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 AstronomyUniversity of Oklahoma

Norman, OK 73019

Electronic Materials ConferenceSanta 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.)

Tin Concentration in Growth Solution, x(%)

0 2 4 6 8 10

Tin Concentration in layer, xS (%)

En

erg

y (m

eV

)

60

80

100

120

140

160

180

200

220

240

260

280

Wa

vele

ngt

h (

mic

ron

s)

5

6

7

8

910

12

15

20

Wav

en

um

be

r (c

m-1

)

2200

2000

1800

1600

1400

1200

1000

800

600

0 2 3 4 5 6 7 8 9

RoomTemperature

109 - 125 K

Pb1-xSnxSe

Lattice Parameter (Angstroms)6.0 6.1 6.2 6.3 6.4 6.5

77 K

Wav

elen

gth

( m

) 3.3

5.0

8.0

12.0

18.0

77 K

Ban

dga

p E

ner

gy (

meV

)

60

80

200

400

600

100

77 K

Wav

enu

mb

ers

(cm

-1)3000

2000

1300

800

500

PbTe

PbSe

Pb1-xSrxSe

Pb1-xSnxSePb1-xSnxTe

PbSe1-xTex

Pb1-xSrxTe

Wav

elen

gth

(mic

rons

)

14.0

11.0

9.0

8.0

7.0

6.0

5.5

5.0

4.5

Ene

rgy

(meV

)

50

75

100

125

150

175

200

225

250

275

Temperature (K)

0 50 100 150 200 250 300 350

Wav

enum

bers

600

800

1000

1200

1400

1600

1800

2000

2200

2400

PbSe0.78Te0.22

Photoluminescence

PbSe0.78Te0.22

Absorption Edge

Pb0.95Sn0.05Se0.80Te0.20

Photoluminescence

Pb0.95Sn0.05Se0.80Te0.20

Absorption Edge

IV-VI Laser Materials

PbSrSe p-type

PbSe Substrate~~

~~

~~

PbSe n-type PbSrSe n-type

Double Heterostructure Laser

Breath Analysis with IV-VI Lasers

Heat Sink

Heat Sink

IV-VI Laser

Wavenumber (cm-1)

1912.6 1912.7 1912.8 1912.9 1913.0

Vo

ltag

e (

V)

-5.0

0.0

5.0

Vo

ltag

e (

V)

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

Wavenumber (cm-1)

1912.68 1912.74 1912.80 1912.86 1912.92 1913.00

Inte

nsi

ty(c

m-1

/mo

lecu

le x

cm

-2)

1e-27

1e-26

1e-25

1e-24

1e-23

1e-22

1e-21

1e-20

1e-19

CO2

(A)

CO2

(B)

NO

CO2 + H2O

H2O - Spectral Reference

HITRAN '96

eNOeCO2

Upper Airway or Nasal NO (nNO)

Lower Airway NO (eNO)

40

20

0eNO

Co

nce

ntr

atio

n (

pp

b)

0

2

4

6

0 5 25 45Time (seconds)Start Exhalation End Exhalation

eCO

2C

on

cen

trat

ion

(%

)

End Tidal CO2

eNOeCO2

eNOeNOeCO2eCO2

Upper Airway or Nasal NO (nNO)

Lower Airway NO (eNO)

40

20

0eNO

Co

nce

ntr

atio

n (

pp

b)

0

2

4

6

0 5 25 45Time (seconds)

0 5 25 45Time (seconds)Start Exhalation End Exhalation

eCO

2C

on

cen

trat

ion

(%

)

End Tidal CO2

Asthma Diagnosis

Time (seconds)0 5 10 15 20 25 30 35 40 45 50

Con

cent

ratio

n (a

rb. u

nits

)

Non-Asthmatic

Asthmatic

Exhaled NO Exhaled CO2

• 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.

Laser Focus World, June 2002, P. 22

Roller et al., Optics Letters 27, 107 (2002).

