Poster presented at the 16th International Conference on Small-Angle Scattering, held on 13-18th September, 2015, Berlin (Germany).A subsea flexible pipe has been subjected to cyclic rapid decompression. The poly(vinylidene fluoride) (PVDF) layers have flowed radially into gaps of adjacent metallic coils forming whitened noses. Microbeam small-angle X-Ray scattering (SAXS) scanning is applied. PVDF layers (inner: wear, outer: barrier) in two zones (undamaged and damaged) are tested. Far from noses and damage zone the samples are isotropic without voids. Their morphological parameters are determined and compared to virgin material. Approaching the noses, the structure turns into highly oriented microfibrils perpendicular to the local flow into the gaps. Here voids extend parallel to the microfibrils. At edges of the metallic structure they turn more perpendicular to the layer. Crystallite orientation extends out to both sides of the whitened nose regions, but in the undamaged samples tilting of the orientation direction and void-formation are restricted to the white regions: successive mechanisms of cold drawing are mapped into space. Under the damaged spot voids and crystallite orientation are observed everywhere[1]

Abid, I.

Benazzi, D.

Borga, M.

Delruyn, J.

Medjdoub, F.

Meneghesso, G.

Meneghini, M.

Puesche, R.

Zanoni, E.

Aquino, F.

Stribeck, A.

Zeinolebadi, A.

Buchner, S.

Santoro, Gonzalo

Digital.CSIC

Small-Angle X-Ray Scattering Investigation of PVDF Microstructure due to Exposure to Supercritical CO2

M. Borga

M. Meneghini

D. Benazzi

R. Puesche

J. Delruyn

I. Abid

F. Medjdoub

G. Meneghesso

E. Zanoni

Archivio istituzionale della ricerca - Università di Padova

GaN-on-Silicon buffer decomposition experiment: analysis of the vertical leakage current

International audienceIn this work an extensive analysis on the leakage current of three samples obtained by stopping the epitaxial growth of a GaN-on-Silicon stack is presented. We studied the current leakage behavior and the breakdown voltage as a function of the ambient temperature, as well as the trapping phenomena that lead to a hysteresis on a double-sweep measurement. We demonstrate that the leakage current through the AlN nucleation layer is mainly related to the conduction within defects and dislocation, while as the epitaxial layer become thicker the dislocation density drops as well as both the leakage current level and the device-to-device variability. Hysteresis measurements demonstrate that the trapping within the vertical stack depends on both the ambient temperature and the injected charge during the upward sweep. Moreover, we shown that the presence of a carbon doped layer cause positive charge to be trapped within the epitaxial layers

MENEGHINI, M.

Benazzi, D

Püsche, R

Derluyn, J

Abid, I

Archive Ouverte en Sciences de l'Information et de la Communication

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GaN-on-Silicon buffer decomposition experiment:
analysis of the vertical leakage current
M. Borga, M. Meneghini, D Benazzi, R Püsche, J Derluyn, I Abid, F.
Medjdoub, G. Meneghesso, E. Zanoni
To cite this version:
M. Borga, M. Meneghini, D Benazzi, R Püsche, J Derluyn, et al.. GaN-on-Silicon buffer decomposition
experiment: analysis of the vertical leakage current. 43rd Workshop on Compound Semiconductor
Devices and Integrated Circuits, WOCSDICE 2019, Jun 2019, cabourg, France. ￿hal-02356883￿
GaN-on-Silicon buffer decomposition experiment:  
analysis of the vertical leakage current 
M. Borga
1
*, M. Meneghini
1
, D. Benazzi
1
, R. Püsche
2
, J. Derluyn
2
, 
 I. ABID
3
, F. Medjdoub
3
, G. Meneghesso
1
, E. Zanoni
1
 
