Laser beam induced currents in polycrystalline silicon thin films prepared by
interference laser crystallization
B. Rezek
∗†
, C. E. Nebel, M. Stutzmann
Walter Schottky Institut, Technische Universität München, Am Coulombwall, 85748 Garching, Germany
(January 21, 2002)
Polycrystalline silicon layers are prepared by interference laser crystallization (ILC) in the super
lateral growth regime (SLG). To characterize their microscopic photoelectrical properties, light beam
induced current (LBIC) is used, employing a focused laser beam for local generation of photocarriers
in the layers with spatial resolution of ≈ 0.4 µm. The results are correlated with surface morphology
obtained by atomic force microscopy. In the single pulse ILC, the temperature profiles are optimized
by changing the proportion of interfering beam intensities. The typical grains are of triangular shape,
with a length of 1.5 µm and width < 0.5 µm. The photocurrent response is dominated by variations
in the sample thickness. In the multiple pulse ILC, thin films with grains of quadratic shape and of
size exceeding 5 µm are obtained by shifting the sample through an interference pattern, thus taking
advantage of lateral epitaxial regrowth. Here, by use of a lock–in, LBIC can detect position and
local electronic properties of individual grain boundaries. Grain boundaries are clearly identified by
180◦ shifts of the photocurrent phase close to maxima of photocurrent amplitude. The photocurrent
is attributed to local fields at grain boundaries. These fields extend about 1.4 µm into the grains.
The barrier height at the boundary is about 110 mV.
73.61.Cw, 73.50.Gr, 68.37.-d, 68.55.Ac
I. INTRODUCTION
Silicon deposited as a thin film has found use in large area electronic devices, the most prominent representatives
of which are flat panel displays and solar cells. Up to now, on low cost substrates such as glass or polymer foils
silicon thin films can be grown only in amorphous or microcrystalline form and the electronic transport properties of
these films do not reach the quality of monocrystalline silicon. Solar cell and thin film transistor applications require
polycrystalline silicon thin films of high electronic quality, a prerequisite for which is a large grain size and a low
defect density. Since the average grain size achieved by conventional chemical vapour deposition techniques remains
in the range of several tens of nanometers, which for instance limits mobility of charge carriers to about 1 cm2 /Vs
compared to 1300 cm2 /Vs in monocrystalline silicon, different methods for crystallization of thin amorphous silicon
films have been investigated over the last few years.
Solid phase crystallization can produce rather large grains [1], but the process takes hours to complete and temperatures necessary during the growth (500 − 600◦ C) are too high for most low cost substrates. In continuous wave
laser processing [2], the growth of large grains is achieved by scanning of a focused laser beam, which is also slow
and expensive. Pulsed laser crystallization is a comparatively fast process [3], it is compatible with the conventional
amorphous silicon technology and can be applied to films grown on low cost transparent substrates.
Single pulse excimer laser processing in the so–called super lateral growth (SLG) regime [4,5] resulted in grains
considerably larger than the amorphous silicon layer thickness. In a very narrow processing window of laser energy
density (around 500 mJ/cm2 ) the silicon film is molten almost fully, but not completely. The remaining solid particles
act as seeds for the subsequent liquid phase crystallization. The maximum grain size is limited by the seed distance
and by the competing grain growth via spontaneous nucleation. See Tab. I for some relevant parameters of amorphous
and polycrystalline silicon.
In contrast to the narrow processing window and randomness of homogeneous pulsed laser treatment, control of
the grain sizes and of the location of grain boundaries can be achieved by fully melting the film only in selected areas,
while adjacent solid regions can initiate SLG.
∗
†
Electronic mail: rezek@fzu.cz
On leave from: Institute of Physics ASCR, Cukrovarnická 10, 162 53 Prague, Czech Republic
1
Among various methods of spatially selective melting, single pulse interference laser crystallization (ILC) is recognized as a cheap and fast process for preparation of large areas without using masks [6]. As shown in Fig. 1, the layer
melts only in parts where the periodic intensity profiles exceed the melting threshold of amorphous silicon. In the
transition region from supercooled liquid to solid silicon, crystallization seeds are generated. From these seeds SLG
starts and proceeds towards the center of the liquid at the interference maxima. However, the lateral grain size is
still limited by spontaneous nucleation [7] and variation of temperature profiles by asymmetric interference did not
show any significant influence on resulting grain sizes [8]. Very large and well ordered grains could be produced when
the interference pattern was sequentially shifted across the sample, resulting in sequential epitaxial regrowth of the
grains [9].
