Download Ge/Si avalanche photodiode(APD) become a promising candidiate

The Simulation of Ge-on-Si APD
Xiaohui Yi
Abstract-Ge/Si avalanche photodiode(APD) become a promising candidiate
operating at 1.31-1.55um due to complementary metal-oxide-semiconductor
compatibility, high-absorption coefficient of Ge, low impact ionization rate of
silicon,and high thermal conductivity. To the next research, we designed a Ge/Si
APD and as follow simulated a series of performances by using commecial
software Silvaco, then we obtain the dark current level, breakdown voltage, dc
gain, and electric field profile, as well as frequency response and gain-bandwidth
product.
Index Terms-Avalanche photodiode, germanium, silicon, simulation.
1. INTRUDUCTION
In optical fiber communication systems avalanche photodiodes can be applied to
receive photo information and improve the detection sensitivity due to its internal
current multiplication mechanism.As we see, in the past several years, standard
InP-base APDs instead of silicon-base APD have been produced successfully in a
large scale. But It looks less promising than Si-base devices. Because there is a lower
multiplication noise and also complete integration for APDs produced by silicon. As
we all know, silicon material is extremely important for integrate circuit, but it is
difficult to produce active devices because of the intrinsic properties-indirect gap. We
should find a new materia which can be easily combine with Si.
Ge and Si both belongs to elements. they have a common crystal
structure-diamond structure. Germanium is one of the most attractive material due to
the strong absorption of 1.55um，and the Ge absorption range can be extended up to
1.6um by introduce bandgap shrinkage to tensile strain. Consequently, it is a
promising candidate for high quality material applied in a large scale. Ge/Si APD with
a
separated
absorption-charge-multiplication(SACM)
structure
decouple
the
absorption region and avalanche region, has a extremely high performance. For
instance, high internal gain, high gain-bandwidth product. Silicon has a low k(defined
as the ratio of the ionization rate of electron and hole), which has been proved play a
important role in multiplication coefficient and also excess noise. In silicon, the
multiplication happen but photo-generated carriers are produced in Ge.
There is a 4.2% mismation between Si and Ge, which is considered as the most
annoying problem. Epitaxial growth of high quality thick germanium becomes very
difficult for the reason of high concentration of threading dislocation in epitaxial layer
and dismatch dislocation in the interface.
Fortunately, a great many approach have been demonstrated. High-quality
epitaxial Ge-on-Si growth has also been achieved through suitable direct Ge growth
without using SiGe buffers layers. In the case, a two-temperature Ge growth
technique is used to prevent islanding during the ultrahigh vacuum chemical vapour
deposition(UHV-CVD) process system, with subsequent annealing to significantly
decrease the threading dislocation density. In the first growth temperature, a thin
epitaxial Ge buffer layer of 30-60 nm is directly grown on Si at 320-360℃. At such
low temperatures, the low surface diffusivity of Ge kinetically suppresses the
islanding of Ge. The main growth temperature is>600℃, which is purpose on
achieving higher growth rates and better crystal quality.
the equivalent circuit models of APDs have been reviewed in the past years.
Some passive devices like resistance, capacitance and conductivity and a few active
devices like current source have been introduced. The process of designing such
complicated modern devices as APDs requires deep understanding of semiconductor
physic. How ever, even in such case, it is extremely difficult to predict overall
properties of structure. Modern technology can be helpful in designing photodiodes
via more and more sophisticated simulation software. This type of software package
belongs to a category defined as technology computer-aided design(TCAD).
We use commercial software Silvaco to simulate my devices. Silvaco is a famous
company software, which is devote to process simulation and device simulation of
semiconductor. It have been used to electrical circle automation. In 2006, silvaco
entered into Chinese market, and it has develop the basic theory research for
semiconductor physic.
2.EXPERIMENT
In general, silvaco is consist of a large number of simulationmodule,for instance,
Athena, Atlas , Devedit . Atlas is the module of semiconductor device simulation,
which include several sub-module. It can simulate but not limit the performance of
electricity, phonics, thermotics, direct current, and alternating current. Atlas is a
powerful device simulation module connected with a series of sub-module. We can
define easy devices directly by atlas, a module of silvaco, which can be invoked by
deck-build. But for a complicated structure, it will employ more convenient to
describe the device by the DevEdit, in which the device is constructed by a series of
node. In the experiment, we use DevEdit to get structure of device.
First of all, we will define a mesh in both x direction and y direcion. Then, the
region wich a unique number will be introduced in the mesh. Next, doping
concentration and type(acceptor or donnor) will be specified. Then, at least there is
one electrode in the surface of the device.
Fig. 1 shows the schematic configuration of the Ge/Si SACM APD. There is a
thin charge layer with 0.1um between the multiplication layer and the absorption layer.
This designment can obtains sufficient gain via a high electric field in the
multiplication layer while the electric field in the absorber is low enough to ensure
carrier drift without multiplication.
Fig. 1. Schematic of the Ge/Si SACM-APD structure modeled in this paper.
The Ge/Si APD is modeled by solving the Poisson’s equation coupled with the charge
continuity equations. In this model the total charge caused by the presence of traps is
subtracted from the right hand side of the Poisson’s equation as follows:

