Growth and Characterization of
IV-VI Semiconductor Multiple
Quantum Well Structures
Patrick J. McCann, Huizhen Wu, and Ning Dai*
School of Electrical and Computer Engineering
*Department of Physics and Astronomy
University of Oklahoma
Norman, OK 73019
Electronic Materials Conference
Santa Barbara, CA
June 27, 2002
Outline
•
•
•
•
•
IV-VI Semiconductors
Biomedical Applications
MBE Growth and Characterization
Square and Parabolic MQWs
Summary
IV-VI Semiconductors
(Pb-Salts)
•Unique Features
– High Dielectric Constants  Defect Screening
– Can be Grown on Silicon  Low Cost, Integration Possibilities
– Symmetric Band Structure  High Electron and Hole Mobilities
•Applications
– Thermoelectric Coolers (Low Lattice Thermal Conductivity)
– Infrared Detectors (Silicon Integration Possible)
– Spintronics (Quantum Dots with Magnetic Impurities)
– Tunable Mid-IR Lasers (Medical Diagnostics, etc.)
IV-VI Laser Materials
275
2400
PbSe1-xTex
8.0
12.0
100
2000
1300
800
Pb1-xSnxTe
Pb1-xSnxSe
80
5.0
1800
200
5.5
1600
175
Absorption Edge
PbSe0.78Te0.22
1400
125
1200
Pb0.95Sn0.05Se0.80Te0.20
1000
18.0
6.1
6.2
6.3
6.4
Lattice Parameter (Angstroms)
50
3
4
5
6
7
8
50
100 150 200 250 300 350
Temperature (K)
Tin Concentration in layer, xS (%)
2
11.0
14.0
Photoluminescence
600
0
0
9.0
Pb0.95Sn0.05Se0.80Te0.20
6.5
9
280
2200
260
6
200
180
7
160
8
9
10
140
109 - 125 K
120
100
12
80
15
60
20
0
2
4
6
8
10
Tin Concentration in Growth Solution, x (%)
1800
1600
1400
1200
1000
Wavenumber (cm-1)
Pb1-xSnxSe
2000
Wavelength (microns)
220
5
Room
Temperature
240
PbSrSe
PbSe
PbSrSe
~
~
p-type
n-type
n-type
PbSe Substrate
~
~
~
~
800
600
8.0
75
800
6.0
7.0
100
Absorption Edge
500
60
150
Photoluminescence
6.0
Double Heterostructure Laser
Wavelength (microns)
PbSe
225
2000
Energy (meV)
5.0
PbTe
3000
Wavenumbers
3.3
77 K Wavenumbers (cm-1)
Pb1-xSrxSe
200
4.5
PbSe0.78Te0.22
77 K Wavelength (m)
400
250
2200
Pb1-xSrxTe
Energy (meV)
77 K Bandgap Energy (meV)
600
Breath Analysis with IV-VI Lasers
Wavenumber (cm-1)
1912.8
1912.9
1913.0
0.0
-5.0
1.0
Heat Sink
0.8
Voltage (V)
0.6
Heat Sink
0.4
0.2
0.0
-0.2
Intensity
(cm-1/molecule x cm-2)
-0.4
IV-VI Laser
1e-19
1e-20
1e-21
1e-22
1e-23
1e-24
1e-25
1e-26
1e-27
Upper Airway or
Nasal NO (nNO)
NO
HITRAN '96
CO2
(A)
CO2
(B)
CO2 + H2O
40
End Tidal
CO2
20
Lower Airway
NO (eNO)
1912.74
1912.80
1912.86
Wavenumber (cm-1)
1912.92
1913.00
4
2
0
0
0
5
Start Exhalation
1912.68
6
eNO
eCO2
25
Time (seconds)
45
End Exhalation
eCO2 Concentration (%)
1912.7
H2O - Spectral Reference
eNO Concentration (ppb)
Voltage (V)
1912.6
5.0
Asthma Diagnosis
Concentration (arb. units)
Exhaled NO
Exhaled CO2
Laser Focus World,
June 2002, P. 22
Roller et al., Optics
Letters 27, 107 (2002).
Asthmatic
Non-Asthmatic
0
5
10
15
20
25
30
35
40
45
50
Time (seconds)
• High exhaled NO indicates airway inflammation.
–
People with asthma suffer from chronic airway inflammation.
• Quantum cascade mid-IR lasers have not been able to do such
measurements even though several attempts have been made.
IV-VI Epitaxial Layers
• High quality layers can be grown on silicon
–
–
McCann et al., Journal of Crystal Growth 175/176, 1057 (1997).
Strecker et al., Journal of Electronic Materials 26, 444 (1997).
• Room temperature cw photoluminescence
–
–
McCann et al., Applied Physics Letters 75, 3608 (1999).
McAlister et al., Journal of Applied Physics 89, 3514 (2001).
• Optical devices on silicon
–
–
–
Through-the-substrate inter-chip optical interconnects (PC Magazine, January 21, 2002).
Modulators for free-space optical communication.
Infrared imaging arrays.
BaF2
0.1
0.2
0.3
0.5
0.8
1.3
2.0
3.3
5.0
8.0
12.0
18.0
26.0
40.0
EuTe
EuSe
Silicon
1
Pb1-xEuxSe1-yTey
PbTe
PbSe
0.1
Pb1-xSnxSe
Pb1-xSnxTe
Pb1-xSnxSe1-yTey
0.01
5.2
IV-VI MBE Chamber at OU
5.4
5.6
5.8
6.0
6.2
6.4
6.6
6.8
Lattice Parameter (Angstroms)
Sources: PbSe, Sr, Se, PbTe, BaF2, CaF2, Ag, Bi2Se3
Si(111) (77) after
oxide desorption
In Situ RHEED
BaF2 (111) substrate
(11) at 500 °C
After growth of
2 nm CaF2
After growth of 6 Å
of PbSrSe on BaF2
After growth of
600 nm BaF2
After growth of 3 µm
of PbSrSe on BaF2





