Defects in Nitride Lasers

Transcription

1 Defects in Nitride Lasers Julita Smalc-Koziorowska Institute of High Pressure Physics UNIPRESS, PAS, Sokolowska 29/37, Warsaw, Poland TopGaN Ltd., Sokolowska 29/37, Warsaw, Poland Thessaloniki

2 Outline Defects in Nitride Lasers Blue laser chip Nitride Semiconductors possible applications. Nitride Lasers in Poland - TopGaN Ltd. Possible defects in epitaxial layers. Defects in laser diodes everyday life. Elimination of defects in laser diodes. Laser packages Thessaloniki

3 Nitride semiconductors possible applications -direct wide band gap semiconductors GaN 3.4 ev InN 0.7 ev AlN 6.2 ev Optoelectronic applications white LED diodes Decorative lighting Indoor lighting wurtzite structure Automotive lighting Thessaloniki

4 Specifc on-resistance Ron (mω cm²) Nitride semiconductors possible applications Electronic devices Next generation power devices Power distribution in hybrid electric cars GaN field-effect transistors 1000 Si Limit SiC Limit GaN Limit Nitride based photovoltaic cells 0, Breakdown Voltage (V) Thessaloniki

5 Nitride Semiconductors Laser Diode applications RGB systems Mobile mini projectors Laser TV Car Headlights Movie projectors Thessaloniki

6 Nitride Laser Diodes Laser diodes based on new group of semiconductors (gallium nitride) are able to emit the light in the UV and visible range. As for present, lasing was achieved by various groups and companies in the range from 370 up to 530 nm. 1. UV-LDs operating in the range from 370 up to 400 nm, 2. Violet LDs operating at the range of nm. Here the specially important wavelength is 405 nm, know from DVD standard BluRay, 3. Blue-violet range nm, 4. True blue range nm. This wavelength is usually the wavelength of choice for a blue component of laser projectors, 5. Blue green range nm. This region is interesting (between the others) replacement of argon ion lasers, 6. Green region nm. These devices play a crucial role in the laser diode displays (green component). Thessaloniki

7 Nitride Lasers in Poland TopGaN Ltd was registered in 2001 to commercialize the Blue Laser technology developed by Institute of High Pressure Physics (UNIPRESS) second in Europe demonstration of UV laser diode 2005 first InGaN laser diodes grown by PAMBE method 2010 demonstration of 2.5 W continous work operated laser diodes mini arrays 2012 UV laser diode array emitting 0.5 W optical power TopGaN Ltd. Prof. Sylwester Porowski GaN crystal Thessaloniki

8 Epitaxy of nitride structures Unipress and TopGaN Ltd. Molecular Beam Epitaxy 2 MBE reactors Metal-organic vapour phase epitaxy 4 MOVPE reactors Hydride Vapour Phase Epitaxy 3 HVPE reactors Thessaloniki

9 Devices processing - Unipress and TopGaN Ltd. Metal bonds to laser chip Metal sputtering Laser chips bonding Laser photolitography LD stripe with metallization Thessaloniki

10 Characterization of epitaxial layers and devices TEM FEI Tecnai G2 F20 with HAADF STEM detector SEM Hitachi SU 70 with cathodoluminescence spectometer Thessaloniki

11 Characterization of epitaxial layers and devices Photo and electroluminescence characterization, electrical tests, XRD measurments Electroluminecence test of LD chip Electroluminecence test of epitaxial layer Thessaloniki

12 Areas of activities Unipress and TopGaN Ltd. Superluminescent diodes Epitaxial structures Bulk GaN substrates Freestanding HVPE GaN substrates 3 stripe mini-arrays large laser arrays Laser diodes Thessaloniki

13 Laser Diodes production a long way to get efficient and long term lasing 3 LD stripe mini-arrays o Thessaloniki

14 Possible structural defects in laser diodes Defects caused by lattice mismatch -dislocations, -stacking faults, -cracks, Defects caused by applied growth conditions: -Mg-piramids, Mg- related inversion domains, -V-pits, Trench defects, --InGaN QW thickness variations -stacking faults loops, -voids and In inclusions in InGaN quantum wells. Thessaloniki

15 What can we see in TEM? Specimen preparation volume of the material investigated in TEM. Cutting of stripes of two in-plane orientations 2mm [1120] [1100] Epitaxial layers [1120] [1100] 500 mm Two pieces of sample of various orientations glued together Thessaloniki

