Conduction Path in Nitrides
A. Minj, D. Cavalcoli, A. Cavallini
Physics Department , University of Bologna, Viale Berti Pichat 6/2 I-40127 Bologna, Italy
Introduction
Aims
Study of high leakage current in AlInN due to indium segregation in
structural defects
threading dislocations, cracks
AlInN/AlN/GaN
Conduction mechanisms show thermionic emission from 2DEG assisted
by barrier lowering (CBO-E0) due to image charges
Sample preparation
High 2DEG density due to high polarization field (~1013 cm-2 at 77 K)
Strong spontaneous polariz. allows lattice-matching of AlInN to GaN
High electron mobility due to reduction in electron scattering at AlN/GaN
interface and reduction in alloy scattering
Problems
AlInN
Growth (fig. 1)
MOCVD growth of epitaxial films AlInN, AlN at 750○C
GaN at 1050○C with GaN nucleation on c-Sapphire
Lattice mismatch between Al-N and In-N
Temperature difference between AlN growth and InN dissociation
Slight variation from optimized temperature leading to inhomogeneities such
as formation of In-droplets, In-segregation
AlN
GaN
Applications
Experimental Setup
HEMTs (high frequency, high power)
Blue/Green LEDs, Lasers
Bragg reflectors
Fig. 2 shows AlInN/AlN/GaN
heterostructure present in the sample
and the setup for C-AFM. Surface of
the sample is scanned with a biasedAFM probe in contact mode.
Measurements have been done with
various probes (Pt/Ir, Conductive
diamond and Ag2Ga nano-needles)
Figure 1
Leakage paths via Threading Dislocation
25
20
15
A
10
5
0
Conduction Mechanisms on dislocation free surface
-5
-1 0
Thermionic emission at Metal-AlInN Schottky junction
Poole-Frenkel Effect
Fowler-Nordheim Tunneling
Thermionic emission at AlN/2DEG region
-1 5
AlInN/GaN (0001)
-2 0
-2 5
--1
10
-8
-6
-4
--2
2
0
2
4
6
8
Al-rich
In-rich
10
V b ia s (V )
GaN/Sapphire
ref. 1
AlInN
AlN
I (n A )
GaN
-0.1 nA
2.8 nm
Figure 3a: I-V characteristic at a TD and dislocation free region
I(nA)
1E-9
φb − FAlInN .d AlInN + ∆Ec AlInN
1E-10
0
-5.2 nA
0.0 nm
Iexp
Frenkel Poole Effect
Fowler Nordheim Tunneling
Thermionic emission schottky
1
2
3
4
5
6
7
8
9
10
V applied (volts)
9πℏe 2 n

s
E0 = 

8 ∈0 ∈ 8m* 

n2 D =
AlN
− FAlN .d AlN + ∆Ec AlN
2 /3
GaN
+ E F − Vappl. = 0
m* (E F − E0 )
πℏ 2
2
 πℏ 2 e 2 t 


 n2 D +  eφb − eVapplied − e tσ − ∆Ec , eff  = 0
2.025(ℏ 2 e 4 )1 / 3 n2 D 2 / 3 +  * +


m
∈∈0
∈∈0 



1E-8
Figure 3c: Current map showing high current
density at V-defects
Figure 3b: Topography image obtained in contactmode
loss of confinement
17
17
7.0x10
-3
I(nA)
1E-9
n2D(cm )
Leakage paths via cracks
6
7.5x10
AlN (nm) AlInN (nm)
1
33
7.5
15
0
15
2.5
15
1
15
4
AlInN
I(nA)
2
17
6.5x10
7.5 nm
1 nm
0 nm
2.5 nm
2 nm
0.5 nm
0 nm
1E-10
17
6.0x10
0
AlN
-2
0
4
8
V
-4
-6
-10
-9 -0.4
-0.2
0.0
0 .2
0.4
7
8
9
10
V b ia s (v o lta g e )
Figure 4a: I-V characteristic at a cracks and dislocation free region
17
5.5x10
0
GaN
2
4
6
8
10
Vapplied (volts)
Assumption 1: Thermionic emission
J = A * T 2 . exp[− CBOAlN / GaN / KT ] doesn’t
explain I-V
Assumption 2: 2DEG/AlN as metal/Semi.
Building I-V equation based on
thermionic equation with barrier
lowering due to image charge induced
by 2DEG
Derivation of image charge induced barrier lowering
− q
φ(x) =
4 ∈ (x + x0 )
... see ref. 2
q
φ net ( x ) = F . x + ϕ ( x ) = F . x +
4 ∈ ( x + x0 )
F = q.(σ − n2 D ) / ∈0∈r
dφ net dx = 0
x m = q / 4 ∈ Fmax − x 0
φ net ( x m ) = 2 qF max / 4 ∈ + Fmax . x 0
'
Figure 4b: Topography image obtained in contactmode
Figure 4c: Current map showing the current distribution
around V-defect
Conclusions
With current-AFM, high conductivity due to indium segregation in Vdefects and nano-cracks is shown
Dominant conduction mechanism is shown to be thermionic emission of
electrons from 2DEG region assisted by barrier lowering due to image
charges
J = A * T 2 . exp[− q(φb − ∆φ ) / KT ]
I = A * T 2 exp(−(CBO2 DEG − E0 (V ) − qφ net (V )) / KT )
I = A *.T 2 . exp( −CBO2 DEG / KT ). exp( E0 (V ) + qφnet (V )) / KT )
see ref. 3.
References
[1]. Anas Mouti, Jean-Luc Rouvi`ere, Marco Cantoni, Jean-Francois Carlin, Eric
Feltin, Nicolas Grandjean, and Pierre Stadelmann, Phys. Rev. B 83, 195309
(2011)
[2]. A.A.Grinberg, Phys. Rev. B, 32, 8187, (1985)
[3]. A. Minj, D. Cavalcoli, A. Cavallini, Nanotechnology 23 (2012) 1157012012
Acknowledgment
This work was supported by the EU under project no. PITN-GA-2008-213238
(RAINBOW).