Quantum Well and Quantum Dot Intermixing for Optoelectronic Device Integration

Transcription

1 Quantum Well and Quantum Dot Intermixing for Optoelectronic Device Integration Chennupati Jagadish Australian National University Research School of Physical Sciences and Engineering, Canberra, ACT 0200 AUSTRALIA

2 Overview Introduction Methods of Intermixing Quantum Wells Ion Implantation Induced Interdiffusion GaAs/AlGaAs, InGaAs/AlGaAs, InP/InGaAs QWs Lasers, Photodetectors Impurity Free Interdiffusion GaAs/AlGaAs, InGaAs/AlGaAs, InGaAsN/GaAs QWs Integrated Waveguide-Laser Quantum Dots Suppression of Interdiffusion Implantation Induced Interdiffusion Summary

3 The All-Optical Network Long-Haul (Terrestrial & Submarine) Metro Core All optical routing MEMsswitches Optical mux/demux Tunable lasers and filters Low cost amplifiers (EDFA, EDWA, discrete Raman, SOA) Coarse WDM All optical amplification Improved EDFAs High power pump laser Raman modules Metro Access Customer Premises Integrating fast electronics >40Gb/s with optics Planar & Integrated optics Long wave VCSELs Low cost amplifiers with pump lasers Low cost transceivers

4 Photonic Integrated Circuits / Optoelectronic Integrated Circuits Integrated Circuits Show Superior Performance Over Discrete Devices Multi-functional circuits, e.g. WDM sources Integrated Transceivers Low Cost, Packaging

5 Photonic Integrated Circuits Different Bandgaps on the same chip

6 WDM Source Optical output (to optical fibre) optical amplifier passive optical waveguides multi-wavelength laser diodes dielectric passivation implant isolation

7 Quantum Well Intermixing before after Diffusion of In and Ga across interface creates graded region in the case of GaAs/InGaAs Quantum Wells Changes Bandgap, refractive index, absorption Coefficient

8 Methods Widely Used for Quantum Well (Dot?) Intermixing Impurity Induced Disordering, e.g. Zn, Si Impurity Free Interdiffusion, e.g. SiO 2, SOG Ion Implantation Induced Interdiffusion Defects introduced by these methods enhance atomic interdiffusion Goals: High Selectivity and Low Concentration of Residual Defects while achieving large band gap differences

9 Why Ion Implantation? Widely used in Microelectronics Industry Defect Concentration - Ion Dose, Ion Mass, Implant Temperature, Dose Rate Defect Depth - Ion Energy Selective Ion Implantation using Masks

10 Ion implantation induced quantum well intermixing Ion implantation Point defects: Vacancy Interstitial Intermixing

11 Schematic of 4 QW structure (40 kev Proton Defect Profile) QW1 =1.4 nm QW2=2.3 nm QW3=4.0 nm QW4=8.5 nm

12 10K Photoluminescence Spectra H.H. Tan et.al., Appl. Phys. Lett. 68, 2401 (1996).

13 Energy Shifts vs. Proton Dose 900 o C, 30 sec QW1 =1.4 nm QW2=2.3 nm QW3=4.0 nm QW4=8.5 nm H.H. Tan et.al., Appl. Phys. Lett. 68, 2401 (1996).

14 Thermal Stability of InP/InGaAs QWs with InP and InGaAs 0.90 InGaAs/InP QW, InP cap InGaAs/InP QW, InGaAs cap InGaAs 50nm Peak Energy (ev) InP 250nm InP buffer InGaAs QW InP 200nm InP buffer Annealing Temperature ( o C)

15 Implantation Dose (20 kev P) and Temperature Dependence of Energy Shifts in InP/InGaAs QWs 700 o C, 60 sec Energy Shift (mev) InP capped 25 o C 200 o C Energy Shift (mev) InGaAs capped 25 o C 200 o C Dose (cm -2 ) Dose (cm -2 ) C. Carmody et. al., J. Appl. Phys. 93, 4468 (2003)

16 Damage Accumulation in InP and InGaAs InP Cap InGaAs Cap Normalised Yield 3x10 3 2x10 3 1x10 3 random InP cap unimplanted 1 x cm -2 implanted at room temperature 1 x cm -2 implanted at 200 o C Normalised Yield 3x10 3 2x10 3 1x10 3 random InGaAs cap unimplanted 1 x cm -2 implanted at room temperature and 200 o C Channel Channel

17 Tuning the Emission Wavelength of GRINSCH Quantum Well Lasers GaAs/AlGaAs QW Lasers

18 Tuning the wavelength of QW lasers p-type contact un-implanted dose A dose B Oxide isolation QW Substrate n-type contact dose A < dose B λ 1 λ 2 λ 3 λ1 > λ2 > λ3

19 GRINSCH QW Laser and 220 kev Proton Defect Profile

20 Lasing Spectra and L-I Characteristics of GaAs/AlGaAs QW Lasers (900 o C, 60 sec) H.H. Tan and C. Jagadish, Appl. Phys. Lett. 71, 2680 (1997).

