Transcript 投影片 1

Studies of Band Alignment, Two-dimensional
Electron Gas and Ordering Effects
in InGaPN/GaAs Heterostructures
雷射光譜實驗室
指導教授：黃正雄
博士後：林光儀
3rd Workshop on Low-Dimensional systems and Nanomaterials
page1
Subject
Part 1. Studies of Band Alignment and Two-dimensional
Electron Gas in InGaPN/GaAs Heterostructures.
Part 2. Raman Study of Weak Ordering Effects of
InGaPN.
page 2
Part 1. Outline
Ⅰ. Introduction.
Ⅱ. Photoluminescence
(PL) Spectrum.
Ⅲ. Photoreflectance
(PR) Spectrum.
a) Theory and
Experimental Details
b) Results and Discussion
Ⅳ. Conclusions.
page 3
Ⅰ. Introduction
In0.48Ga0.52P grown lattice matched to GaAs.
Advantages over AlGaAs/GaAs: InGaP/GaAs
has larger valence-band offset △Ev, better etch
selectivity, and less oxidation effect.
Optoelectronic and microelectronic devices:
semiconductor lasers, heterojunction bipolar
transistors (HBTs), and high efficiency tandem
solar cells.
page 4
Ⅰ. Introduction
Nitrogen incorporation drastically reduces the
band gap in InGaAs: long-wavelength optoelectronic devices.
A similar effect in InGaPN has been reported.
Thus, InGaPN may be a suitable emitter and
collector material of the Blocked-hole HBT
(lowering of conduction band).
page 5
Ⅰ. Introduction
Motivation:
Until now, there are few reports on InxGa1-xP1-yNy
and many fundamental electronic and optical
properties of it are left unknown. Most of these are
limited to low-temperature PL. Thus, detailed studies
are necessary.
page 6
Reference: Y. G. Hong et al.
J. Vac. Sci. Technol. B 19, 1413 (2001).
Single In0.54Ga0.46P1-yNy layer :
With N ≥ 1.2%, no detectable room-temperature PL was
obtained, indicating the presence of a high concentration of
nonradiative centers.
InGaPN/GaAs/InGaPN QW samples :
At 10 K QW PL :
In0.54Ga0.46P/GaAs : △Ec=0.170 eV, △Ev=0.234 eV
In0.54Ga0.46P0.995N0.005 /GaAs : △Ec=0.006 eV, △Ev=0.198 eV
A typeⅠalignment.
page 7
The types of band alignment
Conduction Band
△Ec
Conduction Band
Conduction Band
Conduction Band
EG
Eg
Eg
GaAs
GaAs
InGaP
EG
Valence Band
△Ev
Valence Band
TypeⅠalignment
InGaPN
Valence Band
Valence Band
TypeⅡalignment
page 8
Reference: Y. G. Hong et al.
J. Vac. Sci. Technol. B 19, 1413 (2001).
InGaPN/GaAs/InGaPN QW samples :
There is no GaAs QW PL emission when the barrier has a
higher N concentration, such as 1.2% and 2.4%.
Two possibilities:
1. A type Ⅱ alignment.
Back
2. More nonradiative centers.
page 9
Sample structure
InGaPN samples were grown by gas-source MBE
on GaAs (100) SI substrate.
0.5μm In0.54Ga0.46P1-yNy
0.2μm GaAs buffer
y = 0, 0.005, 0.01,
and 0.02
GaAs SI substrate
page 10
Ⅱ. Photoluminescence (PL) Spectrum
a) Theory and Experimental Details
A luminescence process involves three separate steps：
Excitation：Electron-hole pairs have to be excited by
an external source of energy ( laser pump,
photoluminescence ).
Thermalization：The excited e-h pairs relax towards
quasi-thermal equilibrium distributions.
Recombination：The thermalized e-h pairs recombine
radiatively to produce the emission
( dependent on the luminescence paths ).
page 11
Schematic experimental setup for PL spectrum
DC SIGNAL
DETECTOR
LOCK-IN
AMPLIFIER
MONOCHROMATOR
CONTROL
BOX
COMPUTER
FILTER
CHOPPER
LENS
LASER
LENS
LENS
SAMPLE
page 12
b) Results and Discussion
In0.54Ga0.46P1-yNy PL
-5
6x10
-5
PL Intensity (a.u.)
5x10
y=0
y=0.005
y=0.01
y=0.02
x10
-5
4x10
x50
-5
3x10
-5
2x10
-5
1x10
0
1.5
1.6
1.7
1.8
1.9
2.0
Photon Energy (eV)
page 13
In0.54Ga0.46P1-yNy PL results
y
0
0.005
0.01
0.02
PL
normalized FWHM of
Eg (eV) PL intensity PL (meV)
1.832
1.00
46
1.786
0.05
50
1.750
0.01
54
-
page 14
Ⅲ. Photoreflectance (PR) Spectrum
a) Theory and Experimental Details
LENS
He-Ne 633 nm
or He-Cd 325
nm laser
R
R(; Laser On)  R(; Laser Off )
( ) 
R
R(; Laser Off )
page 15
Low electric field limit
p
 /   1/ 3
3
3
ΔR/R  Re[ Aje ( E  Ecj  i j ) j ]
i j