To elucidate the electronic transport properties of either the grains or the grain boundaries in polycrystalline silicon
thin films, microscopic techniques are required. A photoconductive material can be probed by electron beam [10–12]
or laser beam induced currents [13–15] (EBIC or LBIC). Both techniques combine the capability of determining
basic electronic parameters of a semiconductor, such as diffusion lengths, recombination velocities, a depletion region
widths, and the possibility of high spatial resolution. The major advantage of the LBIC technique, especially for
highly resistive materials, is that the light beam will not add any excess charge to the probed layer. In principle,
both the EBIC and LBIC are based on the generation of electron–hole pairs with an electron or light beam and their
separation by a local electric fields in the sample, e.g. at the grain boundaries.
In this paper, interference laser crystallization using both a single and multiple laser pulses is applied to prepare
polycrystalline silicon thin films with grain sizes larger than the film thickness. Microscopic photoelectrical properties
of the resulting films are then analyzed by LBIC and correlated with surface morphology obtained by atomic force
microscopy (AFM). This allows us to investigate the influence of individual grains and grain boundaries on electronic
transport in the thin films.
II. EXPERIMENTAL
A. Sample preparation
Polycrystalline silicon samples were prepared by pulsed interference laser crystallization of amorphous silicon (a–Si).
A 100 nm thin nominally undoped a–Si layer was deposited by low pressure chemical vapour deposition at 560◦ C on
2 µm of thermal SiO2 supported by a monocrystalline silicon substrate. Using helium for silane dilution, the hydrogen
content in the layer was reduced sufficiently (less than 1 at.%) to avoid layer damage, which can occur due to explosive
hydrogen effusion during laser irradiation. Another possibility to reduce the hydrogen content in the layer is effusion
of hydrogen at elevated temperature (≈ 400◦ C) after the deposition, but this additional technological step increases
overall processing times considerably. In order to protect the amorphous silicon from oxidation before it is processed,
a 50 nm SiO2 cap layer is grown on top of the a–Si.
To produce a simple interference pattern, two coherent laser beams are superposed to form a stripe–like spatial
profile of light intensity on the sample surface. Depending on the intensities I1 , I2 of the two laser beams, the one
dimensional spatial modulation (along the axis x) of the light intensity is given by:
µ
¶
p
2πx
I(x) = I1 + I2 + 2 I1 I2 cos
(1)
p
where p is the modulation period given by:
p=
λ
2 sin (α/2)
(2)
where λ is the wavelength and α the angle between the two interfering laser beams. A large difference between
the intensities I1 and I2 (so called asymmetric interference), results in a smaller spatial gradient of intensity and a
spatially constant temperature background in the sample.
The crystallization was performed in air by a 10 Hz Q–switched Nd:YAG laser equipped with a seeder to obtain a
larger coherence length (about 1 m). The fundamental emission at 1064 nm with a pulse length of 8 ns is frequency
doubled to a wavelength of 532 nm by a KD*P crystal and has a Gaussian shaped beam diameter of 8 mm. A
mechanical shutter is used to select single pulses. The output beam is divided by a beam splitter into two coherent
beams which interfere on the sample surface. The schematic setup is shown in Fig. 2. The angle of incidence between
the two beams has been chosen to 11◦ 30’, which results in a period of 5.3 µm of the stripe–like intensity pattern. The
sample can be scanned perpendicular to the interference lines by a piezo table.
2
The laser energy thresholds for the structural modification of amorphous and polycrystalline silicon (poly–Si) can
be estimated based on thermodynamic considerations using the relation:
Q = cm∆T + mH
(3)
where thermal capacity c, temperature difference ∆T , and enthalpy H can be found from Tab. I and m is the mass
of silicon layer. The energy density required to melt the layer was calculated to 56 mJ/cm2 for a–Si and 78 mJ/cm2
for poly–Si.
Considering absorption coefficients and multiple reflections of the laser beam in the samples, calculations using
refractive indices of air (n = 1), SiO2 (n = 1.4), and silicon (n = 4) show that only 63% of the incident energy is
absorbed in a–Si and 33% in poly–Si. Note also that during melting the material parameters are changing significantly
[16], e.g. reflectivity of liquid silicon is enhanced to 73%. Experimentally, energy densities of 165 mJ/cm2 or 340
mJ/cm2 are detected to melt the a–Si or poly–Si samples, respectively.
1. Single pulse ILC
Single pulse interference laser crystallization of a–Si layers was performed first with a total energy density of
280 mJ/cm2 which was split equally into two interfering laser beams.
Then, to investigate the effects of different thermal gradients on nucleation and grain growth, the proportion of
intensities of the two interfering beams was varied so that the maxima of spatial intensity profiles remained the same
but the minima were gradually increasing. The interference period was kept constant. Three representative intensity
profiles are shown in Fig. 3. Compared to profile A with equal beam intensities, more of the material in profile B or
C is molten. Therefore the grains are expected to grow larger due to the larger fraction of liquid silicon.