21 Multi-Step Implantation Scheme for Improved GaAs/AlGaAs QW Laser Performance

22 Tuning the Detection Wavelength of Quantum Well Infrared Photodetectors (QWIPs)

23 Quantum Well Infrared Photodetectors

24 Quantum well intermixing of QWIPs before after hν 1 hν 2

25 QWIP structure Grown by MBE 2 µm GaAs cap (Si: cm -3 ) 50 nm Al 0.3 Ga 0.7 As barrier 50 nm Al 0.3 Ga 0.7 As barrier nm GaAs (Si: cm -3 ) 50 nm Al 0.3 Ga 0.7 As barrier 50 nm Al 0.3 Ga 0.7 As barrier 1.3 µm GaAs (Si: cm -3 ) 0.5 µm AlAs buffer SI-Substrate

26 Multiple energy implantation scheme Displacement profile Displacement density (a. u.) dose G is the sum of different implants with different energies nomalized by different doses Kev 250Kev 300Kev 350Kev 400Kev G M.B. Johnston et.al, Appl. Phys. Lett. 75, 923 (1999) Implant depth (µm)

27 Defect distribution profile of 0.9 MeV Protons Displacement Density (a. u.) Top contact QWs Bottom contact+substrate Implantation Depth (µm)

28 Tuning the wavelength of QWIP Metal contact Top contact Multi-QWs Metal contact Bottom contact Substrate un-implanted dose A dose B λ 1 λ 2 λ 3 dose A < dose B λ 1 < λ 2 < λ 3

29 QWIP spectral response 1.2 un-implanted 950 o C, 30 sec cm cm -2 Photoresponse (a. u.) cm cm -2 L. Fu et al, Appl. Phys. Lett. 78, 10 (2001) Wavelength (µm)

30 Responsivity Responsivity (ma/w) reference cm cm cm Bias (V) L. Fu et al, Appl. Phys. Lett. 78, 10 (2001).

31 Responsivity Response (ma/w) reference 1e16/RTA/1e16/RTA 2e16/RTA One-step implant-anneal sequence: 0.9 MeV cm -2 / 950ºC 30 s Two-step implant-anneal sequence: 0.9 MeV cm -2 / 950ºC 30 s /0.9 MeV cm -2 / 950ºC 30 s Bias (V) L. Fu et al, Infrared Phys. & Technol. 42, 171 (2001).

32 Relative spectral response un-implanted reference one-step implant-annealed sample two-step implant-annealed sample Photoresponse (a. u.) λ/λ = 15.95% λ/λ = 17.43% L. Fu et al, Infrared Phys. & Technol. 42, 171 (2001) Wavelength (µm)

33 Impurity Free Vacancy Disordering Ga Atom Dielectric Film (Silicon Dioxide) Ga Vacancy QW Silicon dioxide acts as a sink for Ga out-diffusion (i) Creation of Ga vacancies, (ii) Diffusion of Ga Vacancies

34 Why Impurity Free Vacancy Disordering? Maintains Good Crystal Quality Low Concentration of Residual Defects Relatively Simple Technique and No Residual Impurities in the Active Regions

35 Experimental conditions Spin-on glass (un-doped and Ga-doped): 3000 rpm for 30 s, baking at 400 C for 15 min SiO 2 : Plasma enhanced chemical vapour deposition (PECVD) TiO 2 : E-beam evaporation RTA: 700 ºC to 900ºC for 30 s Low temperature photoluminescence

36 IFVD using doped spin-on layers GaAs/AlGaAs 2 QW structure un-doped SiO 2 PL intensity (a. u.) Ga-doped Substrate QWs L. Fu et al., Appl. Phys. Lett. 7, 1171 (2002) Wavelength (nm) L. Fu et. Al., Appl. Phys. Lett. 76, (2000).