1 Department of Information Engineering, University of Padova, 35131 Padova, Italy 
2 EpiGaN, 3500 Hasselt,Belgium 
3 IEMN-CNRS, 59652 Villeneuve d’Ascq, France 
* borgamat@dei.unipd.it 
Abstract  
 In this work an extensive analysis on the leakage 
current of three samples obtained by stopping the epitaxial 
growth of a GaN-on-Silicon stack is presented. We studied 
the current leakage behavior and the breakdown voltage as a 
function of the ambient temperature, as well as the trapping 
phenomena that lead to a hysteresis on a double-sweep 
measurement. We demonstrate that the leakage current 
through the AlN nucleation layer is mainly related to the 
conduction within defects and dislocation, while as the 
epitaxial layer become thicker the dislocation density drops 
as well as both the leakage current level and the device-to-
device variability. Hysteresis measurements demonstrate that 
the trapping within the vertical stack depends on both the 
ambient temperature and the injected charge during the 
upward sweep. Moreover, we shown that the presence of a 
carbon doped layer cause positive charge to be trapped 
within the epitaxial layers.  
Introduction 
GaN-on-Silicon technology is nowadays the most suitable 
solution for the fabrication of high-voltage and high-power 
High Electron Mobility Transistors [1]. The epitaxial growth 
of thick (Al)GaN stack over a silicon substrate is crucial to 
guarantee both robustness and reliability of the devices [2]. 
The first epitaxial layer grown over the silicon is an AlN 
nucleation layer, which quality is fundamental to achieve 
good performance of the final devices. Li et al. [3] presented 
an extensive analysis showing that the conduction through the 
AlN layer is ascribed to defect-related conduction 
mechanisms. As the epitaxial stack become thicker and more 
complex, and several layers are grown, the conduction is 
determined by a Space Charge Limited (SCL) process, which 
involve local defects and traps [4][5]. In this paper we present 
an analysis on the vertical leakage current and the related 
charge trapping on three key structures which compose a 
standard GaN-on-Silicon stack.  
Experimental 
The analysis presented within this paper is focused on 
three samples, namely sample A, sample B and sample C 
(Figure 1). These three samples were obtained by interrupting 
the epitaxial growth of a standard GaN-on-Silicon epitaxial 
stack at different stages of the growth process. Sample A 
consists of a 200nm-thick AlN layer growth over a conductive 
n-doped silicon substrate. Sample B has a similar structure 
than sample A, but over the AlN layer a 350nm-thick AlGaN 
layer is grown. Lastly, in sample C, over the n-doped silicon 
substrate, an AlN layer, a 2350nm-thick (Al)GaN multilayer 
and a 2450nm-thick carbon doped GaN layer are grown. 
Within each sample several ohmic contact were processed, so 
that the conduction between the substrate and the top layer 
could be evaluated.  
The electrical characterization presented within this work 
have been carried out by means of a Keysight B1505 
semiconductor parameter analyzer, equipped with a high 
voltage Source/Measure Unit (SMU) capable to supply 
±3000V with a current of 4mA.  
Results and discussion  
In order to obtain a preliminary overview on the behavior 
of the three samples under test when submitted to an electrical 
stress, we started the analysis by sweeping the voltage of the 
top ohmic contact of several devices for each sample from 0V 
up to the failure of the vertical stack.  
Devices on sample A (Figure 2 (a)) exhibit a remarkably 
noisy current over the applied voltage, as well as a high 
device-to-device variability. This can be ascribed to the high 
defectivity of the AlN layer grown over the Silicon substrate; 
even though the nucleation layer is fundamental for the 
growth of the subsequent (Al)GaN layers, the high lattice 
mismatch between Aluminum Nitride and Silicon as well as 
the low surface mobility of the Al species during the epitaxial 
growth, result in the creation of defects and dislocations. 
These latter cause the conduction to be a local process, thus 
resulting in the high variability observed. 
From Figure 2 (b), it can be noticed that the AlGaN layer 
grown over the AlN nucleation layer, considerably improve 
the stability of the devices, resulting in a more repeatable 
behavior of the leakage currents over the applied voltage. 
Usually, in a standard GaN-on-Silicon buffer, several AlGaN 
layers are interposed between the substrate and the active 
region of the device: the aim of these layers is to compensate 
the tensile stress that generate during the epitaxial growing 
process, as well as to reduce the dislocation density that 
propagate from the nucleation layer toward the buffer. 
Lastly, sample C, is composed by the AlN nucleation 
layer, a complex (Al)GaN stress compensation layer, and most 
important for this analysis, a 2450nm thick carbon doped GaN 
layer. The carbon impurities act as acceptor states, thus 
compensating the intrinsic conductivity of the unintentionally 
doped GaN; the resulting highly resistive layer either 
considerably improve the vertical robustness of the GaN-on-
Silicon stack and prevent the punch through effect. Figure 2 
(c) demonstrate that the leakage current of the devices within 
sample C is very stable up to the failure of the vertical stack; 
moreover, it can be noticed that the robustness of the whole 
stack is remarkably enhanced, and the failure voltage at room 
temperature is above 700V.  
In Figure 3 the current-voltage characteristics at low- and 
at high-temperature of the three analyzed samples are 
compared. The leakage current on sample A is not strongly 
affected by the ambient temperature, meaning that the 
conduction is mainly related to tunneling and hopping 
processes which might occur within the dislocations that 
propagate from the Si/AlN interface toward the ohmic contact 
on the top of the AlN. On the other hand, the leakage current 
on sample B has a higher dependence on the ambient 
temperature, meaning that there is not only a conduction 
through dislocations, but also either thermal emission or band 
conduction play a role. Blue lines in Figure 3 show the strong 
impact of the ambient temperature on the vertical leakage of 
the sample C. The high temperature dependence is compatible 
with the presence of a deep acceptor level (i.e. carbon); a 
tentative explanation is that at high temperature the 
compensation of the residual conductivity of the GaN is less 
effective than at low temperature, resulting in a strongly 
increase of the vertical leakage with the increasing ambient 
temperature.  
Figure 4 shows the hysteresis measurements performed on 
all the three samples presented within this analysis at both 
room temperature and high temperature. The hysteresis 
between the upward and the backward sweep is mostly related 
to charges that are trapped and to the electrostatic effect they 
have either in the charge-injection and in the conduction. On 
sample A (Figure 4 (a)), as the ambient temperature increases 
from 30°C to 150°C, the hysteresis is considerably reduced. 
This can be ascribed to the effect of the temperature on the 
detrapping kinetic, which results in a lower amount of trapped 
charges during the backward sweep with respect to the 
upward sweep. Unlike sample A, sample B exhibit a 
comparable hysteresis at both room temperature and high 
temperature; it is worth noticing that, unlike sample A in 
which the current level during the upward sweep is not 
depending on the ambient temperature, the leakage current in 
sample B strongly depends on the temperature, as discussed in 
the previous paragraph. This leads to a higher carrier 
injection, which results in an increased availability of charge 
that might be trapped. Sample C, beside being the more 
complex structure within the experimental set, thanks to the 
carbon acceptor states, is the only structure where also a 
source of positive charges is present. The role of the carbon in 
the hysteresis of the leakage current through the stack is clear 
in Figure 4 (c): the current during the backward sweep is 
higher than the current during the upward sweep, meaning that 
positive charges which promote the carrier injection are 
trapped within the stack. The high temperature even enhances 
this phenomenon, thanks to the higher availability of positive 
charges within the epitaxial layers.  
Conclusions  
In this work we demonstrate that the current on the AlN 
nucleation layer is mainly related to the conduction through 
defects and dislocation. Moreover, we evaluated the impact of 
the ambient temperature on the vertical leakage through both 
the AlN nucleation layer, and on two more complex 
structures. Lastly, we shown that during an IV sweep the 
presence of a carbon doped layer results in the trapping of 
positive charge.  
Acknowledgments 
This work was partially supported by the project InRel-
NPower (Innovative Reliable Nitride based Power Devices 
and Applications). This project has received funding from the 
European Union’s Horizon 2020 research and innovation 
program under grant agreement No. 720527. 
 
