We investigated the structural and optoelectronic properties of p-n germanium nanocrystals based junctions embedded between GaAs substrate and layers of ZnO:Al or a-Si:H. Scanning electron microscopy and scanning tunneling microscopy were used on these junctions in this work. Calculations of tunneling current on the substrate showed effect of localized defects trapping Fermi level at the surface tending to make a semi-insulating substrate. The average value of the diameter of the Ge nanoparticle is around 12.5 nm. These results lay the foundation for the development of solar cells which active part is made of GeNCs

Dzagli, M.M.

Gadedjisso-Tossou, K.S.

Grandidier, B.

Mohou, A.M.

Sossoe, K.K.

SocioEconomic Challenges Journal (Sumy State University)

JOURNAL OF NANO- AND ELECTRONIC PHYSICS ЖУРНАЛ НАНО- ТА ЕЛЕКТРОННОЇ ФІЗИКИ 
Vol. 8 No 4(1), 04035(5pp) (2016) Том 8 № 4(1), 04035(5cc) (2016) 
 
 
2077-6772/2016/8(4(1))04035(5) 04035-1  2016 Sumy State University 
Optical and Structural Characterization of Pin Photodetector Based on Germanium  
Nanocrystals for Third Generation Solar Cells 
 
K.K. Sossoe1,*, M.M. Dzagli1, K.S. Gadedjisso-Tossou1, A.M. Mohou1, B. Grandidier2 
 
1 Laboratoire de Physique des Composants à Semi-conducteurs (LPCS), Département de Physique, Faculté des 
Sciences, Université de Lomé, BP 1515 Lomé, Togo 
2 Equipe Physique des Nanostructures et Composants Quantiques à l’Institut d’Electronique, de Microélectronique et 
de Nanotechnologie (IEMN), IEMN CNRS, France 
 
(Received 08 October 2016;  published online 29 November 2016) 
 
We investigated the structural and optoelectronic properties of p-n germanium nanocrystals based 
junctions embedded between GaAs substrate and layers of ZnO:Al or a-Si:H. Scanning electron microscopy 
and scanning tunneling microscopy were used on these junctions in this work. Calculations of tunneling 
current on the substrate showed effect of localized defects trapping Fermi level at the surface tending to 
make a semi-insulating substrate. The average value of the diameter of the Ge nanoparticle is around 12.5 
nm. These results lay the foundation for the development of solar cells which active part is made of GeNCs. 
 
Keywords: Germanium nanocrystals, Photodetector, Renewable energy, Scanning tunneling spectroscopy, 
Solar cells. 
 
DOI: 10.21272/jnep.8(4(1)).04035 PACS numbers: 61.72.uf, 68.37.Hk, 68.37.Ef, 
71.15. – m, 73.63.Kv, 81.07.Bc 
 
                                                             
* kodjo_sossoe@yahoo.fr  
1. INTRODUCTION 
 
Economic and ecological perspectives related to in-
creasing energy costs and catastrophic climatic effects 
due to global warming are pushing us to find solutions 
to the issue of renewable energy. These solutions are 
becoming strategic for everyone with the socio-economic 
implications: (i) reduce the production of greenhouse 
gases, (ii) develop new energy solutions at a reasonable 
cost, (iii) have a sustainability of proposed solutions (iv) 
achieve energy independence [1].  
For several years, many devices based on nanocrys-
tals semiconductors have demonstrated the use of quan-
tum dots in solar cells [2]. Nanoparticles have the po-
tential to improve the efficiency of solar cells and photo-
detectors [1]. For example, studies on the introduction of 
quantum dots (QDs) based on III-V semiconductors in 
the pin structures showed that large energy absorption 
range may be obtained which increases the density of 
short-circuit current [3-10]. 
Thus, quantum dots offer a growing interest through 
the emergence of new concepts: 
First, half of the intensity of sunlight is in the infra-
red region of the electromagnetic spectrum [11, 12]. 
Quantum dots can serve as intermediate electronic lev-
els for the absorption of photons of energy less than the 
bandgap of the semiconductor in which they are embed-
ded. By successively absorbing more photons, it is pos-
sible to generate high energy charge carriers to keep a 
high output voltage and therefore improve the output 
power of the cells [13]. 
Second, one can produce more excitons from a single 
photon. The formation of multiple excitons by absorbed 
photon requires that the photon energy is much larger 
than the band gap of the semiconductor. This phenome-
non does not occur easily in semiconductors where ex-
cess energy is simply dissipated as heat before it can 
cause the formation of other electron-hole pairs. In the 
nanocrystals, the dissipated energy rate is significantly 
reduced [14-16]. Thus, by using the quantum confine-
ment properties, it is possible to generate a number of 
charge carriers from a single photon of high energy, 
when quantum dots constitute the active part of a solar 
cell [17]. Saeed et al. showed multiple generations of 
charge carriers through the use of quantum dots [18, 19]. 
Study of electronic properties of these nanocrystals 
are now crucial for achieving optimal generation and 
extraction of photo-excited charge carriers and then 
realize thin films of good quality. Germanium is an in-
teresting material since it has a bandgap smaller than 
that of silicon and can form the quantum dots layer ei-
ther by epitaxial growth techniques or by chemical syn-
thesis. The interest of germanium nanocrystals (GeNCs) 
comes from the high mobility of charges carriers (elec-
trons and holes), the best absorption of light due to its 
direct bandgap [20, 21] as well as compatibility with 
optoelectronics and photovoltaics applications. 
In this study, we exhibit the optical and structural 
characterization of pin photodetector based on germani-
um nanocrystals in n-GaAs/GeNCs/a-Si:H and p-GaAs / 
GeNCs/ZnO:Al samples. These results show the poten-
tial of GeNCs in the manufacturing of 3G solar cells and 
show new types of absorber based on Ge. 
 