37 IFVD using doped spin-on layers InGaAs/AlGaAs structure PL Intensity (a. u.) un-doped Ga-doped L. Fu et. al., J. Appl. Phys. 92, 3579 (2002) wavelength (nm)

38 Integration of a Waveguide and a Laser Diode Using IFVD

39 Lateral waveguiding P ++ GaAs contact layer active (gain) DQW active layer waveguide Ridge in the gain part 1.9 µm Ridge in the waveguide part M. Buda et.al., J. Electrochem. Soc. 150, G481 (2003). 0.1 µm L a =0.87 mm (laser diode) L p = 2.5 mm (waveguide)

40 η i = 95 %, α p =5 cm -1 α a = 2 cm -1, L a = 0.87 mm intermixed 900 o C 60 s w = 4 µm 1 / η L p (mm) ( ) ( )( ) Re 1 e 1 e R 1 2 R 1 ln L L R 1 ln L p p p p p p L L L p p a a p p i d α α α α α + α = η η

41 Quantum Dot Photonic Integrated Circuits

42 Why Quantum Dots? Three Dimensional carrier confinement Leads to Atom Like Density of States Low Threshold Current Lasers High Quantum Efficiency High Thermal Stability (To) Lasers Lasers operating at 1.3 & 1.55 um on GaAs (VCSELs) Normal Incidence, High Operating Temperature and High Performance of QDIPs

43 Density of States Bulk (3D) Quantum Well (2D) Quantum Dot (0D) ρ(e) ρ(e) ρ(e) Energy Energy Energy

44 Self Assembled Growth of Quantum Dots Frank- van der Merwe Growth Mode Volmer-Weber Growth Mode Stranski-Krastanow Growth Mode Layer by Layer Growth Lattice matched Systems, e.g. AlGaAs on GaAs Direct Island Growth Large lattice mismatch Very High Interfacial Energy, e.g GaN on Saphire Layer by Layer followed by Island Nucleation Dissimilar Lattice Spacing, Low Interfacial Energy e.g. InAs on GaAs

45 Growth details Aixtron 200/4 MOCVD reactor rotation IR lamps TMGa, TMIn, AsH 3 & PH 3 Single layer In 0.5 Ga 0.5 As dots Dot growth ~ C GaAs cap at 650 C Characterization AFM and PL 5-6ML 50% InGaAs GaAs GaAs S-I GaAs Substrate 300nm 300nm

46 Amount of Material 6.5ML 5.8ML 5ML 4ML Increase material: Density increases until saturation Size decreases

47 Temperature - AFM 500 C 520 C 550 C Height increases with increasing temperature Less incoherent dots with increasing temperature

48 Quantum Dot Intermixing Large surface area to volume ratio Non-uniform composition profile Large strain field around the dots

49 Experimental Details Samples annealed for 30sec Proximity capped RTA Anneals at 700, 800, 850 and 900 C Spin on Glass, PECVD SiO 2, E-Beam TiO 2 Photoluminescence 10K Cooled Ge detector Argon-ion laser at 514.5nm

50 Thermal stability of single layer QDs Intensity (a.u.) as-grown 750 O C 800 O C 850 O C 900 O C Wavelength (nm) 12K Photoluminescence Spectra

51 IFVD of single layer QDs Spin on Glass (SOG) Intensity (a.u.) as-grown annealed ref undoped SOG ga SOG ti SOG 800 o C, 60 sec Wavelength (nm) 12K Photoluminescence Spectra

52 Suppression of thermal interdiffusion by TiO 2 as function of RTA temperature 160 PL energy shift (mev) RTA TiO 2 +RTA (a) TiO 2 0 FWHM of annealed FWHM of as-grown (b) Annealing temperature (ºC) Substrate L. Fu et.al., Appl. Phys. Lett. 82, 2613 (2003)

53 Suppression of interdiffusion using bilayer SiO 2 + TiO 2 RTA: 850ºC 30 s PL intensity (Normalized) E 1 E 2 as-grown RTA only SiO 2 + RTA SiO 2 + TiO 2 + RTA SiO 2 TiO 2 Substrate Wavelength (nm)

54 Suppression of interdiffusion using bilayer SiO 2 + TiO 2 as a function of SiO 2 thickness PL energy shifts (mev) E 1 E 2 E 1 SiO 2 E 2 -SiO 2 /TiO Thickness of SiO 2 (nm) L. Fu et al, Appl. Phys. Lett. 82, 2613 (2003).