After the laser processing the SiO2 cap layer was etched away in a buffered solution of hydrofluoric acid. With this
procedure the silicon layer is preserved by the oxide layer through the whole laser processing, however, the oxide layer
buckles as it floats on the molten silicon due to relieved stress [16] and it can induce pronounced surface corrugation
[9]. Although the corrugation can be caused also by surface ripples which arise from interference of the primary laser
beam with scattered surface waves [17] or by frozen capillary shock waves [18], almost no ripples were observed on
control samples, which had the SiO2 cap layer removed prior to laser crystallization.
2. Multiple pulse ILC
In the single pulse ILC the maximum grain size can be limited by the spontaneous nucleation in the supercooled
liquid silicon. This limitation can be avoided by the so–called sequential lateral solidification, the principle of which
can be applied also in pulsed ILC as shown in Fig. 4. The sample is sequentially molten by successive laser pulses.
Between two subsequent pulses the sample is moved with respect to the laser beam over a distance smaller than the
SLG range achieved by a single pulse. The translation must be in direction of the lateral growth. In this manner,
continuous epitaxial regrowth of the grains is achieved.
To take advantage of epitaxial regrowth of the grains, the sample was mounted on a piezo table and shifted through
the interference pattern. The shift was perpendicular to the interference lines and consisted of 100 nm steps repeated
53 times. After each shift a pulsed laser illumination was performed with the same energy density of the primary laser
beam of 360 mJ/cm2 . Note that the shift of the interference pattern can also be accomplished optically by adjusting
the phase difference between the interfering beams with a phase shifter.
For multiple pulse interference laser crystallization the SiO2 cap layer was removed just prior to the laser processing,
so that its buckling was avoided and on the other hand, possible incorporation of ambient oxygen into the non–
hydrogenated a–Si was minimized. This procedure resulted in more flat surfaces, still, ripples could be observed at
some locations on the sample surface. They are most likely due to interference of the incident beams with a standing
surface wave [17].
After crystallization, titanium/platinum/gold contacts in coplanar configuration with 20 − 50 µm spacing were
evaporated on the sample masked with photoresist and dipped into 10% HF, so that its photoelectrical properties
could be characterized by LBIC.
B. Laser beam induced current technique
The scheme of the LBIC setup used in the present study is shown in Fig. 5. A helium neon laser (wavelength:
543 nm, output power: 0.25 mW) is used as the light source. The beam is homogenized and focused with a 100x
3
microscope objective. Neutral density filters are used for intensity variation. The actual intensity on the sample is
monitored by a pyrodetector. The sample is mounted on a piezo table with XYZ feedback controlled movements of
20 nm accuracy. An auto–focus control loop ensures constant spatial resolution during the scans. A CCD camera
allows optical control and monitors the focus quality. The entire measurement is controlled by a computer. As the
focus spot is scanned across the sample between the electrodes, photocarriers are generated locally and the resulting
photocurrent is registered by lock–in technique. External bias is usually applied to extract the photocarriers, however,
the built–in fields may also be probed under zero bias.
The spatial resolution of the LBIC setup was determined by moving the focus spot over the edge of a metallic
layer, below which a large area photodiode (p–i–n a–Si:H) was positioned. The derivative of the acquired intensity
profile is approximately gaussian with a typical diameter (Imax /e2 ) of about 0.8 µm. Within this diameter 99 % of
the intensity is confined. Then considering the Rayleigh criterion, an electronic resolution of approximately 0.4 µm is
expected. The lateral LBIC step size was ≤ 0.2 µm to avoid aliasing of the measured profile.
On the silicon layers processed with a single laser pulse, the laser beam induced photocurrent was measured applying
2 V bias to the electrodes and using lock–in detection with a chopping frequency of 141.7 Hz. The beam power was
8 µW. Considering a focus diameter of 0.8 µm, the incident laser beam intensity was about 1600 W/cm2 giving rise to
photocurrents of the order of picoamperes. The high intensity was necessary to generate detectable currents, however,
the photocurrent as a function of intensity then exhibits a sub–linear dependance. This indicates that electronic
properties can be altered by the illumination. In particular, direct illumination of grain boundaries can reduce grain
boundary charge and hence the barrier height and recombination velocity [14]. The high intensity did not induce any
structural changes in the sample, most likely because the illuminated volume is cooled by heat dissipation into the
surrounding material.
On the layers prepared by multiple pulses the laser beam intensity incident on the sample was 40 µW. The generated
photocurrent was measured using lock–in detection with a chopping frequency of 41.7 Hz. In this case no external
bias was applied to the electrodes and both amplitude and phase of the photocurrent were registered.
