Real-time optical characterization of Trimethylindium (TMI) by UV absorption spectroscopy

Transcription

1 Real-time optical characterization of Trimethylindium (TMI) by UV absorption spectroscopy Nikolaus Dietz and Vincent Woods Department of Physics & Astronomy Georgia State University; Atlanta GA Last update: Jan. 1, 25

2 Trimethylindium (TMI) Characterization N. Dietz TMI structure TMI flow characterization continuous flow pulsed TMI injection flow velocity and pressure TMI decomposition dynamics

3 Trimethylindium (TMI) structure TMI - trimethylindium In-C bonds lay in a plane; TMI forms loose tetramers in crystal form TMI breaks down to DMI, MMI and finally surface active Indium Decomposition Symmetry Change Spectral Change Chemical decomposition reduced order model (1) In(CH 3 ) 3 In(CH 3 ) 2 + CH 3 (2) In(CH 3 ) 2 *In(CH 3 ) 1 + CH 3 (3) In(CH 3 ) 1 In + CH 3 Step (1) starts around 6 K, step (2) and (3) around 68 K and above!

4 TMI flow characterization The total flow of gases through the HPCVD reactor is given by Flow of TMI from TMI bubbler is: F = x [slm] F = F + F + F =. 5 y x x [slm] TMI Main_N2 bubbler_n2 TMI The molar TMI flow ratio χ through the reactor is given by 1 5 slm χ = n n TMI total = ntmi n + n +n Main_N2 bubbler_n2 TMI and can be expressed in term of the flows x and y as χ = x -2-5 z + 1 x x where z and x are the percentage of the flow scale (FS) with 5 slm and.5 slm, respectively. TMI flow ratio χ (1-5 ) slm 15 slm 2 slm 3 slm 5 slm molar TMI flow x (%FS)

5 TMI absorption characterization absorption (1-2 cm -1 ) energy (ev) TMI absorption 6% FS 5% FS % FS 2% FS 1% FS 5% FS 1% FS Reactor pressure: 163 mbar N 2 -Main flow: 5 slm N 2 flow trough TMI bubbler: 5sccm = 1% FS Absorption peak-maxima (cm -1 ) Maximum of absorption at λ = nm TMI absorption maxima Slope = 1.72 * 1-3 [cm -1 * %FS -1 ] N 2 flow through TMI bubler (%FS) Absorption strength as function of N 2 -flow through TMI bubbler in %FS wavelength (nm) Spectral resolved absorption on TMI diluted in N 2 -carrier gas as function of N 2 -flow through TMI bubbler in %FS. The total flow through the reactor is maintained at 5 slm at 163 mbar.

6 TMI absorption characterization Absoprtion maxima ( 1-3 cm -1 ) 1 1 Absorption maxima vs. TMI flow ratio χ TMI 21.7 nm Steady-state TMI flow condition α(χ) = exp( χ / ) [cm -1 ] TMI flow ratio χ (1-5 ) Calculate the number of TMI molecules per time unit as function of the observed absorption as N = 1 18 z -3 ( ln( α ) ) TMI with ln( α ) α = α [cm -1 ]

7 TMI flow characterization: Pulsed Injection 12 3%FS transmission (a.u.) 5.. 3%FS 5%FS 1%FS 15%FS absorption ( 1-2 cm -1 ) 8 25%FS 2%FS 15%FS 1%FS 2%FS 5%FS 25%FS 3%FS 3. 3%FS time (s) time (s) Transmission trace monitored at 21.7 nm during TMI precursor pulse injection in the reactor at 1.6 bar and a total flow through the reactor of 5slm. The TMI flow was varied from sccm (3-3%FS). The cycle sequence is 6 s with a.2 s width. Absorption traces for.2 sec TMI pulses injected 6 s apart

8 TMI flow characterization: Pulsed Injection absorption (1-2 cm -1 ) x = 2%FS; z = 5slm N TMI molecules per time unit TMI molecules per pulse Cycle repetition = 6 s; TMI flow x = 2%FS; λ m = 21.7 nm 15 slm 1 slm 5 slm N TMI / Pulse = 2 * time (sec) Absorption trace monitored at λ=21.7 nm during pulsed TMI injection of.2sec duration. The right axis depicts the computed concentration of TMI atoms (N TMI ) per time unit pressure (bar) Number of TMI molecules per pulse as function of flow and reactor pressure for a constant TMI flow x =2%FS.

9 TMI flow characterization: Pulsed Injection absorption ( 1-2 cm -1 ) bar 2.5 bar 5bar 7.5bar 12.2bar 1bar Three distinct features: a systematic shift in the pulse arrival time, a systematic TMI pulse broadening, an initial decrease of the absorption peak maximum with an increase as the pressure further increases time (s) Absorption traces monitored at 21.7 nm during TMI precursor pulse injection in the reactor at constant flow of 5slm. The reactor pressure was varied between 1 and 12 bar. The pulse cycle sequence is 6 s with.2 s TMI injection time.

10 TMI flow characterization: Pulse broadening 6 15 slm absorption ( 1-2 cm -1 ) slm 5 slm time (s) Pulse broadening as observed at the substrate center line as function of the reactor flow rates for a constant reactor pressure of 1 bar (λ = 21.7 nm).

11 TMI flow characterization: Absorption absoprtion pulse maxima ( 1-2 cm -1 ) Absorption peak maxima shift Cycle repetition rate: 6 s λ= 21.7 nm pressure (bar) Absorption maxima for various flows and reactor pressures. The pulse cycle sequence is 6 s with.2 s TMI injection time. 15 slm 1 slm 5 slm With increasing pressure, the amplitude decreases as expected by the pulse broadening effect. As the pressure further increases, the amplitude increases. An significant increase in the absorption cross-section for higher pressure may explain the observed amplitude increase. At present, no data on the absorption cross-section for TMI are known and further investigations are needed to clarify the observation.

12 References [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. 1-19, 2. [2] N. Dietz an V. Woods, unpublished results (2). refer to