Fig. 6. The modulation amplitude vs. electric field of (a) the reflectance ( Re) and (b)
transmittance ( Tr) of the He-Ne laser measured at the glass/GaAs interface. Positive and
negative signs refer to positive and negative logic, respectively. The solid and broken lines
are guides for the eyes
For comparison, Fig. 7 shows the corresponding LC results for the thin-film, i.e., the film
side was excited as in previous works (Erlacher and Ullrich, 2004, Ullrich et al. 2004). The
film reflection exhibits almost no modulation, whereas the maximum of the transmission
Employment of Pulsed-Laser Deposition for Optoelectronic Device Fabrication
591
modulation is comparable with the results in Fig. 6, while, as suspected from the PL results
(Ullrich & Schroeder a, 2001, Ullrich and Schroeder b, 2001), at the interface, more
absorption transitions are excited with the same amount of impinging photons than at the
film surface itself. Furthermore, from the viewpoint of device engineering, with the
employment of the reflected beam effective fan-out is easier to achieve than with the much
weaker transmitted signals, which were approximately 0.8% and 0.2%, for the red and green
laser, respectively.
8
Film
6
e (%)d 4 Positive logic
2
plitu
(a) Re at 633 nm
0
amon -2
lati
-4
Negative logic
odu
(b) Tr at 633 nm
-6
M
-8 0
1
2
3
4
5
6
Electric field (kV/cm)
Fig. 7. Corresponding results to Fig. 6 measured at the film side
Equivalently to Fig. 4, pump-probe experiments have been performed at room temperature
by replacing the green cw laser irradiation with green (532 nm) ultrafast (<100 fs) laser
pump pulses with a repetition rate of 1 kHz, and, instead with the He-Ne laser, the interface
was probed with white light continuum. The reflected spectrum was guided in a fiber
spectrometer. A typical result of the reflection kinetics for 633 nm is shown in Fig. 8, which
depicts the ratio of the reflection without pump over the reflection with pump ( Re 0/ Re).
Turn-on and recovery time are extremely fast resulting in a kinetics profile with a full width
at half maximum (FWHM) of about 600 fs. Hence, in contrast to the thin-film itself whose
recovery time exceeded more than 10 ps (Ullrich et al., 2004), metastable states with unusual
short lifetimes in the fs range are generated at the glass/GaAs interface. It is worth
mentioning that our temporally resolved reflection alteration takes place on a similar time
scale as the transmission kinetics of CdTe nanocrystals (Padilha, 2005) and our observed
recovery time can be compared with that in Bragg-spaced quantum well structures
(Johnston, 2005).
592
Optoelectronic Devices and Properties
1.004
Interface
1.003
1.002
/ Re 0 1.001
Re
1.000
0.999
0.998 0 1 2 3 4 5 6 7 8
Time (ps)
Fig. 8. Ultrafast reflection kinetics of the glass/GaAs interface at 633 nm. The ordinate
shows the signal without pump pulse ( Re 0) over the signal with the pump pulse ( Re)
3. Hybrid photosensitive device formation
The need for operative optoelectronic devices composed of various semiconductors is based
on technological requirements as well as economical arguments, while in many cases
simultaneous improvement of both branches is anticipated. In particular, operative hetero-
pairing of GaAs and Si has been pursued for quite some time owing to its considerable
technological importance for efficient and low-cost photovoltaic and optoelectronic devices
(Alberts et al., 1994). However, GaAs-on-Si formations are not a straightforward task. The
mismatch of thermal expansion coefficients (60%) and lattice constants (4%) causes stress in
the film during deposition and the formation of defect states at the film substrate interface,
respectively. Besides improvement of the electrical and structural properties of the GaAs/Si
heterostructure can be achieved using a ZnSe (Lee et al., 1994) or SrTiO3 (internet, 2001)
buffer layer, direct growth and deposition were pursued by using a wide range of methods
since several decades with varying success and satisfaction. For example, the direct growth
of GaAs on Si employing MBE was studied (Usui et al., 2006).
Concerning the hetero-pairing of PLD GaAs with Si, previous research mainly focused on
rectifying properties and bias dependent alternating photocurrent (APC) of p-GaAs on n-Si
Employment of Pulsed-Laser Deposition for Optoelectronic Device Fabrication
593
(Ullrich and Erlacher a, 2005, Ullrich and Erlacher b, 2005). Here, stoichiometry, surface
morphology, crystallographic properties, and quality of rectification of PLD formed n-GaAs
on p-Si heterostructures were studied. In addition, the bias dependence of APC spectra and
direct photocurrent (DPC) spectra is demonstrated.
