hartmut s. leipner: structure of imperfect materialshsl/realstruktur/realstruktur_2_iv...
TRANSCRIPT
2. Defects and semiconductor technology
Hartmut S. Leipner: Structure of imperfect materials
2.1 Device effects of defects
2.2 Degradation
hsl 2007 – Structure of imperfect materials – Epitaxy and interface defects
„Über Halbleiter sollte man nicht arbeiten, das ist eine Schweinerei, wer weiß ob es überhaupt Halbleiter gibt.“
[Pauli 1931]
2.1 Device effects of defects
2
hsl 2007 – Structure of imperfect materials – Epitaxy and interface defects
The first transistor and IC
Transistor as invented by Bardeen and Brattain at Bell labs, 1947
Integrated circuit of J. Kilby, 1958
3
hsl 2007 – Structure of imperfect materials – Epitaxy and interface defects
Cost reduction
“In 1958, the year of the invention of the integrated circuit, a single transistor sold for about $10. Today, it is possible to buy more than 50 million transistors for that price.”
[Kilby: 2000]
4
hsl 2007 – Structure of imperfect materials – Epitaxy and interface defects
Moore’s law
In 1965 Gordon E. Moore postulated his law: The doubling of transistors per circuit every couple of years.Intel expects that it will continue at least through the end of this decade.
5
hsl 2007 – Structure of imperfect materials – Epitaxy and interface defects
Moore’s laws
log
(num
ber)
Time
Functional form of key semiconductor industry business trends (Tx – transistors)
The exponential expansion of the total number of transistors per chip or area against the time (in years) over the last half-century is known as (the original) Moore’s law.
6
hsl 2007 – Structure of imperfect materials – Epitaxy and interface defects
The field-effect transistor (FET)
SourceGate
Drain
n p n
Structure of a MOSFET (metal–oxide–semiconductor FET). The electrical contact to the gate is separated from the semiconductor by a thin layer of insulator, typically silicon dioxide.
7
hsl 2007 – Structure of imperfect materials – Epitaxy and interface defects
Mobility of electrons in n-type semiconductors
Mobility of electrons μn in Si[Yu, Cardona 2001]
Mobility related to acoustic phonon interaction
Mobility related to ionized donors
Conductivity
μ n (cm
2 V− 1
s−1 )
T (K)
1
µn=1
µv+1
µD
µv ! (m!)"5/3T"3/2
µD ! (m!)"1/2NDT 3/2
! = neµn
8
hsl 2007 – Structure of imperfect materials – Epitaxy and interface defects
Scattering at dislocations
Mobility of electrons in n-type Ge before and after plastic bending. Curve (a) was calculated with the space-charge cylinder model.
[Seeger 1997]
Type and direction of dislocations are
important:
I–U characteristics measured parallel and
perpendicular to the dislocations are differentT
a
μn
Two contributions: Scattering due to the deformation potentialLinear arrangement of charged scattering centers
9
hsl 2007 – Structure of imperfect materials – Epitaxy and interface defects
Electron mobility and dislocation density
Si Substrate
Graded Si1 − yGey, y = 0…0.3
Relaxed Si1 − xGex, x = 0.3
Effect of the grading rate in the buffer on the density of threading dislocations and the electron mobility measured at 0.4 K. The dotted line is an extrapolation showing that the mobility is not limited by the dislocations once their density is below 107 cm−2 (thickness of graded layer > 0.5 µm).
[Ismail 1996]
Thickness of graded layer (µm)
10
hsl 2007 – Structure of imperfect materials – Epitaxy and interface defects
Diode characteristics and dislocations
Reverse bias I–U characteristics for InP photodiodes containing different dislocation densities (given in cm−2)
[Beam et al. 1992]
Rev
erse
cur
rent
(A)
Bias (V)
11
hsl 2007 – Structure of imperfect materials – Epitaxy and interface defects
Dislocations at the p–n junction
♦ Dislocations as electrically active defects cause a shortening of the junction
♦ Transport of minority carriers under reverse bias affected
♦ Energy levels of dislocations (recombination centers): source of electrical noise
♦ Dopant precipitation at dislocations: local field enhancement under reverse
bias, drop in reverse breakdown voltage
♦ Microplasma (high electron–hole density) formation
12
hsl 2007 – Structure of imperfect materials – Epitaxy and interface defects
Charged-dislocation model
Model of Read (1957): The line charge of the dislocation is screened by a cylindrical space charge region, i. e. in an n-type semiconductor a negative dislocation is surrounded by positively charged donors.
