Real-time optical characterization of ) by UV absorption 3...

Real-time optical characterization of

ammonia (NH3) by UV absorption

spectroscopy

Nikolaus Dietz and Vincent Woods

Department of Physics & Astronomy

Georgia State University; Atlanta GA

http://www.phy-astr.gsu.edu/dietzrg/HPCVD.html

Last update: Jan. 10, 2005

Advantage of UV spectroscopy to study precursordecomposition dynamics

• optical detectionbelow 400 nmeliminatesinterference withradiation fromheated substrate /gas phase:

– UV absorption

– Luminescence

– Ramanspectroscopy

– Light Scattering

N. DietzDept. Physics & Astronomy

1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0

radi

atio

n in

tens

ity (

a.u.

)

wavelength (µm)

600K

1200K

1000K

800K

Black Body emission

300 500 700ra

diat

ion

inte

nsity

( lo

g sc

ale

)

wavelength (nm)

600K

1200K

1000K

800K

Ammonia (NH3) Characterization

� NH3 UV-transmission spectra

� Continuous NH3 flow: Sensitivity

� Decomposition of NH3

� Pulsed NH3 flow


Characterization of NH3 UV-absorptionN. DietzDept. Physics & Astronomy

0

500

1000

1500

196 200 204 208 212 216 220 224 228

1%2%3%4%5%10%15%20%25%50%75%100%Referencetr

ansm

itted

inte

nsity

(a.

u.)

wavelength (nm)

NH3 absorptionReactor pressure: 1630 mbarNH3 flow: 1000sccm = 100% FS diluted in N2-Main flow: 5 slm

energy (eV)6.20 5.96 5.74 5.54

Characterization of NH3 UV-absorption : MFC-IN. DietzDept. Physics & Astronomy

0.0

0.1

0.2

0.3

0.4

0.5

0.6

195 200 205 210 215 220 225 230

abso

rptio

n (c

m-1)

wavelength (nm)

energy (eV)

NH3

absorption

Reactor pressure: 1630 mbarNH3 flow diluted in N2-Main flow: 5 slm 100%FS = 1000 sccm

6.20 5.90 5.64 5.39

100%FS

75%FS

50%FS

20%FS

10%FS

1%FS

3%FS

5%FS

209.209 217.105205.379

201.808198.136

213.086

221.599

224.778

Characterization of NH3 UV-absorption: MFC-IIN. DietzDept. Physics & Astronomy

0.00

0.05

0.10

0.15

0.20

0.25

0.30

195 200 205 210 215 220 225

abso

rptio

n (c

m-1)

wavelength (nm)

energy (eV)

NH3 absorption

100%FS = 2 sccm

6.20 5.90 5.64

90%FS

75%FS

10%FS

25%FS

50%FS

205.4

201.8

213.1

209.21

221.6

217.1

Reactor Conditions:Pressure: 1630 mbarTemperature: 23 °CN2 Main Flow: 5 slm

NH3 characterization: Absorption maxima

The molar ammonia flow ratio χ through the reactor is givenbywhere z and y are the percentage of flow full scale (FS) with 50 slm and 1 slm, respectively.

The total flow of gases through the HPCVD reactor is given by


F = F + F = 0. z +1 10 y [slm]Main_N NH2 3-25 ⋅ ⋅ ⋅

χ = n

n =

n

n + n =

yz + y

NH

total

NH

Main_N NH

3 3

2 350 ⋅

10

100

0 2 4 6 8 10 12

abso

rptio

n pe

ak-m

axim

a (

10 -2

cm

-1)

NH3 flow ratio χ (10-2)

Absorption maxima vs. NH3 flow ratio χ

NH3 absorption @ 217.105 nmSteady-state NH3 flow conditionReactor pressure: 1600 mbarN2-Main flow: 5 slm

α(χ)peak = 0.38 • ln(χ+0.011) - 2.0 • χ + 1.73 [cm-1]

Saturation effect

1

10

100

0 2 4 6 8 10 12 14

abso

rptio

n pe

ak-m

axim

a (

10 -2

cm

-1)

NH3 flow ratio χ (10-2)

Absorption maxima vs. NH3 flow ratio χ NH3 absorption @ 221.6 nmSteady-state NH3 flow conditionReactor pressure: 1600 mbarN2-Main flow: 5 slm

α(χ)peak

= [ - 45.0 + 45.01 • exp(χ / 18) ] * 10-2 [cm-1]

NH3 flow: Concentration & SensitivityN. DietzDept. Physics & Astronomy

The number concentration of NH3

molecules per time unit as

function of the observed

absorption at

λ = 221.6 nm

Absorption line at λλλλ = 221.6 nm

sensitive for NH3 concentrations

in the range: 10 +19 - 10+21

0

5

10

15

20

25

0 10 20 30 40 50

Num

ber

of N

NH

3 mol

ecul

es p

er ti

me

unit

( 1

019 )

NH3 Flow Setting (%FS)

