GaN Nano- and MicroLED Technology

Transcription

1 Funded by the European Union - GA Kick-Off Meeting, Barcelona, Jan 17-18,

2 ChipScope project kick-off meeting GaN Nano- and MicroLED Technology Hutomo Suryo Wasisto and Andreas Waag Institute of Semiconductor Technology (IHT), Laboratory for Emerging Nanometrology (LENA), Technische Universität Braunschweig, Braunschweig, Germany Funded by the European Union - GA Kick-Off Meeting, Barcelona, Jan 17-18,

3 Braunschweig University of Technology Technische Universität Braunschweig (TUBS) 1745 founded as Collegium Carolinum 1 University 6 Faculties 120 Institutes Scientists Students (BS: citizens) 65 Mio. Euro External Research Funding 250 Mio. Euro Overall Budget largest portfolio of engineering BA/MA programs in northern Germany member of : Andreas Waag - Braunschweig University of Technology page 3

4 Institut für Halbleitertechnik Dept. Electrical Engineering Junior Reserach Group OptoSense (Wasisto) (Fündling) Andreas Waag - Braunschweig University of Technology page 4 headcount ca. 50

5 Epitaxy Competence TUBS in collaboration with GaN Technology / Foundry Services. Andreas Waag - Braunschweig University of Technology page 5

6 Epitaxy Competence Center - Infrastructure 2 cleanrooms each with ~120 m² labspace Infrastructure: Epitaxy Analysis - Processing ALD, 3xMOCVDs, ICP etching, Photolithography, Nanoimprint, FESEM, CL, PL, AFM, Sputter systems, PVD Sentech dry etcher for Si and nitrides Tescan Mira with Gatan MonoCL AIXTRON G3 2600HT Thomas Swan 3x2 FT soon: Laser-Lift-Off, FIB-SEM, SIMS, TEM Andreas Waag - Braunschweig University of Technology page 6

7 GRK Laboratory for Emerging NAnometrology our mission: precise measurement at the nanoscale Speaker: Waag

8 GaN LEDs: The basis of Solid State Lighting Century: Candle light bulb LED Efficiency: <1% 4-5% 40-50% Andreas Waag - Braunschweig University of Technology page 8

9 Principle of light production in a LED current in Electrons LED conduction band Energy light valence band x current out Electronic transition between 2 energy levels produces photons The energy difference determines the color of the LED Andreas Waag - Braunschweig University of Technology page 9

10 Principle of light production in a LED electron current in Electrons LED conduction band Energy band gap n-type depletin region p-type valence band x hole current in Electronic transition between 2 energy levels produces photons The energy difference determines the color of the LED Andreas Waag - Braunschweig University of Technology page 10

11 Electronic bands in a high power InGaN/GaN LEDs InGaN/GaN/AlGaN LED with electron blocking layer (EBL) top: flat band conditions (during current flow) bottom: including band bending due to doping Typical ingredents: p-and n-doped (Al)GaN confinement layers multiple quantum wells electron blocking layer p-type GaN always on top (low quality)! total width about 1 µm Andreas Waag - Braunschweig University of Technology page 11

12 The dimensions in case of InGaN/GaN LEDs always p-type on-top 1 µm Top 10 µm = GaN buffer + LED structure substrate thickness = µm sapphire wafer dimensions = 2,4,6 inch Andreas Waag - Braunschweig University of Technology page 12

13 White GaN-LED LED emission white = yellow + blue yellow emitting phosphor emission of the phosphor blue emitting LED chip Quelle: OSRAM OS 2003 Andreas Waag - Braunschweig University of Technology page 13

14 Pacakging of LEDs LED = Light Emitting Diode SMD = surface mounted device Through Hole Mounting Andreas Waag - Braunschweig University of Technology page 14

15 Materials for optoelectronics : lattice matching necessary band gap [ev] wavelength /nm) lattice constant [A] Bergbauer, Strassburg et al, Physik-Journal 2011 Andreas Waag - Braunschweig University of Technology page 15

16 The Green Gap : internal qauntum efficiency in th egreen spectral range, the efficiency of both nitride and phosphide LEDs suffers: no high efficiency green LEDs / laser diodes (might change in the next future) Andreas Waag - Braunschweig University of Technology page 16