Radius of the Read cylinder:(Q charge per unit length)
Smearing out of the screening cloud for T > 0
------------
++
+
+
++
++
+
+
+
+
Captured majority carriers
13
hsl 2007 – Structure of imperfect materials – Epitaxy and interface defects
Influence of dislocations on the p–n junction
Scheme according to Holt (1996) illustrating the absorption of the diffusing impurity (e. g. phosphorus) by dislocations and the retardation of the diffusion front. In this way, cusps in the p–n junction are produced. The magnitude of the effect depends on the depth of the dislocation; shallower ones produce the larger effects. A and B are strongly decorated dislocations. The impurity atmosphere at dislocation C is weaker.
Top surface
High concentration zone of phosphorus donors
p–n+ junction
n+ laye
rA
B
C
14
hsl 2007 – Structure of imperfect materials – Epitaxy and interface defects
Defects in GaAs field-effect transistors
SI GaAs substrate
n channel
DrainGateSource
BB
Structure of a FET. Si implantation is used for the production of the n-conductive channel. Lateral isolation is provided by amorphization in B. The symmetrical n+ contacts are made of Ni/Au–Ge, the gate by Ti–Pt–Au. The drain/source–gate separation is 2 µm.
15
hsl 2007 – Structure of imperfect materials – Epitaxy and interface defects
Precipitates in FET structures
Laser scattering tomography (LST) of precipitates (a) decorating dislocations in the substrate and (b) in the channel between drain and gate.
[Castagné et al. 1992]
(a) (b)
LST LST
16
hsl 2007 – Structure of imperfect materials – Epitaxy and interface defects
FET characteristic curvesP
reci
pita
te d
ensi
ty (c
m−
3 )
Breakdown voltage (V)Th
resh
old
volta
ge (m
V)
Precipitate density (cm–3)
2 µm20 µm50 µm
Theory
Influence of the precipitates on the breakdown voltage (a) and the threshold voltage (b) of the GaAs FET. The symbols in (b) represent different lengths of the channels. [Castagné et al. 1992]
(b)(a)
17
hsl 2007 – Structure of imperfect materials – Epitaxy and interface defects
2.2 Degradation
♦ Limitation of the lifetime of semiconductor devices, especially optoelectronic devices
♦ Defect formation during operation
♦ Classification of degradation modes, recombination enhanced defect
generation and motion
18
hsl 2007 – Structure of imperfect materials – Epitaxy and interface defects
Scheme of a laser diode
Basic structure of a laser diode
Optical resonator realized with
the Fabry–Perot condition
mλ = 2nL(m integer number, λ wavelength, n refraction index, L resonator length)
Current
Mirror face
Active zone
Radiation
Opticallyrough
p doped
n doped
d
w
L
Population inversion at the p–n junction as the lasing condition (i. e. stimulated emission larger than optical absorption)
19
hsl 2007 – Structure of imperfect materials – Epitaxy and interface defects
GaN laser diode
Nichia blue laser diode consisting of a stack of AlGaN, GaN, and InGaN
layers. The SiO2 islands are stoppers for threading dislocations.
Top surfaceDouble heterostructure
GaN
Substrate–film interfaceAl2O3
TEM cross-section image of the layer containing threading dislocations [Lester et al. 1995]
1 µm
TEM20
hsl 2007 – Structure of imperfect materials – Epitaxy and interface defects
Influence of dislocations on device efficiency
Dislocation density (cm–2)
Qua
ntum
effi
cienc
y
Light output as a function of the dislocation density in the active region for various laser diodes according to Lester et al. (1995) and Sugahara et al. (1998)
104 105 106 107 108 109
0.20.40.60.81.0
GaAs
GaAlAsGaP GaAsP
GaN
21
hsl 2007 – Structure of imperfect materials – Epitaxy and interface defects
Degradation of light-emitting devices
A representative degradation curve of light output versus time. The scale along the ordinate can vary over hours to ten on thousands of hours.
Ligh
t out
put
Time of operation
22
hsl 2007 – Structure of imperfect materials – Epitaxy and interface defects
Formation of dark-line defects
Micro-electroluminescence distribution of -oriented dark-line defects in a high-power GaAs light-emitting diode of the 1980ies.