j1
A : Amplitude ;  : Phase angle
E : Incident photon energy
Ecj : Interband transition energy
 : Broadening parameter
: Critical point dimensionality
For unbound states : third derivative line-shape
 2.5 ....... 3 dimensional critical point
For bound states : first derivative line-shape
2
page 16
b) Results and Discussion
PR Intensity (R/R)
In0.54Ga0.46P1-yNy He-Cd 325 nm laser pump
y=0
y=0.005
y=0.01
y=0.02
1.5
Go p.38
1.6
1.7
1.8
1.9
2.0
Photon Energy (eV)
page 17
Fitting results
y
0
0.005
0.01
0.02
In0.54Ga0.46P1-yNy
Eg (eV)
PR
PL
1.843 1.832
1.786 1.786
1.751 1.750
1.628
-
The incroporation of 2% nitrogen
reduces 215 meV band-gap energy.
page 18
In0.54Ga0.46P1-yNy He-Ne 633 nm laser pump
PR Intensity (R/R)
y=0
y=0.005
y=0.01
x2
1.25
1.30
y=0.02
1.35
1.40
1.45
1.50
1.55
1.60
Photon Energy (eV)
page 19
In0.54Ga0.46P1-yNy He-Ne 633 nm laser pump
2DEG transition energies
PR Intensity (R/R)
y=0
y=0.005
y=0.01
y=0.02
x2
1.25
1.30
1.35
1.40
1.45
1.50
1.55
1.60
Photon Energy (eV)
page 20
Fitting results In0.54Ga0.46P1-yNy
y
0
0.005
0.01
0.02
Energy levels of 2DEG (eV)
E1
E2
E3
E4
1.393
1.369 1.402
1.297 1.316 1.345 1.363
page 21
The types of band alignment
Conduction Band
△Ec
Conduction Band
Conduction Band
Conduction Band
EG
Eg
Eg
GaAs
GaAs
InGaP
EG
Valence Band
△Ev
Valence Band
TypeⅠalignment
InGaPN
Valence Band
Valence Band
TypeⅡalignment
page 22
The In0.54Ga0.46P1-yNy and GaAs heterojunction. Approximate triangular
potential wells and two-dimensional electron gas are formed at the
junction. (a) For typeⅠalignment; (b) for typeⅡalignment.
Conduction Band
(b)
(a)
Conduction Band
Conduction Band
Conduction Band
In0.54Ga0.46P
GaAs
GaAs
In0.54Ga0.46P1-yNy
y=0.005~0.02
Valence Band
Valence Band
Valence Band
Valence Band
TypeⅠalignment
TypeⅡalignment
page 23
In0.54Ga0.46P1-yNy / GaAs heterojunction band
alignment varies with nitrogen composition
Ec
Conduction Band
Conduction Band
y = 0.005
y > 0.005
Ec
GaAs
buffer layer
In0.54Ga0.46P1-yNy
Valence Band
Ev
Ev
Valence Band
page 24
Fitting results In0.54Ga0.46P1-yNy
y
0
0.005
0.01
0.02
Eg (eV)
PR
PL
1.843 1.832
1.786 1.786
1.751 1.750
1.628
-
FWHM of
PL (meV)
46
50
54
-
Energy levels of 2DEG (eV) Electric field at the
E1
E2
E3
E4 interface (kV/cm)
25.9
17.8
1.393
16.2
1.369 1.402
1.297 1.316 1.345 1.363
14.1
Bowing parameter Mismatch
y
b (y) (eV)
(△d/d)⊥×10-3
0
8.20
0.005
11.15
8.09
0.01
9.07
7.12
0.02
10.72
6.45
page 25
Ⅳ. Conclusions
1. A small amount of N incorporation drastically
decreases the PL intensity due to a high
concentration of nonradiative centers, and
broadens the linewidth.
2. For 2% nitrogen InGaPN, there is no detectable
room-temperature PL, but the band gap is easily
obtained by PR.
3. The incroporation of 2% nitrogen reduces 215
meV band-gap energy at room temperature.
page 26
Ⅳ. Conclusions
4. The 2DEG transition energies are determined,
and all of them are smaller than the band gap of
GaAs.
5. We make sure that the InGaP1-yNy/GaAs lies in
type Ⅱ band alignment as y > 0.005.
J. S. Hwang et al, Appl. Phys. Lett. 86, 061103 (2005).
K. I. Lin et al, J. Appl. Phys. 99, 056103 (2006).
page 27
Part 2. Outline
1. Introduction.
2. Theory and Experimental
Details--Raman Spectrum.
3. Results and Discussion.
4. Conclusions.
page 28
1. Introduction
The spontaneous long-range ordering has been
observed in many Ⅲ-Ⅴ ternary semiconductor alloys.
ex. : InGaAs, InGaP etc.
The degree of ordering in InGaP depends on the
growth conditions such as growth rate, growth
temperature, substrate misorientation, and Ⅴ/Ⅲ flux
ratio
the formation of different crystal structure.
The ordering influences carrier lifetimes and
energy band which can affect the electronic and optical properties of semiconductors.
page 29
1. Introduction
The incorporation of dilute N atoms induces long-
range order in GaAs0.98N0.02.
Motivation:
We expect that the incorporated nitrogen will induce
different degree of order in InGaP. Raman measurements are necessary.
page 30
Sample structure
InGaPN samples were grown by gas-source MBE
on GaAs (100) SI substrate.
0.5μm In0.54Ga0.46P1-yNy
0.2μm GaAs buffer
y = 0, 0.005, 0.01,
and 0.02
GaAs SI substrate
page 31
2. Theory and Experimental Details
First-order Raman scattering (one-phonon process)