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.

MBE Growth on Silicon and 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

IV-VI MBE Chamber at OUSources: PbSe, Sr, Se, PbTe, BaF2, CaF2, Ag, Bi2Se3

In Situ RHEED

Si(111) (77) after oxide desorption

After growth of 2 nm CaF2

After growth of 600 nm BaF2

Lattice Parameter (Angstroms)

5.2 5.4 5.6 5.8 6.0 6.2 6.4 6.6 6.8

Wav

elen

gth (

mic

rons)

0.1

0.20.30.50.81.32.03.35.08.012.018.026.040.0

Ban

dga

p E

ner

gy a

t 77

K (

eV)

0.01

0.1

1

10

Wav

enu

mber

s (c

m-1)

5000030000200001250080005000310020001300800500300200

PbTePbSe

Silicon

CaF2 BaF2

Pb1-xSnxSe1-yTey

Pb1-xEuxSe1-yTey

EuTe

Pb1-xSnxSe Pb1-xSnxTe

EuSe

BaF2 (111) substrate

(11) at 500 °C

After growth of 6 Å of PbSrSe on BaF2

After growth of 3 µm of PbSrSe on BaF2

PbSe/PbSrSe MQWs

PbSe

PbSrSe

PbSe

PbSrSe

PbSrSePbSe

PbSrSe

Buffer layer

Substrate

20

4 nm to 100 nm

2degree)

25.2 25.4 25.6 25.8 26.0 26.2 26.4 26.6

Log

Diff

ract

ion

Inte

nsity

(a.

u.)

1e+0

1e+1

1e+2

1e+3

1e+4

1e+50

+1

+2

+3

+4

+5

+6

-1

-2

BaF2(222)

-3

-4

1e+0

1e+1

1e+2

1e+3

1e+4

1e+5

(a)

(b)

0

+1

+2+3

+4+5

-2

-1

-3

Si Substrate

BaF2 Substrate

HRXRD

MQWs on Si have high crystalline quality MQWs on BaF2 substrates have higher crystalline

quality due to better thermal expansion match

Photoluminescence

Energy (meV)

280 300 320 340 360 380 400 420 440

PL

Inte

nsity

(ar

b. u

nits

)

0.0

0.5

1.0

1.5

2.0

2.5

Wavenumbers (cm-1)

2200 2400 2600 2800 3000 3200 3400 3600

4 nm (x5)

20 nm

12 nm

10 nm

5C Measured SpectraGaussian Fits

BaF2 Substrates

Near-IR (~980 nm) cw diode laser pumping (low intensity, ~250 mW)

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

Energy (meV)

280 300 320 340 360 380

PL

Inte

nsity

(ar

b. u

nits

)

0.0

0.2

0.4

0.6

0.8

1.0

Wavenumber (cm-1)

2100 2200 2300 2400 2500 2600 2700 2800 2900 3000 3100

20 nm

5C

25C

35C

45C

55C

15C

Measured SpectraGaussian Fits

CO2 Absorption

Mid-IR Emitter on Silicon

Wavenumber (cm-1)

2000 2500 3000 3500

PL In

ten

sity

(a

rb. u

nits

)

0.0

0.2

0.4

0.6

0.8

Energy (meV)

220 260 300 340 380 420 460

W331: 800 mA

15C

25C

35CTemperature (C)0 50

En

erg

y (m

eV

)

330

350dE/dT=0.381 meV/K

CO2 Absorption

Measured SpectraGaussian Fits

Si Substrate

Near-IR (~980 nm) cw diode laser (~250 mW)

Emission through Silicon Substrate – Promising optical interconnect architecture

IV-VI MQW

Si Substrate

PL I

nte

nsity (

a.u

.)