!110"
EL EL
23
hsl 2007 – Structure of imperfect materials – Epitaxy and interface defects
Dislocations in a GaAs LED
Dislocations in a GaAs light-emitting diode in EBIC (a) and cathodoluminescence (b) images. Note the vanishing contrast in (b) for the dislocations 1 to 3. [Schreiber et al. 1984]
25 µm1
23
(a) (b)
1
23
EBIC – electron beam induced current
CLEBIC
24
hsl 2007 – Structure of imperfect materials – Epitaxy and interface defects
Dark-line defects and dislocation motion
Dark-line defects of a GaAlAs laser diode in the electroluminescence distribution and in a transmission electron microscope image. B denotes a threading dislocation and A the formation of a dislocation dipole in a direction. g is the diffraction vector. [Hutchinson, Dobson 1980]
!100"
10 µm
!100"
TEM
EL
25
hsl 2007 – Structure of imperfect materials – Epitaxy and interface defects
Growth of dark-line defects
Scheme of the growth of a dislocation network in the active layer of the laser device. (a) Initially, the dislocation PN with the Burgers vector b crosses the layer; (b) climb into the active zone; (c) further climb confined to the active zone causes elongation along the [100] direction.
[Hutchinson, Dobson 1975]P
N
bActive layer
TEM
26
hsl 2007 – Structure of imperfect materials – Epitaxy and interface defects
Dislocation climb in the active layer
Two models proposed for the extension of -oriented dark-line defects in degraded optoelectronic devices via dislocation climb. (a) Model related to absorption of interstitials, (b) emission of vacancies.
(a) (b)
!100"
27
hsl 2007 – Structure of imperfect materials – Epitaxy and interface defects
Influencing defects by carrier recombination
♦ Release of energy by recombination of excess carriers (photons, phonons)
♦ Recombination energy may be directly used for defect reactions
(generation, motion, transformation) – recombination enhanced defect reaction
♦ Energy transfer can be directly an special sites on the dislocation (kinks) –
recombination enhanced dislocation glide
♦ Stimulated activity of the kink migration: increase in the dislocation velocity
28
hsl 2007 – Structure of imperfect materials – Epitaxy and interface defects
Electron-beam-stimulated dislocation motion
Moving dislocations near a scratch on GaAs observed by cathodoluminescence imaging
[Höring et al. 2000]
CL
29
hsl 2007 – Structure of imperfect materials – Epitaxy and interface defects
Recombination enhanced dislocation glide
Temperature dependence of the thermal and recombination-enhanced dislocation velocity υ of polar 60° α and β dislocations in GaAs
[Maeda, Takeuchi 1983]
1000/T (K−1)
v (m
/s)
I
I
with/without irradiation
n-type GaAsτ = 26 MN m−2
αβ
I = 0.26 A m−2
I = 2.6 A m−2
(Q activation energy in the dark, I beam current density, ΔE reduction in the activation energy by the electron beam, τ stress, m stress exponent, υ0 and η prefactors)
! = !0"m exp!! Q
kBT
"+#I exp
!!Q!!E
kBT
"
30
hsl 2007 – Structure of imperfect materials – Epitaxy and interface defects
Defect structures
♦〈110〉-oriented dark-line defects (GaAlAs)
dislocation glide enhanced by nonradiative recombination of carriers
♦〈100〉-oriented dark-line defects (GaAlAs)
recombination-enhanced dislocation climb
♦ Dark-spot defects (InGaAsP)
reaction between metal contacts and the matrix, microdefects
31
hsl 2007 – Structure of imperfect materials – Epitaxy and interface defects
Degradation modes
♦ Catastrophic optical damage (< 1 h)
Local heating at the mirror surface related to high output-power density
♦ Rapid degradation (< 100 h)
Formation of dark-line defects
♦ Gradual degradation (several 1000 h)
Microdefects (loops, point defect clusters)
♦ Facet degradation (> 10000 h)
Oxidation or point defect formation at the mirror facets
32
References
J. C. Kilby in: Handbook of semiconductor manufacturing technology, Marcel Dekker 2000.P. Y. Yu, M. Cardona: Fundamentals of Semiconductors. Berlin: Springer 2001.K. Seeger: Semiconductor Physics. Berlin: Springer 1997.K. Ismail: Sol. State Phen. 47–48, (1996) 409.E. A. Beam et al.: Semicond. Sci. Technol. A 7 (1992) 229.D. B. Holt: Scanning Microsc. 10 (1996) 1047.S. D. Lester et al.: Appl. Phys. Lett. 66 (1995) 1249.T. Sugahara et al.: Jpn. J. Appl. Phys. Pt. 2 37 (1998) L398.J. Schreiber et al. in: 6. Tagung Mikrosonde, Dresden 1984, p. 151.P. W. Hutchinson, P. S. Dobson: Phil. Mag. A 41 (1980) 601.K. Maeda, S. Takeuchi: J. Physique 44 (1983) C4/375.
hsl 2007 – Structure of imperfect materials – Epitaxy and interface defects 33