17 Metalorganic Chemical Vapor Phase Epitaxy (MOVPE) 60 wafers, 2 inch diameter 400 LEDs per wafer LEDs per growth run per MOVPE Andreas Waag - Braunschweig University of Technology page 17

18 MOCVD von AlGaN: the concept of MO chemistry Nakamura 1991 P= mbar TMGa = Trimethyl-Gallium TMAl = Aluminum TMIn = Indium Andreas Waag - Braunschweig University of Technology page 18 high temperatures lead to convection Andreas Waag Lichttechnik 7 Seite 18

19 Moore s Law in Solid State Lighting Semiconductor Technology microelectronics solid state lighting smaller transistors faster transistors higher efficiency higher currents larger light emitting area more functionality per chip more logic units from better efficiency since efficiency is almost in saturation, we need to change our perspective: more optical power per chip more photons to smart production / cost efficiency Andreas Waag - Braunschweig University of Technology page 19

20 Moores Law in Solid State Lighting [Haitz, Tsao, Phys. Status Solidi A 208(2011)1] GaN since efficiency is almost in saturation, we need to change our approach: from better efficiency to smart production / cost efficiency Andreas Waag - Braunschweig University of Technology page 20

21 DROOP DROOP = reduction of efficiency with increasing current density B.Hahn (OSRAM OS) DOE Workshop 2016 low current densities increase efficiency, but also increase cost per photon Andreas Waag - Braunschweig University of Technology page 21 Quelle: OSRAM OS

22 The nanorod approach Core-shell geometry: increasing the light emitting area much larger light emitting area per footprint reduced cost per photon down-scaling of wafer cost, processing cost, packaging cost etc. etc. only if everything works out OK (growth, IQE, outcoupling, processing) Andreas Waag - Braunschweig University of Technology page 22

23 A quasi GaN substrate : superior properties of 3D semiconductor material high aspect ratio / small footprint high crystallinity / zero defects fast growth rate: 50 µm/h high surface-to-volume ratio, large increase of active area vertical architecture possible multi-dimensional platform: LEDs, sensing, power electronics no strain even when grown on mismatched substrates (like on silicon) Andreas Waag - Braunschweig University of Technology page 23 3D GaN (TUBS)

24 S.F.Li, AW et al The nanorod approach nanorods and nanowires are building blocks for nanoscale electronics ZnMgO ZnO well ZnMgO d layer ZnO d nanopillar d well E v E c substrate Andreas Waag - Braunschweig University of Technology page 24

25 Selective Area Growth of GaN NanoLEDs Si 3 N 4 and SiO 2 can be used to achieve good growth selectivity Andreas Waag - Braunschweig University of Technology page µm

26 Photolithography : lateral patterning surfaces must be very flat, when very small structures are to be realised TUBS: > 500 nm possible > 1µm stable Andreas Waag - Braunschweig University of Technology page 26

27 Nanoimprint - Lithography Embossing of a stamp into a polymer Polymerisation by UV irradiation stamp needs to be transparent (better than thermal nanoimprint) flexible polymer stamp (soft-stamp) can deal with non-flat surfaces TUBS: >100 nm Andreas Waag - Braunschweig University of Technology page 27 27

28 Silicon Wafer with Nanorod Fields photolitho or nanoimprint D = 400 nm 2 µm varying pitch deep etched silicon soft mask for nanoimprint Andreas Waag - Braunschweig University of Technology page 28

29 Deep etching by plasma chemistry ICP dry etching (flourine based) for nitrides, oxides and silicon based 3D structures Andreas Waag - Braunschweig University of Technology page 29

30 3D GaN vertical architecture Homogenous growth of Ga-polar GaN columns Ga-polar GaN columns on patterned SiOx/GaN/sapphire Andreas Waag - Braunschweig University of Technology page 30

31 3D GaN vertical architecture Alternative combined top-down approach Cr GaN Sapphire For channel I. Film growth II. Cr mask III. ICP dry etching (SF 6, H 2 ) IV. Wet etching (KOH) Alternative combined top-down approach ICP dry etching and wet chemical etching Results relatively smooth a-plane sidewalls, easy reproducible adjustment of NW diameter, more defined doping control Andreas Waag - Braunschweig University of Technology page 31

32 3D GaN vertical architecture Nanowire size optimization ICP 20 min 1 h 2 h 4 h 6 h 90 C Wet etching optimization temperature, etch time, etch solution Etch rate nm/h Very high aspect ratio up to > 60 Small size diameter < 50 nm Andreas Waag - Braunschweig University of Technology page 32