2. EXPERIMENTAL 
 
2.1 Samples 
 
The prepared samples are n-GaAs/GeNCs/a-Si:H 
and the p-GaAs/GeNCs/n-ZnO:Al. These are precisely 
pin diodes whose substrate is GaAs layer and the in-
trinsic layer is made of GeNCs. Figure 1 shows the 
characteristic structure of the diode which are pro-
duced. The intrinsic layer of GeNCs is sandwiched be-
tween GaAs substrate and amorphous silicon or zinc 
oxide layers. The layer exposed to light is either amor-
phous silicon or zinc oxide. 
 
 K.K. SOSSOE, M.M. DZAGLI, ET AL. J. NANO- ELECTRON. PHYS. 8, 04035 (2016) 
 
 
04035-2 
 
 
Fig. 1 – Structure of the sample p-GaAs/GeNCs/n-ZnO based 
on germanium nanocrystals 
 
2.2 Analysis Methods and Simulation Details 
 
Once the sample is prepared by deposition technics 
of thin layers and to access the structural and electrical 
properties, it is cleaved in ultrahigh vacuum. This al-
lows performing measurements on the wafer. This 
cleavage technics is described in reference [22]. Scan-
ning Electron Microscope (SEM) allowed imaging the 
cleaved surfaces of samples and reporting the nature of 
the layers. Analysis of these images allows access to the 
layer thickness and the size of the nanocrystals. Scan-
ning Tunneling Microscopy (STM) is used to access 
more details of the sample than the SEM. Spectroscopic 
measurements were performed to extract surface states 
densities and the gap of semiconductor inspected. 
WsXM image processing software was used to process 
the images from Scanning Tunneling Microscopy Room 
Temperature (RT-STM). All these analysis were real-
ized in physics laboratories at the Institute of Electron-
ics Microelectronics and Nanotechnology (IEMN) and 
at the High Institute of Electronics of Digital (ISEN) at 
Lille in France. 
Computations of the tunneling current were per-
formed on p-doped GaAs substrate. These computations 
were conducted at Laboratory of Physics of Compo-
nents and Semiconductor at University of Lomé (Togo), 
by improved code of SEMITIP Version 6 program of 
Professor FEENSTRA group [23, 24]. This permitted 
interpretations on the nature of localized defects ob-
served on the substrate after the cleavage. This calcu-
lation involves the use of a finite element method as-
suming a metal tip serving as a hyperbolic shaped 
probe located at a typical distance of 0.7 to 0.8 nm from 
GaAs surface. The tunneling current was computed 
using an approach based on the formalism of Bardeen 
and Tersoff-Hamann approximation [25, 26]. Under 
these conditions the semiconductor was treated in an 
effective-mass approximation. Our simulation takes 
into account surface states associated with dangling 
bonds of Ga after cleavage. Due to relaxation of the 
cleaved surface these dangling bonds induce an energy 
continuum pushed beyond the conduction band. The 
surface states may be associated with localized defects 
are not taken into account by our simulation. Main 
input parameters used to perform the calculation of the 
tunneling current are the radius of curvature of the tip, 
the tip-sample distance, the contact potential, the dop-
ing level of semiconductor and density of surfaces 
states, respectively, R, s, Δ, doping, surface state. 
 
3. RESULTS AND DISCUSSIONS 
 
We present results from our samples through SEM 
and STM images and tunneling spectroscopy on n-GaAs 
substrate. 
 
