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 c.jagadish@ieee.org www.rsphysse.anu.edu.au/eme
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
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
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
Photonic Integrated Circuits Different Bandgaps on the same chip
WDM Source Optical output (to optical fibre) optical amplifier passive optical waveguides multi-wavelength laser diodes dielectric passivation implant isolation
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
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
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
Ion implantation induced quantum well intermixing Ion implantation Point defects: Vacancy Interstitial Intermixing
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
10K Photoluminescence Spectra H.H. Tan et.al., Appl. Phys. Lett. 68, 2401 (1996).
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).
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) 0.89 0.88 0.87 InP 250nm InP buffer InGaAs QW InP 200nm InP buffer 0.86 600 650 700 750 800 850 Annealing Temperature ( o C)
Implantation Dose (20 kev P) and Temperature Dependence of Energy Shifts in InP/InGaAs QWs 700 o C, 60 sec Energy Shift (mev) 90 80 70 60 50 40 30 20 10 0 InP capped 25 o C 200 o C Energy Shift (mev) 30 25 20 15 10 5 0 InGaAs capped 25 o C 200 o C 10 12 10 13 10 14 Dose (cm -2 ) 10 12 10 13 10 14 Dose (cm -2 ) C. Carmody et. al., J. Appl. Phys. 93, 4468 (2003)
Damage Accumulation in InP and InGaAs InP Cap InGaAs Cap Normalised Yield 3x10 3 2x10 3 1x10 3 random InP cap unimplanted 1 x 10 14 cm -2 implanted at room temperature 1 x 10 14 cm -2 implanted at 200 o C Normalised Yield 3x10 3 2x10 3 1x10 3 random InGaAs cap unimplanted 1 x 10 14 cm -2 implanted at room temperature and 200 o C 0 0 200 250 300 350 400 450 200 250 300 350 400 450 Channel Channel
Tuning the Emission Wavelength of GRINSCH Quantum Well Lasers GaAs/AlGaAs QW Lasers
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
GRINSCH QW Laser and 220 kev Proton Defect Profile
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).
Multi-Step Implantation Scheme for Improved GaAs/AlGaAs QW Laser Performance
Tuning the Detection Wavelength of Quantum Well Infrared Photodetectors (QWIPs)
Quantum Well Infrared Photodetectors
Quantum well intermixing of QWIPs before after hν 1 hν 2
QWIP structure Grown by MBE 2 µm GaAs cap (Si: 2 10 18 cm -3 ) 50 nm Al 0.3 Ga 0.7 As barrier 50 nm Al 0.3 Ga 0.7 As barrier 48 4.5 nm GaAs (Si: 2 10 18 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: 2 10 18 cm -3 ) 0.5 µm AlAs buffer SI-Substrate
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 0.45 0.45 0.55 0.7 1 200Kev 250Kev 300Kev 350Kev 400Kev G M.B. Johnston et.al, Appl. Phys. Lett. 75, 923 (1999). 0 1 2 3 4 5 Implant depth (µm)
Defect distribution profile of 0.9 MeV Protons Displacement Density (a. u.) 2.0 1.5 1.0 0.5 0.0 Top contact QWs Bottom contact+substrate 0 2 4 6 8 10 12 14 Implantation Depth (µm)
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
QWIP spectral response 1.2 un-implanted 950 o C, 30 sec 1.0 1 10 16 cm -2 2 10 16 cm -2 Photoresponse (a. u.) 0.8 0.6 0.4 0.2 3 10 16 cm -2 4 10 16 cm -2 L. Fu et al, Appl. Phys. Lett. 78, 10 (2001). 0.0 4 6 8 10 12 14 Wavelength (µm)
Responsivity Responsivity (ma/w) 0.13 0.12 0.11 0.10 0.09 0.08 0.07 0.06 0.05 0.04 0.03 0.02 0.01 reference 1 10 16 cm -2 2 10 16 cm -2 3 10 16 cm -2 0.00-3 -2-1 0 1 2 3 Bias (V) L. Fu et al, Appl. Phys. Lett. 78, 10 (2001).