References  
[1] H. Amano et al., “The 2018 GaN power electronics roadmap,” J. 
Phys. D. Appl. Phys., vol. 51, no. 16, p. 163001, Apr. 2018, 
DOI:10.1088/1361-6463/aaaf9d. 
[2] M. Borga, M. Meneghini, I. Rossetto, S. Stoffels, N. Posthuma, M. 
Van Hove, D. Marcon, S. Decoutere, G. Meneghesso, and E. Zanoni, 
“Evidence of Time-Dependent Vertical Breakdown in GaN-on-Si 
HEMTs,” IEEE Trans. Electron Devices, vol. 64, no. 9, 2017, 
DOI:10.1109/TED.2017.2726440. 
[3] X. Li, M. Van Hove, M. Zhao, B. Bakeroot, S. You, G. Groeseneken, 
and S. Decoutere, “Investigation on Carrier Transport Through AlN 
Nucleation Layer From Differently Doped Si(111) Substrates,” IEEE 
Trans. Electron Devices, vol. 65, no. 5, pp. 1721–1727, May 2018, 
DOI:10.1109/TED.2018.2810886. 
[4] C. Zhou, Q. Jiang, S. Huang, and K. J. Chen, “Vertical 
leakage/breakdown mechanisms in AlGaN/GaN-on-Si structures,” 
2012 24th Int. Symp. Power Semicond. Devices ICs, vol. 33, no. 8, pp. 
245–248, 2012, DOI:10.1109/ISPSD.2012.6229069. 
[5] P. Moens, A. Banerjee, M. J. Uren, M. Meneghini, S. Karboyan, I. 
Chatterjee, P. Vanmeerbeek, M. Casar, C. Liu, A. Salih, E. Zanoni, G. 
Meneghesso, M. Kuball, and M. Tack, “Impact of buffer leakage on 
intrinsic reliability of 650V AlGaN/GaN HEMTs,” Tech. Dig. - Int. 
Electron Devices Meet. IEDM, vol. 2015–Decem, p. 35.2.1-35.2.4, 
2015, DOI:10.1109/IEDM.2015.7409831. 
 