3.1 SEM Analysis 
 
SEM images on the edge of n-GaAs/GeNCs/a-Si:H 
sample (Figure 2 (a)) show n-doped GaAs substrate on 
which are deposited GeNCs. We noted that GaAs (110) 
cleaved surface is atomically flat. Therefore, upper 
structure is not impeding the cleavage. A first layer is 
visible and has a high graininess. It corresponds to the 
nanocrystal layer. It is covered with a compact layer 
with darker contrast: amorphous silicon. 
SEM image of the cleaved sample p-GaAs/GeNCs/n-
ZnO:Al has a roughness-free layer (GaAs substrate) on 
which are deposited GeNCs (Figure 2 (c)). ZnO layer 
which is a transparent structure cannot be seeing on 
this image although it is present (Figure 2 (c)). 
 
 
        
a 
 
 
 
b 
 
               
         
                                           c 
 
 
 OPTICAL AND STRUCTURAL CHARACTERIZATION OF PIN … J. NANO- ELECTRON. PHYS. 8, 04035 (2016) 
 
 
04035-3 
 
          
          d 
 
Fig. 2 – SEM images: (a)- n-GaAs/GeNCs/a-Si:H profile; (b) 
GeNCs from n-GaAs/GeNCs/a-Si:H; (c) p-GaAs/GeNCs/n-
ZnO:Al profile; (d) GeNCs from p-GaAs/GeNCs/n-ZnO:Al 
 
Top view shots on GeNCs layer of n-GaAs/GeNCs/a-
Si:H and on ZnO layer of the p-GaAs/GeNCs/n-ZnO:Al 
are shown in Figure 2 (b) and Figure 2 (d), respectively. 
Analysis of these images provides an assessment of size 
and shape of Ge nanocrystals. These structures have 
spherical shapes with diameters between 7-15 nm, as 
confirmed by statistical analysis shown in Figure 3. 
Overall, the average value of the diameter of the Ge na-
noparticle is around 12.5 nm. Also, one can see that the 
dispersion of nanocrystals is not very homogeneous be-
cause porosities can be observed on images. 
 
 
 
Fig. 3 – Statistics of nanocrystals size 
 
3.2 STM Analysis  
 
We have performed STM images on the  
n-GaAs/GeNCs/a-Si:H sample to appreciate details that 
can’t be accessed by SEM.  Scanning tunneling spectros-
copy (STS) was used to interpret some observations on 
the substrate (Figure 4). 
 
 
 
a 
 
                
       
  b 
 
Fig. 4 – RT-STM Images: (a) A large scale n-GaAs/GeNCs/a-
Si:H; (b) Atomic resolution on GaAs substrate (110). Tunneling 
conditions (a) VT  – 2.5 V, IT  1 pA; (b) VT  − 2 V, IT  4 pA 
 
Figure 4 (a) is a zoom out that shows the three constitu-
ent layers of the sample analyzed. Due to the ease of 
cleavage of the GaAs in the atomic planes, its surface 
does not have asperities as observed on SEM image. 
Contrary, the intrinsic layer of nanocrystals which is not 
homogeneous reveals agglomerates. It is the same on the 
very rough amorphous silicon layer. 
High resolution image, as shown on Figure 4 (b), 
was performed on the GaAs substrate highly n-doped 
shows further details on the substrate. It provides a 
vision of the surface topography. Point defects and the 
state of charge of these defects can be deduced from 
these STM images, according to various polarizations. 
It appears on our image obtained (Figure 4 (b)), atomic 
rows of Ga and As atoms in [1-10] crystallographic di-
rection and surfaces defects like black spots. These 
black spots visible on the surface are located on atomic 
rows of arsenic and are associated with depressions. 
These are localized defects which interrupt the perio-
dicity of the crystal lattice. This happens when one or 
more neighboring atoms of arsenic are absent from 
their usual incorporations sites. The defect is then as-
sociated with one or more vacancies. 
 
3.3 Tunneling Spectroscopy on p-GaAs sub-
strate - tunneling current Simulation 
 
Scanning tunneling spectroscopy measurements 
were performed on highly p-doped GaAs layer. Experi-
mental spectrum is shown in Figure 5 (red line). Two 
trends are observable: the current (C) in the conduction 
band (CB) due to the transfer of electrons from the tip to 
the empty states of conduction band and current (V) in 
the valence band (VB) due to the passage of electrons 
from filled states of the VB to the tip. The steeper slope 
and the high amplitude of the current in the VB com-
pared to that of CB indicate the high doping of the sub-
strate. The Fermi level which is expected to be near the 
maximum of the valence band (Ev) is slightly moved to 
the middle of the band gap at the surface. The fact that 
part of the voltage between the tip and the substrate is 
found in the substrate acts on the band bending. So, one 
can observe a shift of the Fermi level as noticed on our 
spectrum. 
 