16 Specimen preparation Epitaxial layers Mechanical polishing on dimond lapping films down to 20 mm thickness using tripod Ion milling to 0 nm thickness nm thickness transparent for electrons at 200kV accelaration voltage 500 mm 20 mm 20 mm Area of epitaxial structure visible in TEM up to 2 mm 2. In order to observe defects in TEM typically their density must be higher than 1 x10 6 cm -2 If some defects are visible in TEM specimen in both pieces it means that they are present in the whole sample. Thessaloniki

17 Mismatch defects misfit disloactions misfit dislocation Misfit edge disloaction at the interface between sapphire and GaN Threading disloaction propagating from the interaface to the epilayer surface Net of misfit dislocations in InGaN layer plan view TEM Thessaloniki

18 Design of coherent epitaxial structures The critical thickness for introduction of misfit dislocations is relatively large for polar (0001) layers, since the introduction of dislocations on main easy glide planes is not active. Thessaloniki

19 Dislocations in AlGaN layers in laser structures Relaxation of thick AlGaN layers Thessaloniki

20 Avoiding cracking in AlGaN by lateral patterning AlGaN on unpatterned GaN AlGaN on laterally patterned GaN Developed by Marcin Sarzynski, patented by TopGaN/Unipress Thessaloniki

21 Nonpolar vs polar structures The quantum confined Stark effect The polar InGaN quantum well compressively strained presence of internal piezoelectric fields. A tilt of the energy band causes the spatial separation between electrons and holes and the reduction of the effective band gap. Thessaloniki

22 (20-21) semipolar plane A. E. Romanov et al. J. Appl. Phys. 100, (2006) "Strain-induced polarization in wurtzite IIInitride semipolar layers" (20-21) Thessaloniki

23 Semipolar (20-21) laser diodes misfit dislocations APL100, (2012) Relaxation through introduction of misfit dislocations on easy glide planes of wurtzite structure Thessaloniki

24 Dislocation propagation suppresion in semipolar epitaxy QB InGaN, 3% In QW InGaN 23%, GaN (20-21) Si 3 N 4 stripes, period 300 mm and 50 mm QB InGaN, 3% In InGaN, 10% In SLS: AlGaN/GaN Ammono GaN (20-21) M.T. Hardy et al. APL 101, (2012) Łucja Marona Thessaloniki

25 Dislocation propagation suppresion in (20-21) epitaxy Cathodoluminescence intensity maps QWs: 535nm Łucja Marona Height (µm) 0 12 Intensity µm Width (µm) Height (µm) 0 30 Intensity µm 5 Thessaloniki Width (µm)

26 Defects due to growth conditions Doping related defects occurence of Mg pyramids and Mg-doping related inversion domains. Mg pyramid Thessaloniki

27 Defects due to growth conditions Impurities nanotubes Nanopipe in MOCVD GaN Voids in MBE InGaN QWs due to oxygen contamination coming from the substrate E.Jezierska et al. Eur. Phys. J. Appl. Phys. 27, (2004) Thessaloniki

28 V-pits opening of threading dislocations STEM [11-20] STEM [11-20] M. Shiojiri et al. JAP 99, (2006) Reasons for formation of V-pits - Low metal atoms mobility on the growing surface causing gathering of metal atoms at the defects like dislocations,which hinders the further growth along [0001] direction and exposes {1011} planes. M. Shiojiri et al. JAP 99, (2006) Strain relaxation - T.L. Song JAP however we also observe formation of V-pits in pure GaN. TD Thessaloniki

29 V-pits opening of threading dislocations Ax1221 Ax1019 Threading dislocation continous through the V-pit to the structure surface. The V-pit is overgrown during further growth at higher temperature. The formation of V-pits depends on the growth conditions. Thessaloniki

30 V-pits opening of threading dislocations How to eliminate V-pits: Growth with H2 hydrogen increases the mobility of atoms on the growing surface and prevents gatering of metal atoms on defect sides QB 730 o C + H 2 QB 730 o C no H 2 QB 730 o C H 2 QB 730 o C no H 2 Thessaloniki

31 V-pits opening of threading dislocations How to eliminate V-pits: Growth at higher temperature the higher temperature the higher mobility of atoms on the growing surface Thessaloniki

