| 1. |
- Heiss, M., et al.
(författare)
-
Self-assembled quantum dots in a nanowire system for quantum photonics
- 2013
-
Ingår i: Nature Materials. - 1476-4660. ; 12:5, s. 439-444
-
Tidskriftsartikel (refereegranskat)abstract
- Quantum dots embedded within nanowires represent one of the most promising technologies for applications in quantum photonics. Whereas the top-down fabrication of such structures remains a technological challenge, their bottom-up fabrication through self-assembly is a potentially more powerful strategy. However, present approaches often yield quantum dots with large optical linewidths, making reproducibility of their physical properties difficult. We present a versatile quantum-dot-innanowire system that reproducibly self-assembles in core-shell GaAs/AlGaAs nanowires. The quantum dots form at the apex of a GaAs/AlGaAs interface, are highly stable, and can be positioned with nanometre precision relative to the nanowire centre. Unusually, their emission is blue-shifted relative to the lowest energy continuum states of the GaAs core. Large-scale electronic structure calculations show that the origin of the optical transitions lies in quantum confinement due to Al-rich barriers. By emitting in the red and self-assembling on silicon substrates, these quantum dots could therefore become building blocks for solid-state lighting devices and third-generation solar cells.
|
|
| 2. |
- Felici, M, et al.
(författare)
-
High Energy Optical Transitions in Ga(PN) : Contribution from Perturbed Valence Band
- 2005
-
Ingår i: AIP Conference Proceedings / Volume 772. - : American Institute of Physics (AIP). - 0735402574 ; , s. 265-
-
Konferensbidrag (refereegranskat)abstract
- The GaP1–xNx conduction band is investigated experimentally (by excitation photoluminescence) andtheoretically (by pseudopotential supercells) for N concentrations up to x=3.5%and photon energies ranging from the optical absorption edge to3.2 eV. With increasing x: (i) a direct-like absorption edgedevelops smoothly and red-shifts rapidly overtaking energy-pinned cluster states; (ii)a broad absorption plateau appears between the X1c and the1c critical points of GaP; (iii) the 1c absorption edgebroadens and gradually disappears. Empirical pseudopotential calculations for GaP1-xNx randomalloy supercells account well for all the PLE results byconsidering N induced changes in the valence band overlooked sofar. ©2005 American Institute of Physics
|
|
| 3. |
|
|
| 4. |
|
|
| 5. |
|
|
| 6. |
- Lany, S., et al.
(författare)
-
Halogen n-type doping of chalcopyrite semiconductors
- 2005
-
Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 86:4
-
Tidskriftsartikel (refereegranskat)abstract
- We theoretically identify the chemical thermodynamic boundary conditions that will produce n-type CuInSe2 via halogen doping. Remarkably, we find that due to the low formation energies of the intrinsic defects, V-Cu and In-Cu in CuInSe2, the growth conditions that maximize the halogen donor incorporation do not yield n-type conductivity, whereas the conditions that maximize the concentration of the intrinsic donor In-Cu do yield n-type conductivity. Under the latter conditions, however, the contribution of the halogen donors to the net donor concentration stays significantly below that of In-Cu.
|
|
| 7. |
|
|
| 8. |
|
|
| 9. |
|
|
| 10. |
|
|
| 11. |
|
|
| 12. |
- Persson, Clas, et al.
(författare)
-
Anomalous grain boundary physics in polycrystalline CuInSe2 : The existence of a hole barrier
- 2003
-
Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 91:26
-
Tidskriftsartikel (refereegranskat)abstract
- First-principles modeling of grain boundaries (GB) in CuInSe2 semiconductors reveals that an energetic barrier exists for holes arriving from the grain interior (GI) to the GB. Consequently, the absence of holes inside the GB prevents GB electrons from recombining. At the same time, the GI is purer in polymaterials than in single crystals, since impurities segregated to the GBs. This explains the puzzle of the superiority of polycrystalline CuInSe2 solar cells over their crystalline counterpart. We identify a simple and universal mechanism for the barrier, arising from reduced p-d repulsion due to Cu-vacancy surface reconstruction. This discovery opens up possibilities for the future design of superior polycrystalline devices.
|
|
| 13. |
- Persson, Clas, et al.