', k '

', k '

, k

, K
Stokes
   ' 
  
k  k ' K

, k

, K
Anti-Stokes
   '
  
k  k ' K
Raman scattering measurements provide a quantitative and
non-destructive method to study the electronic and phonon
properties of the materials.
page 32
InGaP2 alloy
In the order phase:
In the disorder phase:
 trigonal CuPt structure.
 cubic zinc-blende structure.
 C3v point group.
 Td point group.
z
y
[111]
x
P
{ Download from: www.tp1.physik.uni-erlangen.de/research/ordering/main.html }
page 33
Scattering configuration
Z’(X’,X’)Z’
Z’: incident laser propagation direction
(X’,X’): ( incident laser polarized direction,
scattered light polarized direction)
Z’: scattered light propagation direction
Z’=[001]
X’=[110]
X’’=[100]
x axis
sample surface
Y’=[110]
xy plane
y axis
Y’’=[010]
Z’(X’,X’)Z’
Z’(X’’,X’’)Z’
Z’(X’’,Y’’)Z’
z axis
Z’=[001]
page 34
3. Results and Discussion
In0.54Ga0.46P0.98N0.02 total range, no polarized
3000
InP-like LO
GaP-like LO
Intensity (a.u.)
2500
2000
InP-like TO
1500
DALA
1000
InGaN-like mode
500
0
0
100
200
300
400
500
600
700
800
900 1000
-1
Raman Shift (cm )
LO : Longitudinal optic mode
TO : Transverse optic mode
DALA : Disorder activated longitudinal acoustic phonon
page 35
Definition of valley-to-peak intensity ratio (b/a)
In0.54Ga0.46P1-yNy
2000
Z'(X',X')Z'
GaP-like LO
InP-like LO
Intensity (a.u.)
1500
b
1000
a
InP-like TO
500
0
280
300
320
340
360
380
400
420
-1
Raman Shift (cm )
page 36
Reference: S. F. Yoon et al.,
Microelectronics Journal 31, 15 (2000).
InGaP
GaP-LO
InP-LO
page 37
In0.54Ga0.46P1-yNy
Z’(X’,X’)Z’
2000
Intensity (a.u.)
1500
y=0
y=0.005
y=0.01
y=0.02
1000
500
0
280
300
320
340
360
380
400
420
-1
Raman Shift (cm )
page 38
In0.54Ga0.46P1-yNy Polarization Z(X’,X’)Z’
Valley-to-Peak Intensity Ratio (b/a)
In0.54Ga0.46P1-yNy
(X',X')
Intensity Ratio (b/a)
Lorentz Fit Lorentz Fit Lorentz Fit Lorentz Fit
y=0
y=0.005
y=0.01
y=0.02
0.408
0.367
0.344
0.335
page 39
b/a Ratio vs Composition (X',X')
0.41
average
0.40
b/a Ratio
0.39
0.38
0.37
0.36
0.35
0.34
0.33
0.000
0.005
0.010
0.015
0.020
Nitrogen Composition (y)
page 40
Reference: S. F. Yoon et al.,
Microelectronics Journal 31, 15 (2000).
InGaP
GaP-LO
InP-LO
page 41
In0.54Ga0.46P1-yNy Polarization Z(X’,X’)Z’
Raman intensity
In0.54Ga0.46P1-yNy
(X',X')
Intensity
Lorentz Fit Lorentz Fit Lorentz Fit Lorentz Fit
y=0
y=0.005
y=0.01
y=0.02
InP-like LO mode
