Three-Dimensional Silicon-Germanium Nanostructures for Light Emitters and On-Chip Optical eptember 2011 Interconnects Leonid Tsybeskov Department of Electrical and Computer Engineering New Jersey Institute of Technology Newark, NJ 1 Moscow, Russia
Motivation: Optical Interconnects for Wafer Level Integration and Multi-Chip Modules IBM Announces Chip Breakthrough Using Silicon Nanophotonics, 12/1/10 http://domino.research.ibm.com Different versions of CMOS-compatible integrated optoelectronic circuits (L. Tsybeskov, D. J. lockwood and M. Ichikawa, Proceedings of the IEEE, v. 97, No. 7, July 2009): IBM announcement yesterday 2 The Light Source is Still Missing!!!
Outline Materials Science of Si/SiGe Threedimensional (3D) Nanostructures (Briefly) Physics of Light Emission in 3D Si/SiGe NSs: Can Luminescence be Fast and Efficient? Conclusion: How Far We are from CMOS- Compatible Integrated Light Sources? 3
Low Level Excitation: Si Exciton Luminescence Because of the indirect band gap, free exciton (FE) recombination in c-si has low rate and a long radiative lifetime (phonon-assisted process, τ rad ~ 10-3 s). (a) Energy (E) (b) E Si (c) Conduction band Radiative Photon (hv) recombination Valence band Momentum (k) Energy conservation hv Phonons k NP Energy and momentum conservation Carrier recombination in (a) direct and (b) indirect band gap semiconductors. (c) Phonon replicas in the PL spectrum of c-si. 4
Luminescence in SiGe Crystalline Alloys: Compositional Disorder Relaxes Selection Rule Compositional disorder in SiGe alloys - significant selection rule relaxation and a higher efficiency of no-phonon (NP) PL [Weber and Alonso, Phys. Rev. B, 1989)]. However, PL is nearly completely quenched at T > 150K; the PL thermal quenching is controlled by exciton localization at SiGe compositional fluctuations (~ 15 mev activation energy). 5
Summary: Pros and Cons of Light Emission in SiGe Alloys Pros: Tunable PL, can be close to the important spectral region of 1.5 µm Efficient PL at low temperature (external QE > 1%) EL is obtained in forward biased p-n junctions Cons: Strong PL/EL thermal quenching Solution: Carrier confinement in Si/SiGe quantum wells Problem: ~ 4% lattice mismatch between Si and Ge - defects Solution: Three-dimensional (3D) Si/SiGe nanostructures 6
Si/SiGe Cluster Morphology (3D) Nanostructures: Low Defect Density SiGe Si Stranski-Krastanov nanostructures; pancake shaped SiGe clusters; typical cluster height is ~ 10-20 nm with a base from 50 to 150 nm and a 2D wetting layer. Control over SiGe cluster composition is achievable (i.e., Si 1-x Ge x cluster composition) [D. J. Eaglesham and M. Cerullo, Dislocation-free Stranski-Krastanow growth of Ge on Si(100), Phys. Rev. Lett. 64, 1943 (1990). 7
Si/SiGe Cluster Morphology (3D) Nanostructures: Low Defect Density The properly grown samples (T G =600-650C o ) have little or no dislocations. The nanometer-sized SiGe crystalline alloy clusters exhibit high quantum efficiency PL near 1.3-1.5 µm. Good news! 8
Light Emission in Composition Controlled Si/SiGe 3D Nanostructures Good x=0.096 Si, TO SiGe Not Good >30% PL Intensity (arb. units) x=0.16 x=0.53 SiGe, core Si, TO SiGe SiGe, WL 1 10 100 Excitation Intensity (W/cm 2 ) Kamenev et. al., Appl. Phys. Lett. 84, 1293 (2004) 9
Summary: Light Emission in Composition Controlled Si/SiGe 3D Nanostructures 1. The PL (and EL) spectra in 3D Si/SiGe nanostructures with 30-50% Ge composition have peaks within the important 1.3-1.5 μm spectral range. 2. In the optimized 3D Si/SiGe nanostructures the PL (and EL) intensity thermal quenching can be suppressed up to T 250 K (almost room temperature). 3. The PL external quantum efficiency at low excitation can be up to 5 %. As excitation intensity increases, the PL intensity very quickly saturates (i.e., the PL quantum efficiency decreases) Why??? Kamenev et. al., Appl. Phys. Lett. 84, 1293 (2004) 10
Traditional Explanation: Type II Energy Band Alignment in Si/SiGe Heterostructures Suggestion: Type II energy band alignment is responsible for a week overlap of electron-hole wavefunctions at Si/SiGe hetero-interface. Thus, slow radiative recombination cannot compete with Auger processes even at very low carrier concentration (< 10 14 cm -3 ). Questions: 1. Si/SiGe hetero-interface suppose to have a low energy barrier for electrons (quasi-type II?) 2. The estimated Auger threshold is ~ 10-100 times lower compared to that in bulk Si free exciton. E C E V E C E V TYPE II Si Si Si 1-x Ge x TYPE I Si Si Si 1-x Ge x (a) (b) Why??? 11