Responsivity Response (ma/w) 0.14 0.12 0.10 0.08 0.06 0.04 reference 1e16/RTA/1e16/RTA 2e16/RTA One-step implant-anneal sequence: 0.9 MeV 2 10 16 cm -2 / 950ºC 30 s Two-step implant-anneal sequence: 0.9 MeV 1 10 16 cm -2 / 950ºC 30 s /0.9 MeV 1 10 16 cm -2 / 950ºC 30 s 0.02 0.00-6 -4-2 0 2 4 6 Bias (V) L. Fu et al, Infrared Phys. & Technol. 42, 171 (2001).
Relative spectral response 1.4 1.2 un-implanted reference one-step implant-annealed sample two-step implant-annealed sample Photoresponse (a. u.) 1.0 0.8 0.6 0.4 λ/λ = 15.95% λ/λ = 17.43% L. Fu et al, Infrared Phys. & Technol. 42, 171 (2001). 0.2 0.0 4 5 6 7 8 9 10 11 12 Wavelength (µm)
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
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
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
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). 650 675 700 725 750 775 Wavelength (nm) L. Fu et. Al., Appl. Phys. Lett. 76, 837-839 (2000).
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) 860 880 900 920 940 960 wavelength (nm)
Integration of a Waveguide and a Laser Diode Using IFVD
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)
0.0 0.5 1.0 1.5 2.0 2.5 3.0 0 1 2 3 4 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
Quantum Dot Photonic Integrated Circuits
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
Density of States Bulk (3D) Quantum Well (2D) Quantum Dot (0D) ρ(e) ρ(e) ρ(e) Energy Energy Energy
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
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 ~500-550 C GaAs cap at 650 C Characterization AFM and PL 5-6ML 50% InGaAs GaAs GaAs S-I GaAs Substrate 300nm 300nm
Amount of Material 6.5ML 5.8ML 5ML 4ML Increase material: Density increases until saturation Size decreases
Temperature - AFM 500 C 520 C 550 C Height increases with increasing temperature Less incoherent dots with increasing temperature
Quantum Dot Intermixing Large surface area to volume ratio Non-uniform composition profile Large strain field around the dots
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
Thermal stability of single layer QDs Intensity (a.u.) 1.2 1.0 0.8 0.6 as-grown 750 O C 800 O C 850 O C 900 O C 0.4 0.2 0.0 880 920 960 1000 1040 1080 1120 1160 Wavelength (nm) 12K Photoluminescence Spectra
IFVD of single layer QDs Spin on Glass (SOG) Intensity (a.u.) 1.0 0.8 0.6 0.4 as-grown annealed ref undoped SOG ga SOG ti SOG 800 o C, 60 sec 0.2 0.0 920 960 1000 1040 1080 1120 1160 Wavelength (nm) 12K Photoluminescence Spectra
Suppression of thermal interdiffusion by TiO 2 as function of RTA temperature 160 PL energy shift (mev) 140 120 100 80 60 40 20 RTA TiO 2 +RTA (a) TiO 2 0 FWHM of annealed FWHM of as-grown 1.0 0.9 0.8 0.7 0.6 0.5 0.4 680 700 720 740 760 780 800 820 840 860 (b) Annealing temperature (ºC) Substrate L. Fu et.al., Appl. Phys. Lett. 82, 2613 (2003)
Suppression of interdiffusion using bilayer SiO 2 + TiO 2 RTA: 850ºC 30 s PL intensity (Normalized) 1.2 1.0 0.8 0.6 0.4 0.2 E 1 E 2 as-grown RTA only SiO 2 + RTA SiO 2 + TiO 2 + RTA SiO 2 TiO 2 Substrate 0.0 900 950 1000 1050 1100 1150 1200 Wavelength (nm)
Suppression of interdiffusion using bilayer SiO 2 + TiO 2 as a function of SiO 2 thickness 120 100 PL energy shifts (mev) 80 60 40 20 0-20 -40-60 E 1 E 2 E 1 SiO 2 E 2 -SiO 2 /TiO 2 0 20 40 60 80 100 120 140 Thickness of SiO 2 (nm) L. Fu et al, Appl. Phys. Lett. 82, 2613 (2003).