T=25 oC

2800 mA

2600 mA

2400 mA

2200 mA

2000 mA

1800 mA

1600 mA

MQW/BaF2/CaF2/Si(111)

Photon Energy (meV)

200 250 300 350 400 450 500

T=25 oC

1700 mA

1800 mA

1900 mA

2000 mA

2100 mA

2200 mA

2300 mA

MQW/BaF2(111)

InGaAs (972 nm) diode laser pump current

P u m p in g la s e r in je c tio n c u rre n t (m A )

1 4 0 0 1 6 0 0 1 8 0 0 2 0 0 0 2 2 0 0 2 4 0 0 2 6 0 0 2 8 0 0 3 0 0 0

Ep

ilaye

r te

mp

era

ture

(oC

)2 5

5 0

7 5

1 0 0

1 2 5

M Q W /B a F 2 (1 1 1 )

M Q W /B a F 2 /C a F 2 /S i(1 1 1 )

M Q W b o n d e d o n S i

B aF 2 S u b stra te

S ilico n S u b stra tes3 5 °

P u m p in g la s e r in je c tio n c u rre n t (m A )

1 4 0 0 1 6 0 0 1 8 0 0 2 0 0 0 2 2 0 0 2 4 0 0 2 6 0 0 2 8 0 0 3 0 0 0

Ep

ilaye

r te

mp

era

ture

(oC

)2 5

5 0

7 5

1 0 0

1 2 5

M Q W /B a F 2 (1 1 1 )

M Q W /B a F 2 /C a F 2 /S i(1 1 1 )

M Q W b o n d e d o n S i

B aF 2 S u b stra te

S ilico n S u b stra tes3 5 °

BaF2 Substrate

Silicon Substrates

Less epilayer heating with higher thermal conductivity silicon

substrates

Optical Heating of Epilayers

H. Z. Wu et al., J. Vac. Sci. and Technol. B 19, 1447 (2001).

h(meV)

150 175 200 225 250

0.0

0.1

4K

70K

(2-2)O

(2-2)N

(1-1)O

(1-1)N

(2-2)N

(1-1)N

(1-1)O

20.6 nm

IR Transmission

Differential Transmission Fourier Transform Infrared Spectroscopy

– Subtract transmission spectra collected at two different temperatures

– Peaks yield interband transition energies

H. Z. Wu et al., Applied Physics Letters 78, 2199 (2001).

20 or more pairs ofPbSe/PbSrSe

PbSrSeBand Gap

PbSe Band Gap

[111]20 or more pairs ofPbSe/PbSrSe

PbSrSeBand Gap

PbSe Band Gap

PbSrSeBand Gap

PbSe Band Gap

[111][111]

4 nm to 100 nm

h(meV)

150 200 250 300 350 400 450 500 550

0.0

0.1

0.2

0.3

0.4

4K

70K

150K

210K

295K

(1-1)N (2-2)N(3-3)N

x 2

x 3

x 5

(1-1)o

(2-2)o (3-3)o

Barrier

(1-1)o

(2-2)o (3-3)oBarrier

(1-1)N(2-2)N

(3-3)N

(1-1)o

(2-2)o(3-3)o

Barrier

(1-1)o

(1-1)o

(2-2)o

(2-2)o

(3-3)o

(3-3)o(2-2)N

LQW=20.6 nm

hmeV)

120 160 200 240 280 320 360 400

T/T

0.00

0.02

0.04

0.06

0.08

29.7 nm

20.6 nm

9.7 nm

(1-1)o

(1-1)N (2-2)N(2-2)o

(1-1)N

(1-1)o

(2-2)N(2-2)o

(3-3)N (3-3)o

(2-2)o

(2-2)N (3-3)o(4-4)o

(1-1)N

(1-1)o

Quantum Size Effects

Removal of L-Valley Degeneracy

• Direct gap is at the L-point in k-space– Four Equivalent L-valleys– Symmetric conduction and valence bands

• Potential variation in [111] direction – 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

[111]

[111]

[111][111]

[111][111]

[111][111]

[111][111][111]