235
530
651
1104
GaP-like LO mode
521
868
1034
1542
page 42
Raman Intensity vs Composition (X',X')
1600
GaP-LO Int.
InP-LO Int.
Intensity (a.u.)
1400
1200
1000
800
600
400
200
0.000
0.005
0.010
0.015
0.020
Nitrogen Composition (y)
page 43
Raman selection rule: Td group symmetry the LO-phonon Raman
scattering is forbidden with configuration
(X”,X”), but is allowed with (X”,Y”).
GaAs ( zincblende lattice, Td )
800
700
Intensity (a.u.)
LO mode
(X",X")
(X",Y")
600
500
400
300
TO mode
200
100
230
240
250
260
270
280
290
300
310
320
-1
Raman Shift (cm )
page 44
Define intensity ratio c = the ratio of
intensity of GaP-like LO mode with
(X”,Y”) and (X”,X”) polarization.
Smaller c means that the selection rule
of the crystal presents a larger deviation
from the expected Td symmetry
closer to C3v symmetry (ordered).
page 45
In0.54Ga0.46P
800
Intensity (a.u.)
700
(X",X")
(X",Y")
600
500
400
300
200
100
280
300
320
340
360
380
400
420
-1
Raman Shift (cm )
page 46
In0.54Ga0.46P0.995N0.005
900
Intensity (a.u.)
800
(X",X")
(X",Y")
700
600
500
400
300
200
100
280
300
320
340
360
380
400
420
-1
Raman Shift (cm )
page 47
In0.54Ga0.46P0.99N0.01
800
Intensity (a.u.)
700
(X",X")
(X",Y")
600
500
400
300
200
100
280
300
320
340
360
380
400
420
-1
Raman Shift (cm )
page 48
In0.54Ga0.46P0.98N0.02
1200
(X",X")
(X",Y")
Intensity (a.u.)
1000
800
600
400
200
0
280
300
320
340
360
380
400
420
-1
Raman Shift (cm )
page 49
In0.54Ga0.46P1-yNy GaP-like LO mode
with (X”,Y”) and (X”,X”) Polarization
GaP-like LO mode
Composition polarization Shift (cm-1) Intensity
y=0
y=0.005
y=0.01
y=0.02
(X",Y")
379.9
592
(X",X")
376.6
124
(X",Y")
378.7
663
(X",X")
376.8
326
(X",Y")
377.4
593
(X",X")
375.6
373
(X",Y")
376.2
968
(X",X")
376.0
831
Int. Ratio c
4.77
2.03
1.59
1.17
page 50
GaP-like LO mode Intensity Ratio (X",Y")/(X",X")
5.0
Intensity Ratio
Intensity Ratio c
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.000
0.005
0.010
0.015
0.020
Nitrogen Composition (y)
page 51
Reference: F. Alsina et al.,
Phys. Rev. B 53, 12994 (1996).
η=0.49
DALA : Disorder activated
longitudinal acoustic phonon
~ 200 cm-1
η=0.44
η=0.30
FLA : Folded longitudinal
acoustic phonon
~ 208 cm-1
η<0.2
page 52
page 81
In0.54Ga0.46P1-yNy
Polarization Z(Y’,Y’)Z’
460
y=0
y=0.005
y=0.01
y=0.02
Intensity (a.u.)
410
360
310
260
210
160
140
160
180
200
220
240
-1
Raman Shift (cm )
page 53