Thermal expansion coefficient Material α (ºC -1 ) GaAs 6.86 10-6 SiO 2 0.52 10-6 TiO 2 8.19 10-6
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
Stacked Dots 2000A GaAs cap GaP layer InGaAs dots 10-150A 300A GaAs substrate
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
IFVD of stacked QDs Intensity (a.u.) 1.0 Single Layer 0.8 as-grown ann. ref 0.6 SiO 2 0.4 SiO 2 /TiO 2 TiO 0.2 2 Intensity (a.u.) 0.0 1.0 0.8 0.6 0.4 0.2 0.0 Stacked layer 920 960 1000 1040 1080 1120 1160 Wavelength (nm)
Implantation (Single QD Layers) Protons Arsenic Ions Intensity (a.u.) 1.0 0.8 0.6 0.4 0.2 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.) 1.0 0.8 0.6 0.4 0.2 as-grown annealed 1x10 11 As/cm 2 5x10 11 As/cm 2 1x10 12 As/cm 2 5x10 12 As/cm 2 0.0 900 950 1000 1050 1100 1150 1200 Wavelength (nm) 0.0 900 950 1000 1050 1100 1150 1200 Wavelength (nm) P. Lever et al, Appl. Phys. Lett. 82, 2053 (2003)
Stacked QDs -H implant Intensity (a.u.) 1.2 1.0 0.8 0.6 0.4 as-grown annealed 750 O C, 30s 5x10 13 H/cm 2 1x10 14 H/cm 2 0.2 0.0 1000 1020 1040 1060 1080 1100 1120 1140 1160 1180 1200 Wavelength (nm)
Energy Shifts As Ion Dose, Annealing Temperature (Single Layer of QDs) 140 120 Energy Shift (mev) 100 80 60 40 20 0 annealed at 700 O C, 30s annealed at 750 O C, 30s annealed at 800 O C, 30s 10 11 10 12 Dose (As/cm 2 )
Energy Shifts vs Implant Temperature (Protons and Arsenic Ions) 140 120 Energy Shift (mev) 100 80 60 40 5x10 11 As/cm 2 1x10 12 As/cm 2 5x10 14 H/cm 2 1x10 15 H/cm 2 20 0 0 20 40 60 80 100 120 140 160 180 200 220 Implant Temperature ( O C)
Energy Shift and PL Intensity Vs. Displacement Density Energy Shift (mev) 140 120 100 80 60 40 20 H As 0 10 19 10 20 10 21 Displacements (cm -3 ) Intensity (norm) 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 10 19 10 20 10 21 Displacements (cm -3 ) H As
Quantum Dot Lasers with and without GaP 1/η diff 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 With GaP layers No GaP layers Ref QW laser Dot laser Internal Efficiencies Without GaP 89.7% GaP 74.6% Loss 7.3cm-1 0.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 Length (mm) QW laser Internal efficiency 99% Loss 2.6cm-1
Quantum Dot Lasers (without GaP) Intensity (a.u.) 8 6 4 2 1mm 1.5mm 2mm 3mm 0 1070 1080 1090 1100 1110 1120 1130 1140 1150 1160 Wavelength (nm) Lasing in ground state for long devices Broad lasing spectrum Modulations of 8nm and 2nm separation Transverse and Lateral modes
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
Quantum Dot Infrared Photodetectors Photoresponse (a.u.) 10 8 6 4 2 0 6V 3V 1.5V 3 4 5 6 7 8 9 10 Wavelength (µm)
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
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
Conferences/Networks/ Journal Special Issues Conference on Optoelectronic and Microelectronic Materials and Devices (COMMAD 2004), Brisbane Dec 8-10, 2004, http://www.commad2004.itee.uq.edu.au/ Australian Nanotechnology Network (http://www.ausnano.net) IEEE Journal of Selected Areas in Communications, Special Issue on Nanotechnologies for Communications Deadline: April 1, 2004, Submit manuscripts to: c.jagadish@ieee.org 2 nd International Conference on Integrated Optoelectronics Electrochemical Society Meeting, Honolulu, Hawaii, Oct. 2004 2005 LEOS Annual Meeting in Australia (Sydney or Cairns)