At. % Si/SiGe Interfaces in 3D Nanostructures: Interface Abruptness and Carrier Recombination Horizontal Si At. % Vertical Si Ge Ge Kamenev et al., - PL data suggest that SiGe clusters have Ge-rich core (APL, PRB 2004-2008) Baribeau et al., - Analytical TEM (EDX) measurements of Si/SiGe heterointerface composition: diffused interfaces 12
Si/SiGe Heterointerfaces: Interface Abruptness and Carrier Recombination E E C E C E V E V Si SiGe E V Si SiGe Abrupt Type II heterojunction based mid IR Lasers (III-V MBE) Diffused A weak overlap of e-h wavefunctions x 13
Si/SiGe Interfaces in 3D Nanostructures: Si/SiGe Interfaces and Carrier Recombination 1- slow electron-hole recombination due to spatial separation and a weak e-h wavefunction overlap; 2 - a faster recombination, presumably involving holes at excited energy states in smallsize SiGe nanoclusters; 3 - hole diffusion due to cluster-to-cluster tunneling; 4 - hole diffusion due to thermionic processes; 5 - Auger hole excitation with possible charge transfer in real space ( Auger Fountain ). A wide variety of recombination processes (Tsybeskov et al, 2008-2010). 14
Si/SiGe Interfaces in 3D Nanostructures: Si/SiGe Interfaces and Carrier Recombination 1- slow electron-hole recombination Low-temperature (T=4 K) PL transients in 3D Si/Si1 xgex nanostructures with different Ge concentrations. The inset shows PL spectra at 4 K for samples with x=0.16 and x=0.53. Kamenev et al., PHYSICAL REVIEW B 72, 193306 (2005) 15
Si/SiGe Interfaces in 3D Nanostructures: Si/SiGe Interfaces and Carrier Recombination 1- slow electron-hole recombination The PL relaxation time constant as a function of the PL detection photon energy in 3D Si/Si1 xgex nanostructures with different Ge concentrations (0.16 < x < 0.53). Kamenev et al., PHYSICAL REVIEW B 72, 193306 (2005) 16
Si/SiGe Interfaces in 3D Nanostructures: Si/SiGe Interfaces and Carrier Recombination 3 - hole diffusion due to cluster-to-cluster tunneling; 4 - hole diffusion due to thermionic processes; SiGe PL intensity thermal quenching at different excitation intensities (3-4); Lee et al, 2008 17
Si/SiGe Interfaces in 3D Nanostructures: Si/SiGe Interfaces and Carrier Recombination (a) (b) 0.1 mj/cm 2 10 mj/cm 2 Time (s) Anti-correlation between Si EHD PL and SiGe PL (5); Lee et al, PRB 2009 Auger Fountain is responsible for non-monotonic decay of SiGe PL under pulsed excitation (5) 18
Summary: Light Emission in 3D Si/SiGe Nanostructures Light Emission in Low Defect Density 3D Si/SiGe Nanostructures is Governed by Si/SiGe Interface Abruptness How abrupt Si/SiGe interfaces can be fabricated? 19
Current Work: Engineering of Si/SiGe Nano-hetero-interfaces 1. Control of Si/SiGe intermixing and strain by varying SiGe composition and layer thicknesses Detecting High Strain Contrast in Si Layers Using TEM; Low Strain Contrast in Top Layers 20
Current Work: Engineering of Si/SiGe Nano-hetero-interfaces Top Detecting Strain and Interface Abruptness Using Lattice Imaging and EDX 21
Current Work: Engineering of Si/SiGe Nano-hetero-interfaces and PL Properties 1. Control of Si/SiGe intermixing and strain 2. Fast & efficient SiGe PL, which successfully competes with other recombination channels, including Si EHD and Si EHP. EDX confirms abrupt Si/SiGe interfaces; Efficient PL at 1.5 um; PL Intensity (arb. un.) 325 nm excitation τ rad??? Si EHD 0.8 0.9 1 1.1 1.2 Photon energy (ev) T=15K 458 nm excitation Si EHD Strained Si SiGe EHD τ rad ~100 ns Si TO 0.8 0.9 1 1.1 1.2 Photon energy (ev) 22
Current Work: Engineering of Si/SiGe Nano-hetero-interfaces and PL Properties 1. Control of Si/SiGe intermixing and strain 2. Fast & efficient SiGe PL, which successfully competes with other recombination channels, including Si EHD and Si EHP. Efficient PL at 1.5 um; Less than 20 ns radiative lifetime PL Intensity (a.u.) 10 0 10-1 10-2 PL dynamics at ~0.8eV < 20 ns Diffused Si/SiGe Interfaces ~2 µs Abrupt Si/SiGe Interfaces T=16K 0 1 2 3 Time (µs) Normalized PL Intensity (arb. un.) 10 3 10 2 10 1 408 nm Excitation 488 nm Excitation 1 10 100 Excitation Intensity (mw) < 20 ns!!! 23
Future Work: Engineering of Si/SiGe Nano-hetero-interfaces and PL Properties 1. Control of Si/SiGe intermixing and strain 2. Fast & efficient SiGe PL, which successfully competes with other recombination channels, including Si EHD and Si EHP. Si/SiGe 3D Nanostructure based CMOS- Compatible Light Sources 24
Acknowledgement NJIT: N. Modi, S. Mala, E.-K. Lee, B. V. Kamenev, H-Y. Chang NRC Canada: D. J. Lockwood, J.-M. Baribeau and X.Wu HP Labs: T. I. Kamins and A. M. Bratkovsky MRS Fall 2010; Boston, MA Supported by US NSF, Intel, Hewlett Packard, SRC and Foundation at NJIT 25