Reference: L. H. Robins et al.,
MRS Internet J. Nitride Semicond. Res. 4S1, G3.22 (1999).
LO
590 cm-1
LO
734 cm-1
Raman spectra of InxGa1-xN samples.
page 54
In0.54Ga0.46P1-yNy
no polarized
750
Raman Intensity (a.u.)
InGaN-like LO mode
650
550
y=0.02
450
y=0.01
y=0.005
350
y=0
250
150
550
600
650
700
750
800
850
-1
Raman Shift (cm )
page 55
GaN clusters
The InGaN-like LO mode: the GaN bonds (or
clusters) should make more contributions than InN
bonds because the broad structure is located near the
pure GaN LO frequency.
A. Hashimoto et al. have reported: the formation
of spontaneous ordering in GaAsN
the formation of
GaN clusters.
page 56
4. Conclusions
1. A slight decrease in the valley-to-peak intensity
ratio (b/a) and an increase in the Raman intensity of
the InP-like and GaP-like LO modes all indicate that
In0.54Ga0.46P is more ordered with more nitrogen
incorporation.
2. The LO-phonon Raman selection rule for Td point
group (intensity ratio c) also proves the first result.
3. The relatively large b/a ratio point out the level of
ordering present in our InGaPN samples is weak.
page 57
4. Conclusions
4. We assign the broad Raman peak at about 735 cm-1
to the InGaN-like LO mode.
5. The ordered effects in InGaPN samples could be
explained by the formation of different atomic
structure (zinc-blende to CuPt structure) and GaN
clusters.
K. I. Lin et al, Appl. Phys. Lett. 86, 211914 (2005).
page 58
page 59
In0.54Ga0.46P1-yNy/GaAs
GaAs Eg and electric field fit
y=0
PR Intensity (R/R)
0 2
4
1
3
6
y=0.005
5
FKOs
y=0.01
y=0.02
x2
1.25
G p.35
1.30
1.35
1.40
1.45
1.50
1.55
1.60
Photon Energy (eV)
page 60
In0.54Ga0.46P1-yNy/GaAs
GaAs Eg and electric field fit
1.54
y=0
y=0.005
y=0.01
y=0.02
fit
1.52
En
1.50
1.48
1.46
1.44
1.42
0
2
4
6
8
10
Fn
page 61
Fitting results In0.54Ga0.46P1-yNy
y
0
0.005
0.01
0.02
Electric field at the
interface (kV/cm)
25.9
17.8
16.2
14.1
page 62
 /  3  1/ 3
3
Moderate electric field
Franz-Keldysh oscillations (FKOs)
2
1
R / R ~ E ( E  Eg ) exp[( E  Eg )
n = 1, 2, 3…
1/ 2
2 E  Eg 3 / 2
/() ]  cos[ (
)  ]
3 
3/ 2
;
  : electro-optical energy
En : photon energy of the nth extreme of oscillations
Eg : Band gap
;
 : phase factor
  (e2 2 F 2 / 8 )1/ 3
F : electric field ;
 : reduce mass of electron
and heavy hole
page 63
The extremes of the FKOs occur when :
n  23 [(En  Eg ) / ]3/ 2  
3
 2/3
En  [  (n  )]  Eg
2