55 Thermal expansion coefficient Material α (ºC -1 ) GaAs SiO TiO

56 Thermal stress effect Compressive stress on GaAs tensile surface favourable for V Ga diffusion enhanced interdiffusion compressive SiO 2 QDs TiO 2 compressive tensile Substrate Substrate Compressive stress on GaAs surface favourable for V Ga diffusion enhanced interdiffusion Tensile stress on GaAs surface unfavourable for V Ga diffusion inhibited interdiffusion

57 Stacked Dots 2000A GaAs cap GaP layer InGaAs dots A 300A GaAs substrate

58 Growth of stacked layers Top layer of 3- layer stack. Single Layer Density ~4x10 10 /cm 2 Top layer of 3- layer stack. With smoothing Density ~4x10 10 /cm 2 Original conditions Dots smaller than for single layer. Density ~2x10 10 /cm 2 Top layer of 3-layer stack. Low V/III, with GaP layers. Dots slightly flatter. Density ~3x10 10 /cm 2

59 IFVD of stacked QDs Intensity (a.u.) 1.0 Single Layer 0.8 as-grown ann. ref 0.6 SiO SiO 2 /TiO 2 TiO Intensity (a.u.) Stacked layer Wavelength (nm)

60 Implantation (Single QD Layers) Protons Arsenic Ions Intensity (a.u.) as-grown annealed 750 O C, 30s 5x10 13 H/cm 2 1x10 14 H/cm 2 1x10 15 H/cm 2 Intensity (a.u.) as-grown annealed 1x10 11 As/cm 2 5x10 11 As/cm 2 1x10 12 As/cm 2 5x10 12 As/cm Wavelength (nm) Wavelength (nm) P. Lever et al, Appl. Phys. Lett. 82, 2053 (2003)

61 Stacked QDs -H implant Intensity (a.u.) as-grown annealed 750 O C, 30s 5x10 13 H/cm 2 1x10 14 H/cm Wavelength (nm)

62 Energy Shifts As Ion Dose, Annealing Temperature (Single Layer of QDs) Energy Shift (mev) annealed at 700 O C, 30s annealed at 750 O C, 30s annealed at 800 O C, 30s Dose (As/cm 2 )

63 Energy Shifts vs Implant Temperature (Protons and Arsenic Ions) Energy Shift (mev) x10 11 As/cm 2 1x10 12 As/cm 2 5x10 14 H/cm 2 1x10 15 H/cm Implant Temperature ( O C)

64 Energy Shift and PL Intensity Vs. Displacement Density Energy Shift (mev) H As Displacements (cm -3 ) Intensity (norm) Displacements (cm -3 ) H As

65 Quantum Dot Lasers with and without GaP 1/η diff With GaP layers No GaP layers Ref QW laser Dot laser Internal Efficiencies Without GaP 89.7% GaP 74.6% Loss 7.3cm Length (mm) QW laser Internal efficiency 99% Loss 2.6cm-1

66 Quantum Dot Lasers (without GaP) Intensity (a.u.) mm 1.5mm 2mm 3mm Wavelength (nm) Lasing in ground state for long devices Broad lasing spectrum Modulations of 8nm and 2nm separation Transverse and Lateral modes

67 Quantum Dot Infrared Photodetectors Si doped GaAs Si doped GaAs S.I. GaAs 30 QD layers Grown by MBE 30 Stacked dot layers Wide GaAs spacer layer reduced dark current Top and bottom Si doped layers for contacts Quantum dots are Si doped to provide carriers

68 Quantum Dot Infrared Photodetectors Photoresponse (a.u.) V 3V 1.5V Wavelength (µm)

69 Summary Quantum Well and Quantum Dot Intermixing Techniques are promising for Optoelectronic Device Integration Understanding defect generation, diffusion and annihilation processes are important for achieving QWI and QDI Dopant diffusion issues need to be taken into consideration for Device Structures such as Lasers and Photodetectors

70 Acknowledgements Australian Research Council Funding Past and Present Members of Semiconductor Optoelectronics And Nanotechnology Group, ANU (H.H.Tan, L. Fu, P. Lever, M. Buda, P.N.K. Deenapanray, J. Wong-Leung, Q. Gao, C. Carmody, R.M. Cohen, A. Clark, G. Li, M.I. Cohen, S. Yuan, Y. Kim, J. Lalita, X. Q. Liu, K. Stewart, F. Karouta, P. Gareso, A. Allerman, R.van der Heijden, M.F. Aggett ) Many Collaborators in Australia (Mike Gal, M.B. Johnston. P.Burke, L.V. Dao, P.Reece, B.Q. Sun, David Cockayne, Zou Jin, B. Gong, R.N. Lamb) and Overseas

71 Conferences/Networks/ Journal Special Issues Conference on Optoelectronic and Microelectronic Materials and Devices (COMMAD 2004), Brisbane Dec 8-10, 2004, Australian Nanotechnology Network ( IEEE Journal of Selected Areas in Communications, Special Issue on Nanotechnologies for Communications Deadline: April 1, 2004, Submit manuscripts to: 2 nd International Conference on Integrated Optoelectronics Electrochemical Society Meeting, Honolulu, Hawaii, Oct LEOS Annual Meeting in Australia (Sydney or Cairns)