The deposition of GaAs onto Si was carried out with the second harmonic emission at 532
nm of the Q-switched Nd:YAG laser (6 ns, 10 Hz). The substrate was boron (B) doped prime
grade 250-300 μm thick p-type Si (100) wafer and the target was tellurium (Te) doped n-type
GaAs (100) with a doping concentration of 1.3-3.4×1018 cm-3. As usual, the rotating (16 rpm)
GaAs target was kept at 45º with respect to the impinging laser beam. The rotation was
performed in order to ablate the material homogeneously. Parallel to the ablation area of the
target, the substrate was mounted 6 cm above the target. The deposition was carried out in
vacuum of 6.7×10-5 Pa without heating the substrate in order to avoid interface stress and
deformation during cooling. The target was exposed to a laser fluence of 0.98 ±0.1 J/cm2 for
one hour resulting in a GaAs film with a thickness of about 600 nm. In order to perform
APC and DPC measurements, electrical contacts were made with evaporated aluminum (Al)
to the surface and by silver (Ag) paste to the rear of the sample.
Stoichiometry, crystallographic properties, and surface quality of the GaAs film were
investigated by electron probe micro-analyzer (EPMA) technique, x-ray diffraction, and
scanning electron microscopy (SEM), respectively. The current-voltage ( I- V) characteristic,
of the heterojunction was measured in the dark by sweeping the applied bias between the
Al and Ag contacts with a programmable source meter. The APC spectra were measured
with lock-in technique by exciting the sample surface with chopped (107 Hz) light, while
steady-state illumination and a pico-amperemeter were used to record the DPC responses.
The sample was irradiated with a 200 W halogen lamp, whose emission was
monochromatically dispersed with a ½ m monochromator. Excitation light was passed
through a long pass filter and the photon energy was varied from 1.1 to 1.80 eV and the
biases were applied using a programmable power supply. The PC was measured vertically
through the device using contacts 0 and 1 or laterally employing contacts 1 and 2 as
schematically shown in Fig. 9. All the PC spectra shown have been corrected with a
calibrated Si photodiode in order to express the results in terms of responsivity (A/W). All
measurements were carried out at room temperature.
The EPMA results show Ga and As concentrations of 48.80 (at. %) and 51.20 (at. %),
respectively. Hence, low-temperature PLD forms quasi-stoichiometric GaAs films on Si
with a slight As surplus. The slight deviation of stoichiometry reflects the trend known
from MBE formed GaAs, i.e., the growth of GaAs at low substrate temperatures results in
GaAs with excess As incorporated into the GaAs matrix (Ma et al., 2004), which is
amorphous as the x-ray result in Fig. 10 reveals. The only GaAs related peak observed is
the fairly faint feature at 27.25 degree indicating that the film texture is predominately of
amorphous nature embedding small portions of crystallites. We should stress that
previous x-ray investigations on n-GaAs/ p-Si did not show peaks, presumably due to the
extremely thin film. The SEM image, (Acharya et al., 2009), shows droplet-like micron-
sized clusters. With selectively performed Raman measurements, we found that these
clusters are single crystalline GaAs, which are not grown but directly transferred from the
target during PLD (Erlacher et al. 2006). Since these droplets increase the surface area, the
formed hetero-pairing has the potential to be used as chemical sensors and chemical
reaction catalysts.
594
Optoelectronic Devices and Properties
Power supply
Lock-in amplifier
Al (1)
Monochromatic light
Ag (0)
Chopper Al (2)
Si
PLD-GaAs
Fig. 9. Experimental setup used for the APC measurements. DPC was measured without
chopper and the lock-in amplifier was replaced with a pico-amperemeter
350
ts)
(111)
un
300
(co
sity
ten
250
y in
-raX
20020
22
24
26
28
30
2Θ (degree)
Fig. 10. X-ray pattern of the GaAs film
The dark I-V characteristic of the n-GaAs/ p-Si device is shown in Fig. 11. The device showed rectification and the symbols represent the measurements, while the solid line visualizes
close fit of the forwardly biased I-V curve using the common diode equation (Fahrenbruch
&Bube, 1983),