 K.K. SOSSOE, M.M. DZAGLI, ET AL. J. NANO- ELECTRON. PHYS. 8, 04035 (2016) 
 
 
04035-4 
 
 
Fig. 5 – Comparison of experimental tunneling current spec-
trum on p-GaAs layer (red line) and computed spectra (circle, 
star): the edges of VB and CB are indicated by Ev and Ec, the 
Fermi level is given by the position of 0 V. Tunneling conditions 
(a) VT  − 2 V, IT  100 pA 
 
In order to assist with the interpretation of the spec-
troscopic tunneling measurement, the tunneling current 
produced by tungsten tip near the GaAs (110) surface 
was computed with the Semitip code. The value at-
tributed to the doping shows that the sample is highly 
doped. The density attributed to the dangling bonds is 
that used by Feenstra et al. [23, 24] on a cleaved surface 
of GaAs (110). Computation of I(V) characteristic of the 
tunneling current presents here two spectra according 
to the distance tip-surface (Figure 5, circle, star). It is 
noticed that the amplitude of the tunneling current is 
not equal to that obtained experimentally according to 
the two distances tip-surface. However, in conduction 
band (positive bias), the current amplitude is greater 
than that obtained experimentally for a value of a dis-
tance equal to 0.70 nm. In addition, the computed spec-
tra show between 1V and 1.5V discrepancy compared to 
the experimental spectrum. It also notices that the crys-
talline quality of the substrate surface may be the cause 
of the discrepancy observed between theory and experi-
ence in valence band. 
 
3.4 Measure the Dark Current 
 
I(V) characteristic obtained in the dark on the pin 
structure of the p-GaAs/GeNCs/ZnO shows a threshold 
voltage around 0.3 V (see Figure 6). This threshold volt-
age value confirms that this is a germanium diode. At 
reverse bias, a leakage current occurs. This leakage re-
lated to minority carriers is created by thermal excita-
tions. On one hand, this leakage arises from – 0.2 V and 
seems to slightly increase in intensity with polarization. 
This leakage would come from minority carriers’ 
transport due to the electric field present in the deple-
tion zone. 
Thermally generated electrons in the p region reach 
the depletion region without undergoing recombination. 
They are driven by the electric field of the depletion 
region towards the n region. This gives rise to a nega-
tive current. 
The same thing happens to holes generated ther-
mally in the n region which will go along the p region. 
On the other hand the aluminum contact could also 
contribute to the presence of the leakage current ob-
served in negative bias. The dark current is considered 
 
 
 
 a 
 
 
 
 
 b 
 
Fig. 6 – Dark current in the p-GaAs/GeNCs/ZnO (a) and its 
associated band diagram (b) 
 
as the sum of diffusion, generation-recombination and 
tunneling currents [27]. The band diagram of the pin 
structure is also shown in Figure 6. 
 
4. CONCLUSION 
 
In this present work, different techniques of optical 
and structural analysis on semiconductor surfaces are 
performed on two pin photodiodes, the n-GaAs/GeNCs/a-
Si:H and the p-GaAs/GeNCs/n-ZnO:Al. A dark current 
measurement is performed on the p-GaAs/GeNCs/n-
ZnO:Al. These results announced interest in the use of 
germanium nanocrystals in the production of solar cells. 
Computations results of the tunneling current on the 
substrate have shown the effect of localized defects trap-
ping the Fermi level at the surface tending to make the 
substrate semi-insulating. This affected the measured 
dark current. 
 
ACKNOWLEDGEMENTS 
 
This study was financially supported by the Agence 
Universitaire de la Francophonie (Grant No. BAO-2012-
05-52220FT2E0).  The authors thank Maxime Berthe, 
Jean-Philippe Nys, Yannick Lambert, Pierre Capiod, D. 
Stiévenard Etienne Quesnel for their technical input and 
fruitful discussions. We thank Prof. R. Feenstra for the 
use of his SEMITIP Program under the tunneling current 
simulations.
 
 
 OPTICAL AND STRUCTURAL CHARACTERIZATION OF PIN … J. NANO- ELECTRON. PHYS. 8, 04035 (2016) 
 
 
04035-5 
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Optical and Structural Characterization of Pin Photodetector Based on Germanium Nanocrystals for Third Generation Solar Cells

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Optical and Structural Characterization of Pin Photodetector Based on Germanium Nanocrystals for Third Generation Solar Cells

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