Optical Constants of Ge and GeO 2 from Ellipsometry

Transcription

1 Optical Constants of Ge and GeO 2 from Ellipsometry T. Nathan Nunley, Nalin Fernando, Jaime Moya, Nuwanjula S. Samarasingha, Cayla M. Nelson, Stefan Zollner Department of Physics, New Mexico State University, Las Cruces, NM, USA Wo ist Las Cruces? 216 DPG Frühjahrstagung Regensburg, 9. März 216, HL 59.1 NIR/VIS/QUV ellipsometry: 19 to 25 nm, 77 to 8 K NSF: DMR AFOSR: FA

2 Graduate Students: Lina Abdallah, Travis Willett-Gies, Nalin Fernando, Tarek Tawalbeh (Theory), Nuwanjula Samarasingha, Nathan Nunley Undergraduate Students: Cesar Rodriguez, Khadijih Mitchell, Cayla Nelson, Jaime Moya, Jackie Cooke Flat & uniform films, at least 5 by 5 mm 2, low surface roughness, films on single-side polished substrate zollner@nmsu.edu New Mexico State University 2

3 Biography Regensburg Germany Motorola (Mesa, Tempe) Arizona, Freescale, IBM New York, 91-92;7-1 Motorola, Freescale Texas, Las Cruces, NM Since 21 New Mexico State University 3

4 5 SiGe:C Metrology: How thick is my film? Si 1-x Ge x alloys Si 1-x Ge x alloys Si 9.6% 14.93% 21.1% 26.86% Si 1-x Ge x Si 1-x Ge x alloys Energy (ev) Spectroscopic Ellipsometry Log Intensity (a.u.) theta (seconds) 1 thickness measurements Si x=9.6% x=14.93% Need precise values of.25 refractive index 21.1% 26.86%.2 Ge atomic fraction Si cap (emitter) Energy (ev) SZ, Hildreth, Liu, Zaumseil, Weidner, Tillack, J. Appl. Phys. 88, 412 (2) simulation data High-resolution XRD SiGe:C base Si substrate Depth (a.u.) 4

5 Why Germanium? First transistor built with Ge. High frequency applications (bipolar). Excellent infrared photodetector. Recent interest: PMOS channel material. Training students in semiconductor physics Why not? SciFi CMOS cartoon: DPG Frühjahrstagung Regensburg, 9. März 216, HL 59.1

6 Multi-Sample Analysis Single sample: Ellipsometry of one GeO 2 /Ge sample Unknown Ge optical constants Unknown GeO 2 (native oxide) optical constants and thickness This problem is under-determined (not enough data). Multi-sample analysis: Grow thermal oxides on Ge with different thicknesses. All oxides identical; only thickness varies between samples. Fit all data simultaneously (over-determined). Ellipsometry measurements : J.A. Woollam VASE ellipsometer with Berek compensator..5 to 6.6 ev (with halogen lamp). 6º-75º angle of incidence. Fit with parametric oscillator model DPG Frühjahrstagung Regensburg, 9. März 216, HL 59.1 QTH lamp

7 Substrate Cleaning and Thermal Oxidation Substrate cleaning: Remove most of the native oxide. Leave stable (but thin) native oxide. No harsh chemicals (BHF, Br:Meth). UV ozone clean at 15ºC for 1 hour, followed by cool-down incubation. Ultrasonic clean in DI water followed by isopropanol (2 min each). Thermal oxidation: 2 by 2 mm 2 undoped Ge pieces. Single-side polished. Anneal in O 2 (2.7 atm) at 55ºC to avoid oxide (GeO) desorption. 1-1 hours; 35 to 13 nm thick. Some spots, but OK. GeO 2 unstable and water-soluble, measure soon. ULVAC MILA-5 RTA Hg lamp Novascan PSD DPG Frühjahrstagung Regensburg, 9. März 216, HL 59.1 GeO 2 on Ge

8 Deal-Grove Oxidation Model d 2 Ad B t B.E. Deal and A.S. Grove, J. Appl. Phys. 36, 377 (1965) DPG Frühjahrstagung Regensburg, 9. März 216, HL 59.1

9 X-ray reflectance of typical sample (55ºC, 1 h, 33 nm) Ge SrTiO GeO 2 3 GeO Ge GeO 2 GeO 2 Ge (Å) GeO 2 density r: 9% of bulk Lower r near surface Higher r near substrate Thickness: 33 nm Roughness:.5 nm (2%, neglect) Layer Electron Density (eå -3 ) Bulk Electron Density (eå -3 ) Thickness (nm) Roughness (nm) GeO GeO GeO Ge Substrate DPG Frühjahrstagung Regensburg, 9. März 216, HL 59.1

10 Ge wafer with native oxide (2.3 nm) Measurement right after cleaning, excellent fit to data. Herzinger-Johs parametric oscillator model for Ge < 1 > < 2 > Photon Energy (ev) Jellison-Sales method for transparent glasses: D at 75º below band gap determines oxide thickness (2.3 nm) DPG Frühjahrstagung Regensburg, 9. März 216, HL 59.1

11 Ge wafer with 34 nm thermal oxide (one hour) in degrees D in degrees %Depolarization Photon Energy (ev) Photon Energy (ev) Cleaned, then oxidized for one hour at 55ºC, 33 nm by XRR. 5% non-uniformity. Poor fit in deep UV (above 6 ev) DPG Frühjahrstagung Regensburg, 9. März 216, HL 59.1

12 1 Ge wafer with 89 nm thermal oxide (5 hours) in degrees D in degrees %Depolarization Photon Energy (ev) Photon Energy (ev) Cleaned, then oxidized for five hours at 55ºC. 7% non-uniformity. Poor fit in deep UV (above 6 ev) DPG Frühjahrstagung Regensburg, 9. März 216, HL 59.1

13 1 Ge wafer with 136 nm thermal oxide (1 hours) in degrees D in degrees %Depolarization Photon Energy (ev) Cleaned, then oxidized for ten hours at 55ºC. 1% non-uniformity. 4 nm spectral bandwidth. Poor fit in deep UV (above 6 ev) Photon Energy (ev) DPG Frühjahrstagung Regensburg, 9. März 216, HL 59.1

14 Preliminary optical constants for Ge and GeO 2 Determined using multi-sample analysis (2, 34, 52, 89, 136 nm) NMSU NMSU UNC-CH Similar to Jellison/UNL data Higher amplitude than Aspnes Broader spectral range (.5 to 6.6 ev) Bigger differences in UV. Much broader spectral range. Absorption begins at 6.5 ev. Tauc-Lorentz oscillator fit DPG Frühjahrstagung Regensburg, 9. März 216, HL 59.1

15 Remaining issues Depolarization of reflected light Thickness non-uniformity (2 mm sample size) Insufficient monochromator resolution (4 nm) Does the density vary between samples? Need to improve XRR fits for some samples. Is there a density gradient in the oxide? Do we need to consider surface roughness (no AFM yet)? Is there an interfacial layer at the GeO 2 /Ge interface? Is a simple Tauc-Lorentz oscillator sufficient for GeO 2? Need FTIR-SE to study phonons DPG Frühjahrstagung Regensburg, 9. März 216, HL 59.1

16 Summary Optical constants for Ge and GeO 2 Developed UV-ozone clean for thermal oxidation of Ge. Performed thermal oxidation of Ge at 55ºC for 1 to 1 hours. Multi-sample ellipsometry fit of ellipsometric angles. Dielectric function of Ge and GeO 2 from.5 to 6.5 ev. Ge GeO 2 NSF: DMR AFOSR: FA New Mexico State University