Digital stress compensation for stacked InAs/GaAs QDs solar cells

Transcription

1 Digital stress compensation for stacked InAs/GaAs QDs solar cells D. Alonso-Álvarez, A. G. Taboada, Y. González, J. M. Ripalda, B. Alén, L. González and F. Briones Instituto de Microelectrónica de Madrid (IMM-CNM-CSIC) A. Martí and A. Luque Insituto de Energía Solar (IES-UPM) A. M. Sanchez and S. Molina Universidad de Cádiz (UCA) CAM TNT 008 OVIEDO

2 Outline. Introduction: GaAs/InAs QDs Solar Cell. MBE growth for zero average stress: Digital stress compensation with GaP 3. Structural quality: XRD, AFM and TEM 4. Optical properties: photoluminescence and photocurrent 5. Conclusions monolayers

3 GaAs InAs QDs Solar Cell: A basic device to study the physics of the IB concept AlGaAs window Emitter FDL InAs QDs + GaP Base BSF Antonio Luque and Antonio Martí, Phys.Rev.Letters, VOLUME 78, NUMBER 6 30 JUNE 997

4 LEDs Objetive: Lasers Increase the light absorption of QDs based devices Infrarred photodetectors Solar cells Increase density Increase the QD density per layer Reduce size Increase transition energy Increase the number of QDs layers Devices usually have a limited active region thickness Increase the density of QDs layers Increase of defects due to excesive strain Strain compensation

5 InAs/GaAs QDs with GaP strain compensation monolayers Reference Nº Layers QDs (InAs) SC layers Spacer Application Nuntawong et al. Appl. Phys. Lett. 85, 3050 (004) 0 3 ML 8 ML (In 0.36 Ga 0.64 P) 8 ML (In 0.30 Ga 0.70 P) 9 nm General (MOCVD) Nuntawong et al. Appl. Phys. Lett. 86, 935 (005) 5 3 ML ML (GaP) 4 ML 6 ML 8 ML 7 nm 7.58 nm 8. nm 8.66 nm Lasers (MOCVD) Tatebayachi et al. Appl. Phys. Lett. 88, 07 (006) 6.6 ML 6 ML GaP 7 nm Lasers (MOCVD) Fu et al. Appl. Phys. Lett. 9, (007) ML (In 0.5 Ga 0.5 As).83 ML GaP 50 nm QD IR photodetectors (MOCVD) Lever et al. J. Appl. Phys, (004) ML (In 0.5 Ga 0.5 As).83 ML GaP 30 nm General (MOCVD) Oshima et al. Photovoltaic Energy Conversion, Conference Record of the 006 IEEE 4th World Conference on Volume, May 006 Page(s): ML 40 nm GaN As nm Solar Cells (H-MBE) This Work 50 ML + ML GaP 8 nm Solar Cells (MBE)

6 Strain components duetoa biaxial stress Strain energy density in the layer ε xx ε = ε xy yy ε zz = ε = ε = = νε xz = ε xx a0 a a = 0 C U = C + C = C yz 0 ε Aε Stack design for zero stress condition Matrix: a (GaAs) =.83 Å QDs: a (InAs) = 3.03 Å Compensation: a (GaP) =.73 Å n n GaP InAs x InAs QDs (8 nm) + 7 nm GaAs + QD (AFM) a ε = ε + a Strain relation between Average strain two layers energy density pseudomor phically grown on GaAs Zero stress condition a a a Aε n a + Aε na n a U av na + n n Aε = Aε Reference sample Strain compensated sample 33.3 Å GaAs ML GaP 6 Å GaAs 5.3 Å GaAs ML GaP N. J. Ekins-Daukes et al. Procedings of the 8th IEEE PV Specialist Conference, IEEE Press, New York, USA, 000. pp

7 MBE growth sequence Without GaP SC layers With GaP SC layers RHEED Intensity (a.u.) QDs RHEED spot (a.u.) nd QDs layer nucleation rd th Time (min) No QDs No QDs nucleation observed by RHEED beyond the 4th layer Acumulated strain creates too many defects Uniform QDs nucleation time Evidence of correct strain compensation Time (min)

8 Structural characterization: : XRD Symetric (004) reflection Average sin ε = perpendicular strain sin θb ( θ Δθ ) B Intensity (a.u.) Refference Sample SC sample Δθ = 58 arcs) Δθ = 49 arcs) Intensity (a.u.) ω/θ (arc seconds) ω/θ (arc seconds) No SC SC Δθ (arcs) Simulation (dynamical x-ray theory) <ε t > Relative strain Reduction (%) Compensation degree (%) GaP thickness < nominal thickness GaAs (-x) P x alloy Both

9 0 stacked layers of InAs QDs Characterization by AFM Without Strain Compensating Layers With Strain Compensating GaP MLs Non uniform QDs distribution QDs Colonies Higher uniformity QDs Density =.7x0 0 cm Average QDs size: -Diameter~ 50 nm - Heigth ~ 9 nm Dislocations 0.00 nm 0.00

10 photoluminescence of 0 stacked QDs layers PL Intensity (a.u.) PL Intensity (a.u.) 0 x InAs QDs 95 mw 5 mw 0 x InAs QDs + SC 35 mw 5 mw Wavelength (nm) 0 x InAs QDs + SC 0 x InAs QDs 5 mw 6 K Wavelength (nm) PL Intensity (a.u.) Quasi monomodal emission.3 ev at 6 K.05 ev at RT Sample: G396-0xQDs with SC LD 808 nm Good carrier confinement provided by P incorporation into the matrix ΔE Th = 450 mev Log Integrated PL Intensity (a.u.) PL Intensity (a.u.) /KT (ev - ) 30 K 0 K 350 K Wavelength (nm)

11 p-gaas x0 8 cm -3, 0 nm Window: p-al 0.8 Ga 0. As x0 7 cm -3, 40 nm Emitter: p-gaas x0 8 cm -3 Field Damping layer: n-gaas 0 7 cm -3, 0.7 μm 50 stacked QDs: δ-dopping n-gaas 3x0 6 cm μm QDs Solar Cell 50 stacked QDs layers 8 nm of spacer Growth by ALMBE Undoped GaAs 0. μm Base: n-gaas 5x0 7 cm -3 Back Surface Field: n-gaas x0 8 cm -3 n-type GaAs Substrate 3 μm μm Critical thickness (ML) Layers θ c =.6± 0.04 ML

12 50 stacked SCQDs Solar Cell: Structural characterization: : TEM Very low defect density Vertically aligned QDs TEM <0>cross-section by A. M. Sanchez and S. Molina Universidad de Cádiz (UCA)

13 AlGaAs window Emitter FDL 50 stacked SCQDs Solar Cell: Photocurrent InAs QDs + GaP Base BSF Relative Spectral Response GaAs WL Wavelength (nm) QDs RT high recombination in the QDs region Reduced extraction of electrons photogenerated in the emitter Poor high energy response Photocurrent (na) K GaAs -.7 V WL -.6 V -.5 V -5 V 0. V Wavelength (nm)

14 Conclusions Stacks of 0 and 50 InAs QDs layers with only 8 nm of spacer have been grown using ML GaP for strain compensation. XRD, TEM and PL experiments suggest good structural and optical quality. Photocurrent signal up to. μm is observed in the 50 layers solar cell due to light absorption in the nanostructures.

15 Thank you for your attention!!!