Studies of Band Alignment, Two-dimensional Electron Gas and Ordering Effects in InGaPN/GaAs Heterostructures

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Studies of Band Alignment, Two-dimensional
Electron Gas and Ordering Effects in InGaPN/GaAs
Heterostructures
??????? ??????? ?????? 3rd Workshop on
Low-Dimensional systems and Nanomaterials
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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.
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Part 1. Outline
?. Introduction. ?. Photoluminescence (PL)
Spectrum. ?. Photoreflectance (PR)
Spectrum. ?. Conclusions.
a) Theory and Experimental Details b)
Results and Discussion
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?. 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.

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?. Introduction

• Nitrogen incorporation drastically reduces the
  band gap in InGaAs long-wavelength
  optoele-ctronic 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).

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?. Introduction
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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

In0.54Ga0.46P/GaAs ?Ec0.170 eV, ?Ev0.234 eV
In0.54Ga0.46P0.995N0.005 /GaAs ?Ec0.006
eV, ?Ev0.198 eV
At 10 K QW PL

• A type?alignment.

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The types of band alignment
Conduction Band
?Ec
Conduction Band
Conduction Band
Conduction Band
EG InGaP
Eg GaAs
Eg GaAs
EG InGaPN
Valence Band
Valence Band
?Ev
Valence Band
Valence Band
Type?alignment
Type?alignment
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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 ? alignment.
2. More nonradiative centers.

Back
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Sample structure
InGaPN samples were grown by gas-source MBE on
GaAs (100) SI substrate.
y 0, 0.005, 0.01, and 0.02
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?. Photoluminescence (PL) Spectruma) Theory and
Experimental Details

• A luminescence process involves three separate
  steps
• ExcitationElectron-hole pairs have to be excited
  by
• an external source of
  energy ( laser pump,
• photoluminescence ).
• ThermalizationThe excited e-h pairs relax
  towards
• quasi-thermal
  equilibrium distributions.
• RecombinationThe thermalized e-h pairs recombine
  
• 
  radiatively to produce the emission
• (
  dependent on the luminescence paths ).

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Schematic experimental setup for PL spectrum
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b) Results and Discussion In0.54Ga0.46P1-yNy
PL
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In0.54Ga0.46P1-yNy PL results
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?. Photoreflectance (PR) Spectruma) Theory and
Experimental Details
He-Ne 633 nm or He-Cd 325 nm laser
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Low electric field limit

• For unbound states third derivative line-shape

• For bound states first derivative line-shape

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b) Results and DiscussionIn0.54Ga0.46P1-yNy
He-Cd 325 nm laser pump
Go p.38
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Fitting results In0.54Ga0.46P1-yNy
The incroporation of 2 nitrogen reduces 215 meV
band-gap energy.
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In0.54Ga0.46P1-yNy He-Ne 633 nm laser pump
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In0.54Ga0.46P1-yNy He-Ne 633 nm laser
pump2DEG transition energies
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Fitting results In0.54Ga0.46P1-yNy
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The types of band alignment
Conduction Band
?Ec
Conduction Band
Conduction Band
Conduction Band
EG InGaP
Eg GaAs
Eg GaAs
EG InGaPN
Valence Band
Valence Band
?Ev
Valence Band
Valence Band
Type?alignment
Type?alignment
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The In0.54Ga0.46P1-yNy and GaAs heterojunction.
Approximate triangular potential wells and
two-dimensional electron gas are formed at the
junction. (a) For type?alignment (b) for
type?alignment.
Conduction Band
(a)
Conduction Band
In0.54Ga0.46P
GaAs
Valence Band
Valence Band
Type?alignment
Type?alignment
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In0.54Ga0.46P1-yNy / GaAs heterojunction band
alignment varies with nitrogen composition
Ec
y 0.005
y gt 0.005
Ev
In0.54Ga0.46P1-yNy
Ev
Valence Band
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Fitting results In0.54Ga0.46P1-yNy
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?. 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.

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?. Conclusions

1. The 2DEG transition energies are determined, and
   all of them are smaller than the band gap of
   GaAs.
2. We make sure that the InGaP1-yNy/GaAs lies in
   type ? band alignment as y gt 0.005.

J. S. Hwang et al, Appl. Phys. Lett. 86, 061103
(2005). K. I. Lin et al, J. Appl. Phys. 99,
056103 (2006).
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Part 2. Outline

• Introduction.
• Theory and Experimental
• Details--Raman Spectrum.
• Results and Discussion.
• Conclusions.

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1. Introduction

• The spontaneous long-range ordering has been
• observed in many ?-? 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
  ?/? flux
• ratio
• The ordering influences carrier lifetimes and
• energy band which can affect the electronic
  and opti-
• cal properties of semiconductors.

the formation of different crystal structure.
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1. Introduction

• The incorporation of dilute N atoms induces
  long-
• range order in GaAs0.98N0.02.