C. Atomic force microscopy
After LBIC measurements the surface morphology of the samples was characterized with a Digital Instruments
atomic force microscope operated in the tapping mode. Secco etching [19] (solution: 3 parts H2 O, 2 parts HF 50 %,
1 part 0.15 mol solution of K2 Cr2 O7 in H2 O) was usually applied to accentuate the grain boundaries in laser crystallized
layers, so that their granular structure could be evaluated by atomic force microscopy.
III. RESULTS AND DISCUSSION
A. Single pulse interference laser crystallization
Single pulse irradiation of a–Si layers with two interfering laser beams resulted in polycrystalline silicon, which was
arranged in stripes. The crystalline stripes were separated by the residual amorphous silicon close to the interference
minima as shown in Fig. 6.
The effect of different temperature gradients, which were induced by interference profiles shown in Fig. 3, on the
resulting layer structure is shown in Fig. 7. The amorphous regions along the stripe edges as well as fine grains in the
middle of the stripes have been dissolved by the Secco etching. Grain boundaries of larger grains are accentuated.
The grains are of a triangular shape. The maximum length of the grains grown in SLG regime is about 1.5 µm for
all intensity profiles. The width of the grains is less than 0.5 µm.
Taking into account that SLG starts at the solid/liquid phase boundary and ends in the middle of the molten zone,
the grain size expected from profiles in Fig. 3 would be 1.8 µm for profile A and 2.2 µm for profile B. For profile
C the initial material was already polycrystalline. Therefore the material remains solid below a laser energy of 340
mJ/cm2 , does not form a supercooled melt, and the expected grain length would be about 2.0 µm.
Experimentally, however, the grain size produced by all three intensity profiles is comparable. Instead of larger
grains, a region of small grains (< 100 nm) developed in the central part of the stripes. If we assume that in all three
experiments the SLG speed is 14 m/s (see Ref. [7]) and the grain length is 1.3 − 1.5 µm, SLG takes place for about
100 ns independently of the applied intensity profile. After this time, spontaneous nucleation in the center of the
liquid limits further SLG.
The spatial photocurrent profile acquired along the polycrystalline silicon stripes prepared with equal beam intensities is shown by the light gray curve (1) in Fig. 8(a). After LBIC characterization AFM surface profiles were
4
measured: profile (2) was obtained prior to the Secco etching and profile (3) was acquired on the etched surface. For
comparison note that AFM and LBIC have been measured on nearby but not identical spots. Comparing the two
AFM profiles in Fig. 8(a), we can see that Secco etching considerably roughened the originally smooth surface. This
is due to an enhancement of the grain boundaries as well as an enhancement of dislocations in grains and an overall
surface roughening induced by etching. Therefore, grain size and boundaries cannot be clearly identified from the
AFM profiles. Analysis of the two–dimensional AFM images in Fig. 7 revealed that the grain size along the scanning
direction does not exceed 0.5 µm. A periodic modulation of the profiles is a result of the SiO2 cap layer buckling
during the laser crystallization (see Sec. II A).
A resemblance between the LBIC (1) and Secco etched surface profiles (3) in Fig. 8(a) indicates that the fine
structure in the LBIC photoresponse is associated with the granular structure of polycrystalline silicon. However,
because the grain sizes are comparable to or smaller than the focus diameter, always several grains and grain boundaries
are illuminated by the focused laser beam. Since the material is also nominally undoped, with a conductivity ≈
10−5 (Ωcm)−1 at room temperature, depletion regions of grain boundaries can extend rather deeply into the grains
[20] and further reduce the contrast in photoresponse coming from the grain boundaries and grains. This suggests
that the fine structure observed in the LBIC profile cannot be directly interpreted as photoresponse of individual
grains or grain boundaries.
Although AFM and LBIC spatial profiles have not been measured simultaneously, their correlation may be evaluated
from their Fourier spectra, which are shown in Fig. 8(b). Since the grain size is smaller than 0.5 µm, information
related to grains and grain boundaries may be expected at spatial frequencies higher than 2 µm−1 , while corrugations
of the layer are transformed into lower spatial frequencies. In the low frequency part a peak at a spatial frequency
of 1.06 µm−1 in the LBIC Fourier spectrum agrees well with peaks in the Fourier spectra of AFM profiles before
(1 µm−1 ) and after Secco etching (1.1 µm−1 ). There is also a match in the spectra at 0.4 µm−1 . It suggests that
corrugations of the microcrystalline layer are reflected in the LBIC spatial profile.