SiO2 desorption at 700°C allows epitaxial
growth of nearly lattice-matched CaF2 on Si
CaF2 growth on Si is layer-by-layer
BaF2 growth on CaF2 is layer-by-layer
PbSrSe growth on low surface energy BaF2
is initially 3D (island)
PbSrSe layer eventually becomes 2D after
growth of more than 1 µm
50000
30000
20000
12500
8000
5000
3100
2000
1300
800
500
300
200
Wavenumbers (cm-1)
CaF2
10
Wavelength (microns)
Bandgap Energy at 77 K (eV)
MBE Growth on Silicon and BaF2
PbSe/PbSrSe MQWs
1e+5
1e+4
20
PbSrSe
PbSe
PbSrSe
1e+3
Log Diffraction Intensity (a.u.)
4 nm to 100 nm
PbSe
PbSrSe
PbSe
PbSrSe
-1
-3
1e+2
Si Substrate
+1
+2
-2
+3
+4
+5
1e+1
1e+0
1e+5
0
BaF2 (b)
Substrate
1e+4
BaF2(222)
-1
+1
1e+3
+2
-2
-3
Buffer layer
(a)
0
HRXRD
+3
+4
1e+2
+5
-4
Substrate
+6
1e+1
1e+0
25.2
25.4
25.6
25.8
26.0
26.2
26.4
2degree)
 MQWs on Si have high crystalline quality
 MQWs on BaF2 substrates have higher crystalline
quality due to better thermal expansion match
26.6
Photoluminescence
Near-IR (~980 nm) cw diode laser pumping (low intensity, ~250 mW)
Energy (meV)
280
300
320
340
360
Energy (meV)
380
400
5C
420
440
280
320
340
360
20 nm
Measured Spectra
Gaussian Fits
2.5
300
380
Measured Spectra
Gaussian Fits
1.0
2.0
PL Intensity (arb. units)
PL Intensity (arb. units)
CO2 Absorption
20 nm
BaF2 Substrates
1.5
1.0
12 nm
0.5
10 nm
0.0
4 nm (x5)
0.8
0.6
5C
15C
0.4
25C
35C
0.2
45C
2200
55C
0.0
2400
2600
2800
3000
3200
3400
3600
2100
2200
2300
2400
-1
2500
2600
2700
2800
-1
Wavenumbers (cm )
Wavenumber (cm )
 Strong Quantum Size Effect
 Strong CW Emission at 55°C
 Interference Fringes Dominate Spectra
–
–
Spacings depend on index of refraction and epilayer thickness
Strong optical resonance indicates stimulated emission processes
2900
3000
3100
Mid-IR Emitter on Silicon
Energy (meV)
220
Si Substrate
260
340
PL Intensity (arb. units)
Near-IR (~980 nm) cw
diode laser (~250 mW)
420
Si Substrate
330
CO2 Absorption
0