(författare)
-
Compositionally induced valence-band offset at the grain boundary of polycrystalline chalcopyrites creates a hole barrier
- 2005
-
Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 87:21
-
Tidskriftsartikel (refereegranskat)abstract
- First-principles calculations of model grain boundaries (GBs) in CuInSe2 and CaGaSe2 show that cation-terminated GBs have a valence-band offset with respect to the grain interior (GI). This offset repels holes from the GBs, thus depriving electrons there from recombination at the GB defects. Anion-terminated GBs have no such valence offset. CuGaSe2 has, in addition, a conduction-band offset at the GB/GI interface, attracting electrons to the GBs. These features explain how polycrystalline chalcopyrite solar cells could outperform their crystalline counterparts.
|
|
| 14. |
- Persson, Clas, et al.
(författare)
-
Deep nitrogen-induced valence- and conduction-band states in GaAs1-xNx
- 2003
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 68:3
-
Tidskriftsartikel (refereegranskat)abstract
- Most studies of the anomalous electronic properties of the GaAs1-N-x alloy have focused on near-edge states, but x-ray spectroscopic experiments [V. N. Strocov et al., Phys. Status Solidi B 233, RI (2002)] have now revealed anomalous properties deep inside the valence and conduction bands. Indeed, whereas N p character is found in GaN in the energy region near the valence-band maximum (VBM), when similar to3% N is introduced into GaAs one finds that N p character exists about 2-3 eV below the VBM and as two narrow peaks just above the conduction-band minimum. First-principles calculations of the N p character in Ga32As31N and GaN show that the valence resonances are due the fact that the VBM of GaAs0.97N0.03 itself lies > 2 eV above that of GaN. Thus, there is no need to involve an N--> As charge transfer to explain the data. This conclusion is further confirmed by our calculated valence--> conduction-band absorption spectra. We also show that the broken-symmetry core-hole calculations are necessary to explain, within the local density approximation (LDA), the energy of the N 1s<--valence-band transitions. As to the conduction-band peaks observed experimentally in the x-ray absorption, our LDA calculations do not reveal the sharp peaks seen in the measured spectra, leading to the possibility that the latter may not be related to substitutional N.
|
|
| 15. |
- Persson, Clas, et al.
(författare)
-
n-type doping of CuInSe2 and CuGaSe2
- 2005
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 72:3
-
Tidskriftsartikel (refereegranskat)abstract
- The efficiency of CuInSe2 based solar cell devices could improve significantly if CuGaSe2, a wider band gap chalcopyrite semiconductor, could be added to the CuInSe2 absorber layer. This is, however, limited by the difficulty of doping n-type CuGaSe2 and, hence, in its alloys with CuInSe2. Indeed, wider-gap members of semiconductor series are often more difficult to dope than lower-gap members of the same series. We find that in chalcopyrites, there are three critical values of the Fermi energy E-F that control n-type doping: (i) E-F(n,pin) is the value of E-F where the energy to form Cu vacancies is zero. At this point, the spontaneously formed vacancies (=acceptors) kill all electrons. (ii) E-F(n,comp) is the value of E-F where the energy to form a Cu vacancy equals the energy to form an n-type dopant, e.g., Cd-Cu. (iii) E-F(n,site) is the value of E-F where the formation of Cd-on-In is equal to the formation of Cd-on-Cu. For good n-type doping, E-F(n,pin), E-F(n,comp), and E-F(n,site) need to be as high as possible in the gap. We find that these quantities are higher in the gap in CuInSe2 than in CuGaSe2, so the latter is difficult to dope n-type. In this work, we calculate all three critical Fermi energies and study theoretically the best growth condition for n-type CuInSe2 and CuGaSe2 with possible cation and anion doping. We find that the intrinsic defects such as V-Cu and In-Cu or Ga-Cu play significant roles in doping in both chalcopyrites. For group-II cation (Cd, Zn, or Mg) doping, the best n-type growth condition is In/Ga-rich, and maximal Se-poor, which is also the optimal condition for stabilizing the intrinsic In-Cu/Ga-Cu donors. Bulk CuInSe2 can be doped at equilibrium n-type, but bulk CuGaSe2 cannot be due to the low formation energy of intrinsic Cu-vacancy. For halogen anion doping, the best n-type materials growth is still under In/Ga-rich, and maximal Se-poor conditions. These conditions are not best for halogen substitutional defects, but are optimal for intrinsic In-Cu/Ga-Cu donors. Again, CuGaSe2 cannot be doped n-type by halogen doping, while CuInSe2 can.