 
 
 
Figure 3: Current-Voltage characterization performed ad low- 
(solid line) and high- (dashed line) temperature on sample A 
(black line), sample B (red line) and sample C (blue line).  
Figure 1: Schematic representation of the three samples tested within 
this work: Sample A (left), Sample B (center) and Sample C (right).   
Figure 2: Current-Voltage characteristic up to the failure of several devices on (a) sample A, (b) sample B and (c) sample C, performed at ambient 
temperature equal to 30°C. 
 
Figure 4:  Double sweep (upward: solid, backward: dashed) Current-Voltage characterization on (a) sample A, (b) sample B and (c) sample C. The 
measurements have been carried out at 30°C (blue line) and at 150°C (red line) 
0 125 250 375 500 625 750
10
-13
10
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10
-9
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10
-3
 A
 
 
C
u
rr
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(A
)
Voltage (V)
solid: 30°C
dashed: 170°C
 B
 C
Silicon 
AlN 
Silicon 
AlN
AlGaN
Silicon 
AlN
AlGaN 
multi-layer 
C: GaN
A
C
200[nm]
350
2350
[nm]
2450
[nm]
B
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(a) (b) (c
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dash: backward sweep
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)
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dash: backward sweep
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dash: backward sweep
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 30°C
 150°C
(a) (b) (c)


Small-Angle X-Ray Scattering Investigation of PVDF Microstructure due to Exposure to Supercritical CO2

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