Temperature (C)
35 C
0.4
25C
0.2
15C
0.0
Emission through Silicon Substrate
– Promising optical interconnect architecture
2000
460
dE/dT=0.381 meV/K
Energy (meV)
Measured Spectra
Gaussian Fits
0.6
380
350
W331: 800 mA
0.8
IV-VI MQW
300
2500
3000
-1
Wavenumber (cm )
3500
50
Optical Heating of Epilayers
T = 2 5 oC
Less epilayer heating
with higher thermal
conductivity silicon
substrates
M Q W /B a F 2 /C a F 2 /S i( 1 1 1 )
2800 m A
2600 m A
2400 m A
P L I n t e n s it y ( a . u . )
2200 m A
2000 m A
1800 m A
1600 m A
125
T = 2 5 oC
M Q W /B a F 2 ( 1 1 1 )
BaF
2 2Substrate
BaF
Substrate
100
o
Epilayer temperature (C)
2300 m A
2200 m A
2100 m A
2000 m A
35°
Silicon
Silicon
Substrates
Substrates
75
50
MQW/BaF 2(111)
1900 m A
MQW/BaF 2/CaF 2/Si(111)
MQW bonded on Si
1800 m A
25
1400
1700 m A
1600
1800
2000
2200
2400
2600
2800
3000
Pumping laser injection current (mA)
200
250
300
350
400
450
500
H. Z. Wu et al., J. Vac. Sci. and Technol. B 19, 1447 (2001).
P h o to n E n e r g y ( m e V )
InGaAs (972 nm) diode
laser pump current
IR Transmission
20.6 nm
(1-1)
(1-1)N
4 nm to 100 nm
0.1
(2-2)

70K
(1-1)
(1-1)
20 or more pairs of
PbSe/PbSrSe
PbSe
Band
Gap
N
O
N
(2-2)
4K
0.0
PbSrSe
Band
Gap
O
150
175
200
h(meV)
(2-2)
N
225
O
250
0.4
[111]
0.3
Differential Transmission Fourier
Transform Infrared Spectroscopy
– Peaks yield interband transition energies
(2-2)o
(3-3)o
x5
295K
(1-1)o
(2-2)o
x3

– Subtract transmission spectra collected at two
different temperatures
Barrier
(1-1)o
LQW=20.6 nm
(3-3)o
210K
(1-1)o
0.2
(3-3)o
o
(2-2)N (2-2)
x2
(1-1)o
0.1
(1-1)N
(2-2)N
(2-2)o
(3-3)N
(3-3)o
(1-1)o
H. Z. Wu et al., Applied Physics Letters 78, 2199 (2001).
(1-1)
N
(2-2)N
150K
Barrier
(2-2)o
70K
Barrier
(3-3)
N
(3-3)o
4K
0.0
150
200
250
300
350
h(meV)
400
450
500
550
Quantum Size Effects
0.08
(1-1)o
(2-2)o
(1-1)o
9.7 nm
T/T
(2-2)N
(1-1)N
0.06
0.04
(1-1)
0.02
20.6 nm
(1-1)
0.00
120
(2-2)o
N
(2-2)
N
(3-3)
N
(3-3)o
(1-1)o
N
(2-2)
N
(2-2)o
(3-3)o
(4-4)o
29.7 nm
160
200
240
280
hmeV)
320
360
400
Removal of L-Valley Degeneracy
Oblique
• Direct gap is at the L-point in k-space
–
–
Four Equivalent L-valleys
Symmetric conduction and valence bands
Oblique
• Potential variation in [111] direction
–
–
Normal
One L-valley is normal to the (111) plane in k-space
Three L-valleys are at oblique angles
• Two different effective masses for
electrons (and holes) in the PbSe MQWs
normal  m
m111
l
Oblique
oblique
m111
 9ml mt (8ml  mt )
mNe = 0.0788
mOe = 0.0475
mNh = 0.0764
mOh = 0.0408
Normal
Oblique
(3-Fold Degenerate)
Interband Transitions
230
4K
Transition Energy (meV)
220
210
(1-1)O
(1-1)O
200
(1-1)N
Oblique Valleys
190
(1-1)O
Normal Valley
180
170
160
8
10
12
14
16
Eg (PbSe) = 150 meV (4K)
18
20
22
QW Thickness (nm)
24
26
28
30
Energy Levels
Conduction Band Energy Levels (meV)
Oblique
200
Energy Levels (meV)
150
Normal
100
Eg (PbSe) = 150 meV at 4 K
50
0
-50
10
15
20
QW Thickness (nm)
25
30
195
4K
TO Phonon Energy
(5.9 meV)
190
LO Phonon Energy
(16.7 meV)
185
180
175
Oblique Valleys
170
165
Normal Valley
160
155
PbSe (Bulk) Conduction Band Edge
150
10
15
20
QW Thickness (nm)
25
30
PL Emission