The situation is different in the high frequency part (> 2 µm−1 ) of the Fourier spectra. Flat grains with sharp
edges at grain boundaries are transformed into a broad frequency range 2 − 25 µm−1 (limited by the AFM sampling
interval) in the AFM Fourier spectra. In the LBIC spectrum the frequency range is limited to 5 µm−1 due to spatial
sampling interval, or even to 2.5 µm−1 if we consider the resolution of 0.4 µm. The lack of distinct peaks in the high
frequency part of both AFM and LBIC Fourier spectra prevents a reasonable comparison.
Further analysis of the spatial profiles shows that the photocurrent varies around its mean value of 5.5 pA by ±16%.
This is in good agreement with the surface height variation of ±15% around a mean value of 100 nm (because AFM
yields only relative values of the surface height, the a–Si layer thickness before crystallization has been taken as the
absolute value). Considering that the absorption depth of green (λ = 543 nm) HeNe laser in polycrystalline silicon is
approximately 2 µm, which is much larger than the layer thickness, the data indicate that thickness variations prevail
over the influence of granular structure in the photocurrent response. Light trapping phenomena can be excluded in
our case, since the surface corrugations are much smaller in height (≈ 50 nm) than in length (≈ 1 µm).
B. Multiple pulse scanned ILC
With the technique of sequential lateral epitaxial regrowth, grains of the typical length of 5 µm (which corresponds
to the applied interference period) and of about 1.5 − 2.5 µm width were grown as illustrated by Fig. 9, where the
surface morphology of such sample after Secco etching is presented. Because of their large size, individual grains can
be probed by LBIC.
The overall shape of the grains is quadratic and the longer side of the grains is perpendicular to the interference
stripes. The long grains are separated from each other where the interference maxima of the last pulse were located
(see Fig. 9(b)). Major grain boundaries build up there and protrude above the rest of the surface as indicated by the
brighter areas in Fig. 9(a).
Spatial profiles of photocurrent amplitudes (solid) and phases (dashed) measured in short circuit mode are shown in
Fig. 10(a), the section of the surface morphology obtained by AFM prior to the Secco etching is shown in Fig. 10(b).
Note that due to experimental limitations the AFM and LBIC data have been acquired on slightly different traces.
The photocurrent and AFM profiles have been scanned perpendicular to the orientation of interference stripes. In
this direction the typical grain size is in the range of 5 µm. Grain boundaries show a slightly increased thickness of
about 15 nm and therefore they can be easily detected by AFM. The photocurrent shows pronounced fluctuations
with similar periodicity. A somewhat larger period in LBIC profiles than the period shown in AFM can be explained
by a slightly different alignment of scanning directions and a different length calibration in AFM and LBIC.
The maxima of photocurrent fluctuations are in close neighbourhood to 180◦ phase shifts, which indicate sign
reversal of the photocurrent. Recalling from the previous section that surface height variation are reflected in the
5
LBIC spatial profile, the increased thickness at grain boundaries contributes to the LBIC signal. However the increase
by 15 % in thickness can hardly explain nearly an order of magnitude increase in photocurrent observed in Fig. 10(a).
Since spatially resolved photocurrents as shown in Fig. 10(a) were observed in short circuit mode without application
of an external field, the photocurrents are generated by built–in fields which originate most likely from the band
bending at grain boundaries. It is interesting to note that within one period (distance between two photocurrent
maxima) of about 6 µm two phase shifts are observed.
An explanation for this phenomenon is schematically shown in Fig. 11. On each side of the grain boundary, the
electric field is oriented in the opposite direction. As the laser spot approaches the grain boundary from one side,
an increasing photocurrent is generated due to the increasing electric field. As soon as part of the light illuminates
the other side of the boundary, a photocurrent with opposite sign starts to flow, reducing the effective photoresponse
measured by the lock–in amplifier. When the laser spot illuminates the center of the boundary, both photocurrents
are about equal and the resulting current is zero. As the scan continues the photocurrent grows again, however with
reversed polarity. Therefore, the phase of the photocurrent at grain boundaries changes by 180◦ . The phase changes
again when the laser spot passes the grain center and approaches another grain boundary. Indeed, this double phase
shift per period is experimentally detected as shown in Fig. 10(a). Obviously, by evaluation of the phase shift of
the photocurrent, LBIC can reveal the positions of grain boundaries in the photocurrent spatial profile with high
resolution. The 180◦ phase shifts close to the maxima of photocurrent indicate the grain boundary positions.