Motivation
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Sample structure
InGaPN samples were grown by gas-source MBE on
GaAs (100) SI substrate.
y 0, 0.005, 0.01, and 0.02
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2. Theory and Experimental Details First-order
Raman scattering (one-phonon process)
Raman scattering measurements provide a
quantitative and non-destructive method to study
the electronic and phonon properties of the
materials.
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InGaP2 alloy

• In the order phase
• trigonal CuPt structure.
• C3v point group.

• In the disorder phase
• cubic zinc-blende structure.
• Td point group.

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Scattering configuration
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3. Results and Discussion In0.54Ga0.46P0.98N0.02
total range, no polarized
LO Longitudinal optic mode TO
Transverse optic mode DALA Disorder activated
longitudinal acoustic phonon
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Definition of valley-to-peak intensity ratio (b/a)
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Reference S. F. Yoon et al., Microelectronics
Journal 31, 15 (2000).
InGaP
GaP-LO
InP-LO
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In0.54Ga0.46P1-yNy
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In0.54Ga0.46P1-yNy Polarization Z(X,X)Z
Valley-to-Peak Intensity Ratio (b/a)
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b/a Ratio vs Composition (X',X')
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Reference S. F. Yoon et al., Microelectronics
Journal 31, 15 (2000).
InGaP
GaP-LO
InP-LO
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In0.54Ga0.46P1-yNy Polarization Z(X,X)Z
Raman intensity
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Raman Intensity vs Composition (X',X')
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Raman selection rule Td group symmetry the
LO-phonon Raman
scattering is forbidden with configuration
(X,X), but
is allowed with (X,Y).
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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).
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In0.54Ga0.46P
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In0.54Ga0.46P0.995N0.005
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In0.54Ga0.46P0.99N0.01
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In0.54Ga0.46P0.98N0.02
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In0.54Ga0.46P1-yNy GaP-like LO mode with
(X,Y) and (X,X) Polarization
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GaP-like LO mode Intensity Ratio (X",Y")/(X",X")
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Reference F. Alsina et al., Phys. Rev. B 53,
12994 (1996).
DALA Disorder activated longitudinal acoustic
phonon 200 cm-1
FLA Folded longitudinal acoustic phonon 208
cm-1
page 81
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In0.54Ga0.46P1-yNy Polarization Z(Y,Y)Z
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Reference L. H. Robins et al., MRS Internet J.
Nitride Semicond. Res. 4S1, G3.22 (1999).
LO
LO
590 cm-1
734 cm-1
Raman spectra of InxGa1-xN samples.
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In0.54Ga0.46P1-yNy no polarized
InGaN-like LO mode
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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.
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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.

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4. Conclusions

• We assign the broad Raman peak at about 735 cm-1
  to the InGaN-like LO mode.
• 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).
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In0.54Ga0.46P1-yNy/GaAs GaAs Eg and
electric field fit
G p.35
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In0.54Ga0.46P1-yNy/GaAs GaAs Eg and
electric field fit
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Fitting results In0.54Ga0.46P1-yNy
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Moderate electric field
Franz-Keldysh oscillations (FKOs)
F electric field ? reduce mass of
electron and heavy hole
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The extremes of the FKOs occur when
here and
and n0,1,2,3
We have
B p.26
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Bowing parameter --- d(y)
For conventional ?-? ternary alloys Vegards
law adding a quadratic correction bx(x-1). Such
as InxGa1-xAs with d0.5 eV.
Eg(InxGa1-xAs) xEg(InAs)(1-x)Eg(GaAs)dx(x-1)
But incorporating a small amount of nitrogen in
?-? semiconductor results in a strong reduction
of Eg. Such as InPN (d 16 eV), GaPN (d 14
eV). However, GaAsN and InGaAsN need composition
dependent bowing parameter d(x). ex. GaAs1-xNx
d(x)1020 eV.
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Bowing parameter --- d(y)
Eg(In0.54Ga0.46P1-yNy) yEg(In0.54Ga0.46N)
(1-y)Eg(In0.54Ga0.46
P) dy(y-1)
And In0.54Ga0.46N band gap1.6338 eV
APL. 80, 4741 (2002) In0.54Ga0.46P
band gap1.8425 eV
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Further Works

• Temperature-dependent PL and PR spectra
• defect level (N clusters) and Varshnis
• formula.
• Valence-band splitting strain and ordering.
• Band-gap reduction strain, ordering, and N
• incorporation.
• N incorporation band-anticrossing model
• E_, E
  energy level.

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I0(?)
I0RI0?R
I0R
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Fitting results In0.54Ga0.46P1-yNy
B p.20
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Reference R. J. welty et al., IEEE 11-7, 33
(2000).
Tunneling-collector HBT
Blocked hole bipolar transistor
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B p.16
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Schematic experimental setup for micro-Raman
spectrum
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A photograph of our micro-Raman spectrum
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In0.54Ga0.46P1-yNy Polarization Z(X,X)Z
Raman shift
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