Novel materials and nanostructures for advanced optoelectronics Q. Zhuang, P. Carrington, M. Hayne, A Krier Physics Department, Lancaster University, UK
u Brief introduction to Outline Lancaster University Department of Physics Research group - Semiconductor Physics and Nanostructures u III-V compound materials and Nanostructures from MBE QDs (self-assembled, V group exchange & droplets) Nanowires Dilute nitrides u Summary
Lancaster University Established 1964 Close to the Lake District 17,500 students 2,250 staff Ranked in top 10 universities in UK Ranked 131 in the World (THEWUR) Top ranked Physics Department in UK in RAE 2008
semiconductor physics and nanostructures u Academic staff Qiandong Zhuang (molecular beam epitaxy) Manus Hayne (physics & applications of QDs) Oleg Kolosov (scanning probe microscopy) Tony Krier (Mid-infrared optoelectronics) Three Fellowships u Main research facilities Epitaxial growth 3 MBE Photoluminescence (PL) Magneto-PL up to 17 T AFM & SEM X-ray Class 100 clean room for nano-fabrication
Growth facilities u MBE-1&3 (VG V80H): III: Ga, Al, In V: As, Sb, N plasma Dopant: GaTe, Be u MBE-2: Wide bandgap nitride nanowires Graphene on BN u Research focuses on III-V semiconductors ranging from MBE growth to fundamental studies, optoelectronic devices (telecom lasers, MID emitters & detectors, solar cells, thermophotovoltaic, next generation QDs flash memories)
Self-assembled GaSb QDs u Why GaSb/GaAs QDs? Type II: flash memories (a) GaAs!" GaSb GaAs Energy z +" Schematic band-gap diagram of GaSb/GaAs including band bending. z is the growth direction. Type-II QDs (Leuven, Berlin, LNLS Brazil) [PRB, APL, PRB, PRB]
Self-assembled GaSb QDs 60000 4.2 K GaAs 10nm Counts (arb. units) 50000 40000 30000 20000 10000 Height: 3 nm; Diameter: 36 nm Density: 4x10 10 cm -2 QDs 0 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 Energy (ev) WL Scanning tunnelling microscope image of a quantum ring cross-section; brighter regions indicate high Sb content. Sb leaks out Strong As-Sb exchange and Sb segregation during capping
u InAs-based QDs for midinfrared light emitters u Fabricate through an exchange technology InSb/InAs QDs by exchange Sb-As exchange Sb Flux InAs GaAs Currently developing InSb/InAs MIR Lasers on InP
GaAs/AlGaAs QDs by droplets u Why droplets for QDs? - No strain is required More flexible in materials design More suitable for fundamental studies Ga droplets Expose to As4 to crystallize Cap with GaAs or AlGaAs 200 nm Al 0.32 Ga 0.68 As 5 ML Ga droplets Diameter: 71 nm; height: 25 nm Density: 6.5±0.5x10 9 cm -2 Sensitive to growth conditions GaAs Sub
GaAs/AlGaAs QDs by droplets u Sharp PL emission u Persists up to room temperature u Geometry of buried dots to be studied by TEM u Aim to transfer to antimonide QDs
Nitride nanowires on Si(111) u Advantages of NWs? u Devices: Solar cells, lighting & water splitting H 2 generation Requirements: u Bandgap must be 1.6-1.7 ev u straddle H 2 O redox potentials u Rapid charge transfer u Stable in aqueous solution H 2 O/H 2 E g 1.23 ev 1.6-1.7 ev Counter Electrode H 2 O/O 2 p-type Semiconductor
Nitride nanowires on Si(111) 2000 1500 Intensity (a.u.) 1000 400 mw InN grains InN Nanowires u Monocrystalline u Strain-free 500 200 mw 100 mw 50 mw 0 1500 1550 1600 1650 Wavelength (nm)
Dilute nitrides u A few percent of nitrogen reduces lattice constant reduces band-gap Reduced Auger recombination u Ga(In)AsN: 1.3 ~1.5 um emission on GaAs u Possible for MIR optoelectronics?
Physics of bandgap bowing Shan W et al, Phys. Rev. Lett. 82 1221(1999)
Dilute nitrides u Develop GaAs-based dilute nitride lasers operating at 1.55 um up to a temperature of 85 o C through optimising valence band offset Adding Sb Embedded AlGaAs barrier u Oclaro studentship
Dilute nitrides u InAsNSb on InAs or GaSb: covers MIR and far-infrared with suppressed Auger recombination InAsNSb LED Sb as surfactant improves the dilutes Developing nitride MIR lasers on InP in collaboration with Wisconsin University
Summary u Top 10 universities in the UK, top ranked Physics in RAE 2008 u MBE growth facilities - most of arsenic and antimonide materials and u Active in MBE grown novel materials and nanostructures GaSb/GaAs QDs/Quantum Rings InSb/InAs QDs by exchange GaAs/AlGaAs QDs by droplets Novel dilute nitride materials Wide bandgap nitride NWs on Si u New materials & structures: graphene + BN