Although the main features of the experiment are well represented by the model, there are also some quantitative problems. First, the experimentally detected photoresponse is not symmetric as expected by the model. This
asymmetry is most likely systematically introduced by the regrowth process, where the grains are epitaxially pulled
in one direction. This is illustrated by Fig. 9(a) where there are pronounced intra grain defects on the left side
of the emphasized large grain while the right side seems to be without crystallographic defects. This asymmetry
in defect density is then reflected also in the local photocurrent response, either by affecting the local electric field
and/or by enhanced recombination of photogenerated charge carriers. It should be further noted that a perpendicular
arrangement of the major grain boundary plane with respect to the surface is assumed in the model. A small tilt
of the grain boundary plane can also induce asymmetry in the photoresponse across the grain boundary [10], but in
the 100 nm thin layers this can hardly be significant across micrometer distances. Next, the model does not take
into account grain boundaries, which are oriented along the scanning direction. Their influence on the local field
and carrier kinetics is completely neglected. This is obviously not the case in the investigated layers. Last, there are
fluctuations of the photocurrent observed inside the grains in Fig. 10(a). The fluctuation can arise due to presence of
intra grain defects and/or from layer thickness variations, the influence of which becomes more pronounced farther
inside the grains where the grain boundary field is weaker.
When the local fields were probed, the current lock–in amplifier was connected to the right edge contact while the
left edge contact was grounded. Then a positive current corresponds to left–to–right flow of positive charge carriers.
Since the LBIC scan was carried out also in this direction and the left sides of grain boundaries exhibit positive
current (see Fig. 10(a)), the energy bands must be bent upwards at the grain boundaries. This is evidence for n–type
conduction of the crystallized material.
A pronounced LBIC response was also found close to the contacts. The photocurrent was about five times higher
there than the maxima observed between the contacts, where the LBIC measurement was usually done. This is due
to a Schottky barrier formed at the metal–semiconductor interface. Close to the left edge contact the photocurrent
was positive without external bias. This corresponds to a downward band bending at the contact, which is due to the
work function of titanium (4.33 eV) being lower than at n–type silicon (4.85 eV) [21]. However, the Schottky barrier
height cannot be determined from the work function difference since the density of surface states is at the contact not
known.
The total generated photocurrent in an LBIC measurement is the sum of the drift in the local field and carrier
diffusion. As shown schematically in Fig. 12, in the space charge region near the grain boundary the photogenerated
minority carriers (holes) are attracted by the electric field to the grain boundary where they recombine with majority
carriers (electrons) trapped in the grain boundary localized states [22,23]. The traps are refilled from both sides of
the grain boundary by the majority carriers which cross the barrier by thermal excitation. Farther from the grain
boundary, where the depletion field fades out, locally generated carriers must diffuse into the space charge region to
be collected by the grain boundary field. The current flow in this regime is governed by diffusion of minority carriers.
If the measured LBIC profile is controlled by the carrier diffusion, the width of the depletion layer at the grain
boundary must be much smaller than the grain size and the photogeneration region. The lateral extension of the
photogeneration region can be determined by measuring the distance of two current peaks at a grain boundary [14].
Using the peaks around the boundary located approx. at 9 µm in Fig. 10(a), the generation region could be estimated
to 0.6 µm. This value is even below the typical laser beam focus size of 0.8 µm (determined in Sec. II B). Therefore,
the LBIC signal at the grain boundary will be diffusion controlled for a depletion layer narrower than ≈ 0.6 µm and
for larger ones controlled by drift in a local field. Although the width of the space charge region at the grain boundary
6
is not known it can be estimated from the following.
If the carrier transport mechanism is controlled purely by diffusion, the minority carrier diffusion length L and
recombination velocity are usually deduced by analyzing the decrease of the induced current with the distance x of
the exciting beam from the grain boundary [24]. For vanishing surface recombination the current profile I(x) decays
approximately exponentially as the beam is scanned gradually away from the grain boundary and the diffusion length
can be easily obtained from [25]:
I(x) ∝ exp {−x/L}
(4)
However, the investigated layers are thin compared to the absorption depth and carrier diffusion length and thus
surface recombination is expected to play an important role. The increase in surface recombination results in a
reduction of the apparent diffusion length and advanced fitting procedures must be employed [10,26].
In this case, some simplifying approximations are taken for further evaluation. Assuming only carrier diffusion and
neglecting surface recombination, fitting Eq. 4 to the LBIC profile between two phase shifts gives a rough estimate for
the diffusion length of 0.7 ± 0.1 µm. Thus the carriers generated as far as 0.7 µm from the grain boundary depletion
region can contribute to the current. Taking into account a focus radius of 0.4 µm, grain size of 5 µm and the fact
that a photocurrent response is observed within the whole grain, the depletion region cannot be smaller than 1.4 µm.
This rather large width of grain boundary depletion region suggested from this simple evaluation is consistent with
conductivity and Hall measurements, which indicate intrinsic material. In Van der Pauw geometry a dark conductivity
of σvdp ≈ 1.6×10−5 (Ωcm)−1 was obtained at room temperature. Hall measurements yield a free electron concentration
of n ≈ 4.5 × 1012 cm−3 and a Hall mobility of µH ≈ 450 cm2 /Vs.