E  q 
   p  n  N D  N A
x   

(1)
In (1), n and p are the electron and hole carrier densities, respectively, which are
calculated by solving the carrier continuity equations ((2) and (3)) coupled with
current density equations ((4) and (5)), self consistently:
n 1 J n

 Gn  Rn
t q x
(2)
p
1 J p

 G p  Rp
t
q x
(3)
n
x
p
J p  qpv p ( E )  qD p
x
J n  qnvn ( E )  qDn
(4)
(5)
where Jn and Jp are the electron and hole current densities, Respectively. Gn,p
and Rn,p in (2) and (3) represent generation and recombination processes, such as
photoabsorption and impact ionization as well as trap-assisted (Shockley, Read, Hall)
recombination (RSRH).
To describe the impact ionization process the Selberherr model, which shows a
strong dependence of the impact ionization coefficients on the electric field, is used.
Phonon transitions occur in the presence of a trap (or defect) within the forbidden gap
of the semiconductor. Therefore, carrier recombination processes such as SRH, given
by (6) are also included in the model:
R
SRH
np  ni2



 E  Et 
 E  Ei 
 n  p  ni exp  i
   p n  ni exp  t

 kT 
 kT 


(6)
3. RESULT AND ANALYSIS
Fig. 2 show the impuriy concentration distribution of devices in y direction. The
concentration of each layers all obtain by the analysis and caculation. Its purpose is
deserve appropriate field distribution, as we can see from Fig. 3.
Fig. 2. impuriy concentration distribution of devices in y direction.
The electric field distribution across the device is illustrated in Fig. 3 for an ideal APD,
The electric field inside the silicon multiplication region must be above the critical
value of
4105 V/cm to initiate an avalanche, but in gemanium layer, the electric
field must be lower than 1105 V/cm to ensure there is no any impact ionization and
also be higer than 1 10 4 V/cm to make the velocity of carriers saturated.
Fig. 3. Field distribution in y directuon at 30V bias.
As it is shown in Fig. 4, the most of impact ionization happen in Silicon
mutiplication layer, although there is a little impact ionization process in Ge layer.We
can see from Fig 5. Almost light is absorbed in Ge layers and the absorption rate
exponential decay since the depth increase.
Fig. 4.
generation rate caused by impact ionization
Fig. 5.
light absorption rate in material
Fig. 6. spectral response of Ge/Si APD
Fig. 7. shows the simulated dark current (DC) and total current(TC) of a circular
30 μm-diameter APD versus bias voltage under an input optical power of 5 w / cm 2 at
1310 nm. As the reverse bias applied to the device increases, the depletion region expands into the Ge region. The APD breakdown voltage is defined as the reverse bias
voltage at which the dark current is 100 μA. The dark current for the APD device is
0.3 nA at a bias equal to 90% of the breakdown voltage (Vbd = −34.24 V).
Fig. 7. I-V characteristic of Ge/Si APD
Fig. 7.
Gain vs bias of Ge/Si APD
Fig. 7.
Frequency response of Ge/Si APD
4.CONCLUSION
5.
In this papar we
simulated the Ge/Si SACM APD by Silvaco. The result
shows that Ge/Si APD has a high performance for the future photoelectricity.
REFENRENCES
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photodiodes with 340GHz gain-bandwidth product. Nature photonics, 2008,
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[2] K.E. Ponomarev, A.A. Shklyaev, et al. Shape of epitaxial Ge islands on Si(100)
surfaces, 2013, Electron Devices IEEE Transactions 2013, 978-1-4799-0762-5
[3] Zhang Q, Wu N, Osipowicz T, et al. Formation and thermal stability of nickel
germanide on germanium substrate[J]. Japanese journal of applied physics, 2005,
44(10L): L1389.