32 Quantum well-width fluctuations N. van der Laak, APL_90_121911_(2007) QB 880 o C Cross-section STEM-HAADF image of a commercial green LED showing gross thickness variations arrowed in all four InGaN QWs. Quantum well-width fluctuations could be due to etching of QW during high temeprature growth of quantum barriers, they are also observed in teh growth of QB with hydrogen. Thessaloniki

33 Trench defects and V-pits Ax1160 TRENCH DEFECT Area surrounded by V- shaped trench enclosing a material with different emission properties from the surrounding area. Threading dislocation V-PIT Opening of the threading dislocations into inverted hexagonal pyramid enclosed by {1011} planes. AFM image of the surface of InGaN/GaN QW structure Thessaloniki

34 Structure and properties of trench defects Basal Stacking Fault terminated by Stacking Mismatch Boundary SMB Thessaloniki

35 Structure and properties of trench defects Trench defect - Basal Stacking Fault terminated by Stacking Mismatch Boundary F.C.-P. Massabuau et al. Appl. Phys. Lett. 101, (2012) SMB I1 BSF Thessaloniki

36 Formation of Basal Stacking Faults in GaN quantum barriers HR STEM image [1120] Thessaloniki

37 Elimination of trench defects and V-pits in InGaN/GaN structures QW 23% In QW 21% In QW19,5% In 730 o C 830 o C 880 o C Growth of GaN QBs at high temperatures leads to the reduction of the number of trench defects and opening of threading dislocations into V-pits. Thessaloniki

38 Thermal degradation of InGaN QWs Ax1019 Thessaloniki

39 Thermal degradation of InGaN QWs Decoherent decomposition of InGaN 1.48nm Sławek Kret Thessaloniki

40 Thermal degradation of InGaN QWs Li et al. Appl. Phys. Lett. 103, (2013) Micro-PL images of (a) green LD-I sample Cross-section Z-contrast STEM of LD-I (a) non-degraded region and (b) degraded region. Uniform QWs can be observed in (a), but only broken QW layers as well as precipitates and voids are detected in (b). (c) A magnification of the precipitate and void region in (b). The long dashed arrows in (a) and (b) show the direction of [0001]. Thessaloniki

41 Thermal degradation of InGaN QWs spinodal decomposition G.B. Stringfellow / Journal of Crystal Growth 312 (2010) For relaxed zincblende InGaN layers Thessaloniki

42 Thermal degradation of InGaN QWs spinodal decomposition of strained layers For strained wurtzite InGaN layers C. TESSAREK et al.physi.rev. B 83, (2011) Spinodal (black) and binodal (gray) lines calculated for different strain states of the InGaN layer on a GaN substrate. A strain state of 100% corresponds to pseudomorphicmaterial grown on GaN, whereas 0% means completely relaxed material. With increasing strain in the InGaN layer, the critical temperature decreases and shifts toward higher x. Nevertheless, a miscibility gap exists for growth temperatures below 713 C. Thessaloniki

43 Thermal degradation of InGaN QWs Ah1019 We observe often that only one QW is destroyed Thessaloniki

44 Defects due to growth conditions-sf loops The slow atoms that cannot sample the surface bonding sites will bond on the sites where they first arrive. SFs can thus form as a result of growth accidents that result from them bonding to Type C sites, due to low surface mobility of atoms on (0001) growing surface. Since there is no shift on the (0002) planes casued by the formation of the first BSF, possibly the formation of the second BSF is needed to preserve the correct stacking of this area with the surrounding crystal. F.Y. Meng, M. Rao, N. Newman, R. Carpenter and S. Mahajan, Acta Materialia 56, (2008) Thessaloniki

45 Defects due to growth conditions-sf loops in AlInGaN layers Thessaloniki

46 Defects due to growth conditions-sf loops P1699, AlInGaN EBL, epd 10 8 P1966, AlGaN EBL, epd 10 6 Reduction of the numer of threading defects due to eleimination of In in EBL and cladding AlGaN layers Thessaloniki

47 Acknowledgements Ewa Grzanka, Tadeusz Suski, Michał Leszczyński, Piotr Perlin Robert Czernecki, Łucja Marona, Marcin Sarzyński, Dario Schavion - MOCVD Henryk Turski, Grzegorz Muzioł, Marcin Siekacz, Czesław Skierbiszewski - MBE George Dimitrakopulos - AUTH Program BRIDGE: Elimination of structural defects in nitride semiconductor layers (InGaN and InAlGaN) used as active layers in semiconductor lasers Thessaloniki