The height of the grain boundary barrier can be determined by compensation of local fields by external bias. The
phase shifts observed in Fig. 10(a) vanish at an externally applied voltage of about 4 V. This means that the local
field at one side of the grain boundary was compensated and the photocurrent flows in the direction of the external
field. Due to the intrinsic nature of the material there is not enough free charge within the grain to screen an external
field and the applied voltage is more or less homogeneously spread in the layer [27]. Then the local field may be
approximated by a triangle and the height of the local barrier h can be evaluated from the simple geometry:
h = barrier width
external bias
electrode gap
(5)
Substituting 1.4 µm for the barrier width, 4 V for external bias and 50 µm for electrode gap, the Eq. 5 gives an
estimate of 110 meV for the height of the barrier at a grain boundary. This is in agreement with previously reported
values of 100 − 200 meV [22].
The obtained electronic parameters of polycrystalline silicon layers prepared by multiple pulse interference laser
crystallization are summarized in Tab. II.
IV. CONCLUSIONS
To conclude, photoelectrical properties of polycrystalline silicon layers prepared by interference laser crystallization
were characterized by the LBIC technique with the resolution in the sub–micrometer regime and correlated with
surface morphology obtained by atomic force microscopy.
In the thin films crystallized by single pulse irradiation, the typical grain sizes along the polycrystalline stripes were
smaller than the LBIC focus diameter and the photocurrent response to a local focused laser beam excitation was
dominated by the variations in the sample thickness. The maxima of the photocurrent were correlated with maxima
of the surface height. The pronounced surface corrugation was induced by buckling of the SiO2 cap layer during laser
processing.
Application of asymmetric laser interference crystallization enabled adjustment and optimization of the transient
temperature profiles. However, the duration of super lateral growth was always about 100 ns. After this time
spontaneous nucleation in the center of the liquid region limited further SLG so that the typical length of the grains
of 1.5 µm did not change significantly.
Grains of quadratic shape and of sizes exceeding 5 µm were grown by shifting the sample sequentially through the
interference pattern, thus taking advantage of the lateral epitaxial regrowth. The SiO2 cap layer was removed just
prior to the crystallization so that buckling was avoided, resulting in a less significant contribution of the surface
height variations to the LBIC signal.
In these large grained silicon thin films, LBIC resolved individual grain boundaries with spatial resolution in the
sub–micrometer regime. Applying lock–in detection, grain boundaries were clearly identified by 180◦ shifts of the
7
photocurrent phase close to the maxima of the photocurrent amplitude. The photocurrent was generated by local
fields at the grain boundaries. These fields were estimated to extend about 1.4 µm into the grains and the barrier
height at the boundary was about 110 meV. This indicates that grain boundaries are efficient electronic barriers for
carrier propagation in these layers.
ACKNOWLEDGMENTS
This support of BMBF contract BEO 0329814 and the DFG contract NE 524/1-1 is acknowledged. The cooperation
and assistance of Christopher Eisele with setting up the LBIC apparatus is gratefully appreciated.
[1] T. Matsuyama et al., J. Non-crystal. Solids 198-200, 940 (1996).
[2] N. M. Johnson, D. K. Biegelsen, and M. D. Moyer, in Laser and Electron-Beam Solid Interactions and Material processing,
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8
FIG. 1. The principle of interference laser crystallization. (a) Two interfering laser beams generate a spatially periodic
intensity profile. (b) Where the intensity exceeds a certain threshold the sample is crystallized. (c) This results in alternating
stripes of amorphous and crystalline silicon (shown in cross-section).
FIG. 2. The schematic setup for pulsed interference laser crystallization. The sample can be scanned perpendicularly to the
interference lines by a piezo table.
FIG. 3. One period of interference profiles calculated with Eq. 1 using for A I1 = I2 , for B I1 = 5.3I2 , and for C I1 = 15.7I2 .
The shaded regions describe the regions of solid and supercooled liquid silicon.
FIG. 4. Sequential lateral solidification process.
FIG. 5. The scheme of the LBIC setup.
FIG. 6. CCD image showing polycrystalline silicon stripes separated by amorphous silicon. There are metallic contacts along
the top and bottom edge. The arrow indicates the scanning direction of LBIC. The bright dot is the laser spot.
FIG. 7. AFM surface morphology of Secco etched layers crystallized by: (a) profile A, (b) profile B, and (c) profile C of Fig.
3, respectively.
FIG. 8. (a) Spatially resolved photocurrent (1) measured on the polycrystalline silicon stripes prior to the Secco etching and
corresponding AFM surface profiles measured before (2) and after (3) the etching. (b) Fourier spectra of the spatial profiles.