|
|
| 16. |
- Persson, Clas, et al.
(författare)
-
s-d coupling in zinc-blende semiconductors
- 2003
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 68:7
-
Tidskriftsartikel (refereegranskat)abstract
- Most zinc blende semiconductors have a single anion-like s state near the bottom of the valence band, found in density-of-states (DOS) calculations, and seen in photoemission. Here, we discuss the case where two s-like peaks appear, due to strong s-d coupling. Indeed, away from the k=0 Brillouin zone center, cation d states and anion s states can couple in zinc blende symmetry. Depending on the energy difference DeltaE(sd)=E-s(anion)-E-d(cation), this interaction can lead to either a single or two s-like peaks in the DOS and photoemission. We find four types of behaviors. (i) In GaP, GaAs, InP, and InAs, DeltaE(sd) is large, giving rise to a single cation d peak well below the single anion s peak. (ii) Similarly, in CdS, CdSe, ZnS, ZnSe, and ZnTe, we see also a single s peak, but now the cation d is above the anion s. In both (i) and (ii) the s-d coupling is very weak. (iii) In GaN and InN, the local density approximation (LDA) predicts two s-like peaks bracketing below and above the cation d-like state. Correcting the too low binding energies of LDA by LDA+SIC (self-interaction correction) still leaves the two s-like peaks. The occurrence of two s-like peaks represents the fingerprint of strong s-d coupling. (iv) In CdTe, LDA predicts a single s-like peak just as in case (ii) above. However, LDA+SIC correction shifts down the cation d state closer to the anion s band, enhancing the s-d coupling, and leading to the appearance of two s-like peaks. Case (iv) is a remarkable situation where LDA errors cause not only quantitative energetic errors, but actually leads to a qualitative effect of a DOS peak that exists in LDA+SIC but is missing in LDA. We predict that the double-s peak should be observed in photoemission for GaN, InN, and CdTe.
|
|
| 17. |
- Zhao, Y. J., et al.
(författare)
-
Why can CuInSe2 be readily equilibrium-doped n-type but the wider-gap CuGaSe2 cannot?
- 2004
-
Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 85:24, s. 5860-5862
-
Tidskriftsartikel (refereegranskat)abstract
- The wider-gap members of a semiconductor series such as diamond-->Si-->Ge or AlN-->GaN-->InN often cannot be doped n-type at equilibrium. We study theoretically if this is the case in the chalcopyrite family CuGaSe2-->CuInSe2, finding that: (i) Bulk CuInSe2 (CIS, E-g=1.04 eV) can be doped at equilibrium n-type either by Cd or Cl, but bulk CuGaSe2 (CGS, E-g=1.68 eV) cannot; (ii) result (i) is primarily because the Cu-vacancy pins the Fermi level in CGS farther below the conduction band minimum than it does in CIS, as explained by the doping limit rule; (iii) Cd doping is better than Cl doping, in that Cd-Cu yields in CIS a higher net donor concentration than Cl-Se; and (iv) in general, the system shows massive compensation of acceptors (Cd-III,V-Cu) and donors (Cl-Se,Cd-Cu,In-Cu).
|
|
| 18. |
|
|
| 19. |
|
|
| 20. |
|
|
| 21. |
|
|