33 How to analyse GaN based microrods? Andreas Waag - Braunschweig University of Technology page 33

34 FE-SEM Tescan Mira3 GMH Detectors: ET-SE, In-Beam SE, low-kv BSE, EBAC/EBIC, beam rocking, probe 200 n Gatan MonoCL4 with custom designed mirror Simultaneous detection of up to four signals (dedicated ADCs) for imaging. Special parabolic mirror with a notch for investigation of wafers up to 4 at tilt 30. Improved spatial resolution for characterization by BSE, EBIC, CL and EL (via Manipulators) Nondestructive electro-optical investigation of 3D structures on whole wafers Andreas Waag - Braunschweig University of Technology page 34 [J. Ledig et al., Nondestructive inspection of 4 ' wafers in bird's eye view by an FE-SEM imaging & microscopy, vol. 18, no. 2, 2016.]

35 Contacting inside an ensemble of core-shell LEDs electro-optical characterization SE and EBIC image visualizing the light emitting region of the center structure contacted by a tungsten probe tip. CL excitation mapping and spectra from excitation along the sidewall. Andreas Waag - Braunschweig University of Technology page 35 [J. Ledig et al., Nondestructive inspection of 4 ' wafers in bird's eye view by an FE-SEM imaging & microscopy, vol. 18, no. 2, 2016.]

36 STEM/CL of GaN:Si microrod: influence of defects on quantum well Low High Optimisation of shell growth: PL-IQE ~ 60% Andreas Waag - Braunschweig University of Technology page 36 CL: M.Müller, F.Bertram, J.Christen

37 Processing of Core-Shell GaN blue-emitting microrod LEDs Andreas Waag - Braunschweig University of Technology page 37 Schimpke, AW, Lugauer, Strassburg, et al, phys.stat.sol.a(2016)

38 Towards Nanorod and Microrod LEDs Processes for 3D LED Chip Fabrication M. Strassburg, M. Mandl, T. Schimpke, T. Hero, D. Scholz, C. Kölper, M.Sabathil OSRAM Opto Semiconductors GmbH 440 nm 3D & Core-shell AlOx and BCB deposition BCB etch AlOx etch ITO deposition (p-contact) and metal contact preparation Mesa definition (exposure of n-gan cores for n-contact) Pad metallization Andreas Waag - Braunschweig University of Technology page

39 White light generation by phosphor conversion using close coupled phosphor Improved light extraction from LED die and thermal control of converter Significant reduction of grain size compared to conventional converter material without reduction in efficiency is required Deposition of µ-grain phosphor between high-aspect ratio microrods Blue microrod LED chips Standard phosphor GaN x Active Area Phosphor 2 µm 4 µm Schimpke, AW, Lugauer, Strassburg, et al, phys.stat.sol.a(2016) Andreas Waag - Braunschweig University of Technology page 39 EU-FP7

40 White emission from 3D-LEDs by conversion Chip-processing Process steps adopted to the array Blue emission from processed 3D-LEDs White emitting LED 3D-geometry enables new concepts: micro grained converter inside the gaps of the 3D-LED column array forward scattering improved cooling of the converter First demonstration of a converted white emission based on core-shell LEDs micro grained transparent converter conductive oxide isolation layer n-contact metal 3D-LED n-doped GaN-contact layer sapphire substrat p-contact metal Leuchtstoff Andreas Waag - Braunschweig University of Technology page 40 Schimpke, AW, Lugauer, Strassburg, et al, phys.stat.sol.a(2016)

41 Towards Nanorod and Microrod LEDs Bright emission from Core-shell microrod LED Full-4inch wafer processing 4 Andreas Waag - Braunschweig University of Technology page 41 I = 5mA

42 Laser-Lift-Off: removal of GaN LED thin film from the saphire substrate An Excimer-Laser destroys the interface between GaN and saphire substrate. GaN LED can then be transferd to another carrier (e.g. a glass or silicon carrier) Andreas Waag - Braunschweig University of Technology page 42

43 Summary: nanoleds for solid state lighting 3D approach could be of more general importance to GaN technology Core-shell LEDs are a smart way to go beyond scaling laws of conventional planar technology. Gan FIN technology could be interesting: LEDs, vertical electronics, sensing Andreas Waag - Braunschweig University of Technology page 43