here
3

Fn  [  (n  )]2 / 3
2

and

 1

 2
and n=0,1,2,3…
We have
En  Fn  Eg
B p.26
page 64
Bowing parameter --- δ(y)
For conventional Ⅲ-Ⅴ ternary alloys :
Vegard’s law adding a quadratic correction bx(x-1).
Such as : InxGa1-xAs with δ＝0.5 eV.
Eg（InxGa1-xAs）＝ xEg（InAs）+（1-x）Eg（GaAs）+δx(x-1)
But incorporating a small amount of nitrogen in Ⅲ-Ⅴ
semiconductor results in a strong reduction of Eg.
Such as : InPN (δ =16 eV), GaPN (δ =14 eV).
However, GaAsN and InGaAsN need composition
dependent bowing parameter δ(x).
ex. : GaAs1-xNx δ(x)=10~20 eV.
page 65
Bowing parameter --- δ(y)
Eg（In0.54Ga0.46P1-yNy）＝ yEg（In0.54Ga0.46N）+
（1-y）Eg（In0.54Ga0.46P）+ δy(y-1)
And In0.54Ga0.46N band gap＝1.6338 eV [APL. 80, 4741 (2002)]
In0.54Ga0.46P band gap＝1.8425 eV
Use PR fitting results:
In0.54Ga0.46P1-yNy
y
bowing parameter δb ( y )
0
no
0.005
11.15
0.01
9.07
0.02
10.72
(eV)
page 66
Further Works
1. Temperature-dependent PL and PR spectra:
defect level (N clusters) and Varshni’s
formula.
2. Valence-band splitting: strain and ordering.
3. Band-gap reduction: strain, ordering, and N
incorporation.
4. N incorporation: band-anticrossing model
E_, E+ energy level.
page 67
R
R(; Laser On)  R(; Laser Off )
( ) 
R
R(; Laser Off )
LENS
I0(λ)
I0R+I
I0R0ΔR
page 68
Fitting results In0.54Ga0.46P1-yNy
y
0
0.005
0.01
0.02
Energy levels of 2DEG (eV)
E1
E2
E3
E4
1.393
1.369 1.402
1.297 1.316 1.345 1.363
B p.20
page 69
Reference: R. J. welty et al., IEEE 11-7, 33 (2000).
Tunneling-collector HBT
Blocked hole bipolar transistor
page 70
y = 0%
175 K
PR Intensity (R/R)
y = 0.5%
y = 1.0%
225 K
y = 2.0% 200 K
293 K y = 0%
×3
y = 0.5%
×5
y = 1.0%
Ec1
1.5
B p.16
175 K
1.6
×5
Ec2
×5
1.7
y = 2.0%
Ec3
1.8
1.9
2.0
2.1
Photon Energy (eV)
page 71
Schematic experimental setup for
micro-Raman spectrum
CCD
(Charge
Coupled
CONTROL
BOX
Device)
SPECTRA
RAMAN
LINK
DOUBLE
MONOCHROMATOR
SHUTTER
LASER
COMPUTER
MICROSCOPE
OPTICAL FIBER
SAMPLE
page 72
A photograph of our micro-Raman spectrum
CCD
RAMAN
DOUBLE
MONOCHROMATOR
page 73
In0.54Ga0.46P1-yNy
In0.54Ga0.46P1-yNy
Polarization Z(X’,X’)Z’ Raman shift
Lorentz Fit
Lorentz Fit
Lorentz Fit
Lorentz Fit
y=0
y=0.005
y=0.01
y=0.02
InP-like LO mode
358.6
358.3
358.8
358.1
(cm-1)
GaP-like LO mode
379.4
377.9
377.5
376.8
(cm-1)
(X',X')
Shift
page 74
Raman Shift vs Composition (X',X')
380.0
359.5
359.0
379.0
358.5
378.5
358.0
378.0
357.5
377.5
357.0
377.0
356.5
-1
379.5
Raman Shift (cm )
-1
Raman Shift (cm )
InP-LO
GaP-LO
376.5
356.0
0.000
0.005
0.010
0.015
Nitrogen Composition (y)
0.020
page 75