Normal

Oblique

Oblique

Obliquelmnormalm 111

)8(9111 tltloblique mmmmm

mNe = 0.0788 mO

e = 0.0475

mNh = 0.0764 mO

h = 0.0408

Normal Oblique (3-Fold Degenerate)

Interband Transitions

QW Thickness (nm)

8 10 12 14 16 18 20 22 24 26 28 30

Tra

nsiti

on E

nerg

y (m

eV)

160

170

180

190

200

210

220

230

Normal Valley

Oblique Valleys

4 K

(1-1)N

(1-1)O (1-1)O

Eg (PbSe) = 150 meV (4K)

(1-1)O

Energy Levels

QW Thickness (nm)

10 15 20 25 30

Ene

rgy

Leve

ls (

meV

)

-50

0

50

100

150

200

Eg (PbSe) = 150 meV at 4 K

QW Thickness (nm)

10 15 20 25 30C

ondu

ctio

n B

and

Ene

rgy

Leve

ls (

meV

)

150

155

160

165

170

175

180

185

190

195

PbSe (Bulk) Conduction Band Edge

LO Phonon Energy (16.7 meV)

TO Phonon Energy (5.9 meV)

Normal Valley

Oblique Valleys

4 K

Normal

Oblique

PL Emission

Oblique Valleys

Lowest energy level has a low density of states– Lower threshold for population inversion– Stimulated emission at low excitation rates– Four-level laser design

Density of States

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)

Photon Energy (meV)

150 200 250 300 350

77 K

110 K

150 K

200 K

293 K

x2

(3-3)N (3-3)O(1-1)N

(1-1)O

(2-2)N(2-2)O

(2-2)O

(2-2)O

(2-2)O

(3-3)O

(3-3)O

(3-3)O

(1-1)O

(1-1)O

(1-1)O

(1-1)O(2-2)O

CO2

(2-2)N

H2O Absorption

(1-1)N

(2-2)N

CO2 Absorption

Eg(77K) = 180 meV Eg(77K) = 605 meV

LQW= 40 –100 nm LB= 30 nm

Pb1-xSrxSe

x = 14%

x = 0%

Ec

Ev

Eg(77K) = 180 meV Eg(77K) = 605 meV

LQW= 40 –100 nm LB= 30 nm

Pb1-xSrxSe

x = 14%

x = 0%

Ec

Ev

Parabolic MQWs

mnE

e

gc

QW

EQ

Lnji

*

2

21 )(2

Expect Evenly-Spaced Harmonic Oscillator Eigenvalues

hmeV)

150 200 250 300 350

100 nm

80 nm

60 nm

T=77 K

40 nm

(1-1)N

(1-1)O

(2-2)N

(2-2)O

(3-3)O

(3-3)O

(2-2)N(2-2)O

(3-3)N

(3-3)O

(4-4)O (5-5)O (6-6)O

(2-2)O

(2-2)O

(2-2)N

(2-2)N

(1-1)O(1-1)N

(1-1)N

(4-4)O

(4-4)O

(5-5)O

(3-3)N

(1-1)O

(1-1)O

Parabolic MQW Analysis

• 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 = 1.910-15 cm2

k (a)-0.3 -0.2 -0.1 0.1 0.2 0.3

Energy (meV)

-750

-500

-250

250

500

750

Nonparabolic

Parabolic

Nonparabolic

Parabolic

Equally Spaced Energy Levels(Harmonic Oscillator)

E(2-2)

EHO = ½ [E(2-2) - E(1-1)]

n=1n=2

n=5n=4

n=3

n=6

E(1-1)

EHO

Eg = E(1-1) - EHOE(2-2)

EHO = ½ [E(2-2) - E(1-1)]

n=1n=2

n=5n=4

n=3

n=6

E(1-1)

EHO

Eg = E(1-1) - EHO

)1(2

222

km

kE c

cc

)1(2

2v

v

22

v km

kE

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.