Reactor Flow 5 slm Reactor pressure p: 1.6 barλ = 221.6 nm

N = 7 10 z 1 - 32NH3

21

( . ). ln

lnλα

=⋅ ⋅ ⋅ ′

⋅ ′221 617

nm αα

α

[s ]

with

-1

′ == - 80 80.01

α@ .221 6nm

NH3 flow: Concentration & SensitivityN. DietzDept. Physics & Astronomy

Sensitivity to NH3concentrations

λ = 224.8 nm ( 10 +21 - 10+24 )

λ = 221.6 nm ( 10 +18 - 10+21 )

λ = 217.1 nm ( 10 +17 - 10+19 )

λ = 213.07 nm ( 10 +16 - 10+18 )

λ = 209.21 nm ( 10 +15 - 10+17 )

λ = 205.38 nm ( 10 +14 - 10+17 )

λ = 201.81 nm ( ? - 10+16 )

λ = 198.14 nm ( ? - 10+16 )

λ = 194.62 nm ( ? - 10+15 )

100

101

102

100 101 102 103 104

201 nm205.4 nm209.2 nm213.1 nm217.1 nm221.6 nm

abso

rptio

n pe

ak m

axim

a (

10-2 c

m-1)

NH3 Flow ratio χ (10-5)

NH3 Absorption Peak Maxima

Reactor Conditions:Pressure: 1.6 barTemperature: 23 °C

217 nm

222 nm

MFC - I

MFC - II

Absorption peak maxima

correlate linearly with

NH3 flow ratio χχχχ in a

double log scale!

Decomposition of NH3N. DietzDept. Physics & Astronomy

0.2

0.4

0.6

0.8

200 210 220 230

RT 690K800K900K990K1070K1140K1200K

abso

rptio

n (c

m-1)

wavelength (nm)

energy (eV)

NH3 decomposition

6.20 5.90 5.64 5.39

NH3 = 20 sccmdiluted in 5slm N2 main flow

0.1

0.3

0.5

0.7

250 450 650 850 1050 1250

abso

rptio

n pe

ak-a

mpl

itude

(cm

-1)

Temperature [K]

20 sccm NH3

diluted in 5 slm N2 carrier gas

205.4 nm

213 nm

217.1 nm

221.6 nm

224.8 nm

209.2 nm

NH3 flow characterization: Pulsed Injection

Transmission traces monitored at λ = 221.6 nm during NH3 precursorpulse injection in the reactor at 1.6 bar and a total flow through the

reactor of 5slm. The NH3 flow was varied from 10 – 500 sccm(1 - 50%FS). The cycle sequence is 6 s with a 1 s NH3 pulse width.

Absorption traces for 1 sec NH3pulses injected 6 s apart


2

3

4

5

0 2 4 6

tran

smis

sion

(a.

u.)

time (s)

50%FS

NH3 pulse

40%FS

30%FS

20%FS

10%FS

5%FS

1%FS

5

10

15

20

25

0 2 4 6 8ab

sorp

tion

(10

-2 c

m-1)

time (s)

NH3 pulse

40%FS

30%FS

20%FS

10%FS

5%FS

1%FS

50%FS


Absorption tracesmonitored at λλλλ = 221.6 nm

Reactor main flow: 5slm

NH3 flow through thereservoir: 200 sccm

Reactor pressure: 1bar - 12.5 bar

Distinct features:• a systematic shift in the pulse arrival time,

• a systematic NH3

pulse broadening,

• a change in NH3

absorption cross section for p > 8 bar.


0

4

8

12

0 2 4 6 8 10 12

abso

prtio

n (

10-2 c

m-1)

time (s)

1.6bar

2.5 bar 5bar

NH3 pulse (1s @ 20% FS) NH3 pulse

7.5bar

10bar

12.2bar


Main flow increased to 10 slm! � NH3 pulses sharpened


0

4

8

12

0 2 4 6 8 10

abso

prtio

n (

10-2 c

m-1)

time (s)

2.1bar

5 bar7.5bar

NH3 pulse (1s @ 20% FS) NH3 pulse

10bar

12.5bar


Precursor pulse delay /

broadening as function

of flow rate at constant

reactor pressure

Monitoring: λλλλ = 221.6 nm

Cycle repetition rate: 6 sec

NH3 pulse width: 1 sec

NH3 flow: 20%FS = 200 sccm

Reactor pressure: p = 10 bar


0

2

4

6

8

10

0 2 4 6 8 10 12

abso

prtio

n (

10-2 c

m-1)

time (s)

15 slm

NH3 pulse

10 slm

5 slm

NH3 pulse

[1] N. Dietz, M. Strassburg and V. Woods, “Real-time Optical Monitoring of Ammonia

Decomposition Kinetics in InN Vapor Phase Epitaxy at Elevated Pressures”,

AVS 51st International Symposium, Anaheim, CA, Nov. 14-19, 2004.

[2] N. Dietz an V. Woods, unpublished results (2004).

refer to http://www.phy-astr.gsu.edu/dietzrg/HPCVD.html

ReferencesN. DietzDept. Physics & Astronomy

Real-time optical characterization of ) by UV absorption 3...

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