p regions of high dopant
concentration, the bands are bent sharply near the surface of a TEM sample. Also, it can be confirmed from thecarrier concentration profiles that the carriers are
decreased near the surface, forming a depletion layer. On
the other hand, the n region is under significantly different conditions from those of bulk substances. That is tosay, the carriers disappear as can be seen in Figure 12(d), and the bands are globally bent as shown in the band
diagram in Figure 12 (c). The Fermi level is locatedapproximately at the middle of the bandgap, thus indicating remarkably different characteristics compared tointrinsic n semiconductors. From the above, the experimental results suggest that whereas the depletion layer+
exists only near the TEM sample surface for the p and nregions, it extends to the entire TEM sample for the n（b）

1.0×1020
1.0×1019
1.0×1018

1.0×1017
1.0×1016
1.0×1015

0100Potential Energy (eV)

1.5

（c）

1
0.5
0
−0.5
−1
−1.5
−2

0

100

200

（d）

1.0×1020
1.0×1019
1.0×1018

1.0×1017
1.0×1016
1.0×1015

0100Potential Energy (eV)

（e）

1
0.5

0
−0.5
−1
−1.5
−2

0

100

200

（f）

1.0×1020
1.0×1019
1.0×1018

1.0×1017
1.0×1016
1.0×10150100

Depth（nm）
（eV）
c

200

Depth（nm）
Carrier concentration（cm−3）

Depth（nm）
1.5200

Depth（nm）
Carrier concentration（cm−3）

Depth（nm）

200

Depth（nm）
−3
（cm
）
v

（eV）
f

（cm−3）（cm−3）

Figure 12 Simulations of the band diagram and carrier concentration in the TEM sample. (a) band diagram and (b) carrier concentra+tion for the n region along line A. (c), (d) those for the n region along line B. (e), (f) those for p region along line C.

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Observation of Carrier Distribution in Compound Semiconductors Using Off-axis Electron Holography

aged layer several nanometers in thickness due to Ar milling is thought to be present. Under the damaged layer, anelectric inactive layer formed by Ga ion milling or Ar milling exists; under which a depletion layer is present with athickness that depends on the dopant concentration. With
respect to the sample observed here, the entire n regionof the sample is supposed to be depleted.

6.

CONCLUSION

The electrostatic distributions in GaAs have been successfully observed using electron holography. This means
indirect observation of carrier distributions, since electrostatic distribution corresponds to carrier distribution in thecase of semiconductors. It is shown that the use of acleaved sample allow clear observation of p-n junction.
The technique is advantageous in that, although it provides only one-dimensional information, it makes preparation of samples and interpretation of measurement results
easy without the need for consideration on FIB damaged
layer. In the case of sample preparation using FIB, on theother hand, it is advantageous to remove the FIB damaged layer by means of Ar ion milling, as long as observation is concerned. In our experiments for this case, theresults suggest that an electric inactive layer is present on
both sides of the TEM sample. In the observation of GaAs,
it was possible to distinguish the dopant concentration
-3
-3
16
18
regions of 1.3×10 cm and 3.0×10 cm in n-typesemiconductor, as well as p-n junction. These techniques
are applicable to observation of actual devices, making
themselves a useful means for improving the performanceand reliability of semiconductor devices.

ACKNOWLEDGMENTS
We thank our joint researchers at the Nanostructures
Research Laboratory, the Japan Fine Ceramics Center,
including Dr. T. Hirayama, Dr. K. Yamamoto, and Dr. Z.
Wang (currently with Micron Technology).

e−Thin amorphous layer
Inactive layer

p

n

n−

Depletion layer
Inactive layer
Thin amorphous layer

REFERENCES
1) D. Gabor: “A new microscopic principle,” Nature, 161 (1948), 777.
2) A. Tonomura, A. Fukuhara, H. Watanabe and T. Komoda: “Opticalreconstruction of image from Fraunhofer electron hologram,” Jpn. J.
Appl. Phys., 7 (1968), 295.
3) G. Mollenstedt and H. Duker: “Fresnelscher interferenzversuch
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Figure 13 Cross-sectional TEM sample structure deduced byour study.

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