FIG. 9. (a) AFM surface morphology of a silicon thin film prepared by sequential epitaxial regrowth (in the x–direction).
Grain boundaries have been accentuated by Secco etching. The black rectangle emphasizes one large grain. In the bottom
image (b) The intensity distribution of the last laser pulse (interference period p = 5.3 µm).
FIG. 10. (a) Spatial variations of the photocurrent (solid line) and of its phase (dashed line) in short circuit mode (U = 0).
(b) AFM surface profile prior to Secco etching. The pronounced peaks are related to grain boundaries.
FIG. 11. Model of photocurrent response induced by a laser beam, which scans across two grain boundaries. Due to band
bending a photocurrent is generated with reversed polarity on each side of the boundary. Using lock–in detection, the phase of
the photocurrent changes by 180◦ at transitions through zero.
FIG. 12. The concept of beam induced currents in the vicinity of a grain boundary in an n–type semiconductor. Trapping
of electrons at grain boundary defects leads to formation of depletion regions on both sides of the grain boundary. (a) While
diffusion processes prevail farther away from the boundary, (b) drift in the local field and recombination dominate in the space
charge region near the grain boundary.
9
TABLE I. Optical and thermodynamic parameters of silicon.
reflectivity R at 532 nm
absorption coef. α [cm−1 ] at 532 nm
optical band gap Eg [eV]
density ρ [g/cm3 ]
thermal capacity c [J/gK] at 293 K
melting point Tm [K]
enthalpy Hm [J/g]
amorphous Si
0.40
1.25 × 105
1.7
2.20
1.09
1400
1320
crystalline Si
0.38
5 × 103
1.1
2.33
0.89
1685
1805
liquid Si
0.73
1 × 106
2.52
1.1
TABLE II. Electronic parameters of polycrystalline silicon layers prepared by multiple pulse interference laser crystallization.
conductivity type
dark conductivity σvdp at RT
free carrier concentration n at RT
Hall mobility µH at RT
diffusion length estimate
width of grain boundary depletion region
barrier height at the grain boundary
n–type
≈ 1.6 × 10−5 (Ωcm)−1
≈ 4.5 × 1012 cm−3
≈ 450 cm2 /Vs
≈ 0.7 µm
≈ 1.4 µm
≈ 110 meV
10
Fig. 1, Rezek
(a) laser beam
interference
¯
(b) periodic intensity profile
I
x
crystallization
threshold
growth
(c)
mc-Si
a-Si
substrate
Fig. 2, Rezek
piezo
table
Nd:YAG
laser
mirrors
sample
beamsplitter
Fig. 3, Rezek
900
spontaneous
nucleation
800
intensity [mJ/cm2]
700
600
SLG
SLG
500
C
400
300
supercooled
B
200
100
solid
A
0
0
1
2
3
distance [mm]
4
5
Fig. 4, Rezek
shift
st
1 pulse
2 nd pulse
result
Fig. 5, Rezek
Fig. 6, Rezek
Fig. 7, Rezek
(a)
(b)
(c)
4.5 mm
Fig. 8, Rezek
120
7
5.5 pA
±16%
100
(1)
80
6
5
60
4
(2)
40
100
nm 20
±18%
0
secco
100 -20
nm
-40
±15%
3
2
photocurrent [pA]
relative height
[nm]
(a) LBIC & AFM profiles
1
(3)
0
0
1
3
2
4
5
6
7
8
9
10
11
distance[mm]
amplitude [pA]
amplitude [nm]
(b) FFT of LBIC & AFM profiles
0.4
15
0.3
0.40
0.2
LBIC
1.06
0.1
0.0
AFM
AFM, secco
10
5
0
0.5
1.0
1.5
2.0
2.5
3.0
frequency [1/ mm]
3.5
4.0
Fig. 9, Rezek
(a)
(b)
AFM and LBIC scan direction
I
x
p = 5.3 mm
Fig. 10, Rezek
(a) LBIC spatial profile
0.8
16
18
180
150
120
90
60
30
0
-30
-60
-90
-120
-150
-180
20
16
18
20
period
phase shift [deg]
photocurrent [pA]
1.0
0.6
0.4
0.2
0.0
0
2
4
6
8
10
12
14
distance [mm]
(b) AFM profile
25
relative height [nm]
20
period
15
10
5
0
-5
-10
-15
0
2
4
6
8
10
12
14
distance [mm]
Fig. 11, Rezek
grain boundary
grain boundary
energy
bands
photocurrent
0°
phase
amplitude
- 180°
Fig. 12, Rezek
(a)
(b)
traps refilled
ee-
eediffusion
h+
drift
h+
recombination
+ +
+
space charge
EF
- - - - defects
(trapped electrons)