Oblique Valleys
Density of States
Lowest energy level has a low density of states
– Lower threshold for population inversion
– Stimulated emission at low excitation rates
– Four-level laser design
Lasing Thresholds
IV-VI Mid-IR VCSELs
• Bulk Active Region
– Optical pumping threshold: 69 kW/cm2
Z. Shi et al., Appl. Phys. Lett., 76, 3688 (2000)
• MQW Active Region
– Optical pumping threshold: 10.5 kW/cm2
C. L. Felix et al., Appl. Phys. Lett. 78, 3770 (2001)
Parabolic MQWs
x = 14%
Ec
Expect Evenly-Spaced Harmonic
Oscillator Eigenvalues
Pb1-xSrxSe
x = 0%
Eg (77K) = 180 meV
Eg (77K) = 605 meV
E i nj  2(n  12 ) LQW
2Qc Eg
*
me
Ev
LB = 30 nm
H2O Absorption
CO2
293 K
(1-1)O
(2-2)O
x2
O
(1-1)
(2-2)O
200 K
(1-1)O

LQW = 40 – 100 nm
CO2 Absorption
N
(2-2)
(2-2)O
150 K
(1-1)O
N
(1-1)
(3-3)O
N
(2-2)O
(3-3)O
(3-3)O
(2-2)
110 K
(1-1)O
77 K
150
(1-1)N
200
(2-2)
250
N
(2-2)O
(3-3)N
300
Photon Energy (meV)
(3-3)O
350
Parabolic MQW Analysis
(1-1)O
(1-1)N
T=77 K
(2-2)N
40 nm
(1-1)O
N

(1-1)
60 nm
N
(3-3)N
EHO
(4-4)O
(3-3)O
E(1-1)
E(2-2)
O
(2-2)N
80 nm
(2-2)O
(2-2)
(2-2)
(1-1)O
(1-1)N
(2-2)
(3-3)N
(3-3)O
(4-4)O
Eg = E(1-1) - EHO
(5-5)O
EHO = ½ [E(2-2) - E(1-1)]
(1-1)O
(2-2)N
(2-2)O
(3-3)O
(4-4)O
(5-5)O
(6-6)O
Equally Spaced Energy Levels
(Harmonic Oscillator)
100 nm
150
n=6
n=5
n=4
n=3
n=2
n=1
O
200
250
300
350
hmeV)
•
•
Measured bandgaps in strained PbSe
(caused by lattice mismatch with PbSrSe)
compared to 77 K bandgap for bulk PbSe
allows determination of deformation
potentials: Dd = 6.1 eV and Du = -1.3 eV.
Energies for the higher confined states in
100 nm sample allows determination of
band non-parabolicity parameters:
c = v = 1.910-15 cm2
 2k 2
Ec 
(1   c k 2 )
2mc
Energy (meV)
750
Parabolic
Nonparabolic
500
250
-0.3
-0.2
-0.1
0.1
0.2
0.3
k (a)
-250
Nonparabolic
-500
Parabolic
-750
Ev 
 2k 2
(1   v k 2 )
2mv
Summary
• IV-VI semiconductors are versatile materials for a
variety of applications.
– A mid-IR laser spectroscopy application for asthma diagnosis
has been developed.
• PbSe-based MQW structures have attractive properties
for improved mid-IR laser technology.
– L-valley degeneracy removal.
– Energy level structure in MQWs on (111)-oriented substrates
enables low population inversion thresholds.