1 Properties of GaN Films Grown by Atomic Layer Deposition Using Low-temperature III-nitride Interlayers J. R. Gong Department of Materials Science and Engineering Feng Chia University June 4, 2004

2 Co-workers C. L. Wang B. H. Shih Y. L. Tsai I. H. Chien W. T. Liao S. W. Lin

3 OUTLINE  Applications of III-nitrides  Fundamental aspects of ALD  LT-III-nitride interlayers — LT-GaN interlayer — LT-AlN interlayer — Ternary LT-AlGaN interlayer  Conclusions

4 Elemental and compound semiconductors Column IV: Si, Ge, SiGe, SiC Column III and V: GaAs, InP, InAs, InSb, GaN and alloys Column II and VI: ZnSe, CdS, HgTe and alloys

5 Semiconductor bandgaps UV-wide bandgap (GaN, ZnSe) IR-narrow bandgap (InSb, HgTe) Direct (mostly III-V): light emission possible  LEDs, Lasers Indirect (mostly Si): light emission forbidden  transistors, ICs

6 Bandgap engineering UV region

7 Research and development history of GaN

8  Direct band gap  The adjustability of band gap from 1.9eV (InN) to 6.2eV (AlN)  Good radiation hardness  High temperature resistance Advantages of III-nitrides

9 Applications of III-nitride devices  HBLEDs — traffic signal — full-color outdoor display — back light for LCD  LDs — DVDs  High Power Electronics

10 Markets for nitride-based LEDs

11 Reacting speed of LEDs is 20 times faster than traditional light bulbs. LED traffic signal

12 Outdoor full-color LED display

13 LCD backlight

14 LED car indicators

15 LED general lighting

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17 LED Chip substrate

18 Atomic Layer Deposition

19 Photographs of the home-made ALD growth system

20 R.F. Coil Quartz Exhaust Susceptor TMG NH N H Hydrogen Purifier Three-way Valve RegulatorValve Mass Flow Controller TMA A schematic diagram of the ALD system for the growth of III-nitride films

21 A schematic diagram of the rotating susceptor for ALD process

22 Fundamental aspect of atomic layer deposition (ALD)  An ideal ALE growth cycle produces a monolayer AB compound. (B)(A) AX (C) BY (D) AB (monolayer) AB(sub.) AX AB(sub.)

23 Influence of low temperature GaN intermediate layers on the properties of GaN films

24 A schematic structure of HT-GaN films without LT-GaN interlayer 150, 380, 600 nm  HT: 1000 ℃

25 (a)(c)(b) SEM micrographs of the surface morphologies of HT-GaN films grown on (0001) sapphire substrates 150 nm 380 nm 600 nm

26 Schematics of HT-GaN films inserted with LT-GaN interlayers (a)(b)(c)(d)  LT: 500 ℃ HT: 1000 ℃ 

27 (a)(b) (c)(d) SEM surface morphologies of HT-GaN films inserted with a LT-GaN interlayer 0 nm 20 nm 7 nm 70 nm

28 The role of LT-GaN interlayer on the growth of HT-GaN film The arrangement of Ga adatoms is merited by the suppression of surface kinetics at low growth temperatures, which is believed to stop the extension of mosaic structure from the underlying 150 nm-thick HT-GaN film during the growth of LT-GaN interlayer. A LT-GaN interlayer thickness deviated away from its optimised value was observed to deteriorate the quality of the subsequently grown HT-GaN film.  

29 RT PL spectra of HT-GaN films inserted with different LT-GaN interlayer thicknesses (The inset shows the effect of interlayer thickness on the PL emission energy)

30 (0002) DCXRD curve of a HT-GaN film inserted with a 20-nm-thick LT-GaN interlayer

31 Cross-sectional TEM image of a HT-GaN film inserted with a 20-nm-thick LT-GaN interlayer

32 A schematic structure of GaN films having various LT-GaN interlayer thicknesses sapphire AlN buffer LT-GaN HT-GaN 0.9  m HT-GaN 0.6  m 25Å<d<300Å

33 RT PL spectra of GaN films inserted with LT- GaN interlayers having different thicknesses

34 PL linewidth of GaN films inserted with LT-GaN interlayers having various thicknesses

35 Influence of low temperature AlN intermediate layers on the properties of GaN films

36 A schematic structure of GaN films having various LT-AlN interlayer thicknesses sapphire AlN buffer LT-AlN interlayer HT-GaN 0.9  m HT-GaN 0.6  m 25Å<d<125Å

37 RT PL spectra of GaN films inserted with AlN interlayers having different thicknesses

38 PL linewidth of GaN films inserted with LT-AlN interlayers having various thicknesses

39 Influence of low temperature AlGaN intermediate layers on the properties of GaN films

40 A schematic structure of GaN films having various LT-Al x Ga 1-x N interlayer thicknesses sapphire AlN buffer LT-Al x Ga 1-x N HT-GaN 0.9  m HT-GaN 0.6  m 25Å~200Å

41 RT PL spectra of GaN films having 2.5 nm-thick LT-AlGaN interlayers with different Al contents

42 RT PL spectra of GaN films having 5 nm-thick LT- AlGaN interlayers with different Al contents

43 RT PL spectra of GaN films having 7.5 nm-thick LT-AlGaN interlayers with different Al contents

44 RT PL spectra of GaN films having 10 nm-thick LT-AlGaN interlayers with different Al contents

45 PL linewidth of the GaN films versus the Al content of the 2.5 nm-thick LT-AlGaN interlayer

46 PL linewidth of the GaN films versus the Al content of the 5nm thick LT-AlGaN interlayer

47 PL linewidth of the GaN films versus the Al content of the 7.5nm thick LT-AlGaN interlayer

48 PL linewidth of the GaN films versus the Al content of the 10nm thick LT-AlGaN interlayer

49 RT PL spectra of GaN films inserted with different Al 0.6 Ga 0.4 N interlayers thicknesses

50 PL linewidth of GaN films inserted with LT- Al 0.6 Ga 0.4 N interlayers having various thicknesses

51 Conclusions  HT-GaN films inserted with LT-GaN interlayers having optimized thickness show improved surface morphology and enhanced near band-edge PL intensity when compared with that of a HT-GaN film without any LT-GaN interlayer.  The insertion of LT-GaN interlayers in HT-GaN films was found to reduce the compressive strain in HT-GaN films.

52 Conclusions  The insertion of a LT-Al x Ga 1-x N interlayer in a HT-GaN film was found to improve the optical properties of the film considerably when the thickness of interlayer is below a certain value.  It appears that the optimized interlayer thickness for the HT-GaN films having LT-Al x Ga 1-x N interlayers with a specific Al-content decreases as the Al composition in the interlayer increases.  The high Al-content LT-Al x Ga 1-x N interlayer was observed to block some of the threading dislocations (TDs) originated from the underlying GaN layer based on the studies of cross-sectional TEM.

53 Thanks for your patience!