High-Yield Plasma Synthesis of Luminescent Silicon Nanocrystals

Thin films comprising semiconductor nanocrystals are emerging for applications in electronic and optoelectronic devices including light emitting diodes and solar cells. Achieving high charge carrier mobility in these films requires the identification and elimination of electronic traps on the nanocrystal surfaces. Herein, we show that in films comprising ZnO nanocrystals, an electron acceptor trap related to the presence of OH on the surface limits the conductivity. ZnO nanocrystal films were synthesized using a nonthermal plasma from diethyl zinc and oxygen and deposited by inertial impaction onto a variety of substrates. Surprisingly, coating the ZnO nanocrystals with a few nanometres of Al 2 O 3 using atomic layer deposition decreased the film resistivity by seven orders of magnitude to values as low as 0.12 O cm. Electron mobility as high as 3 cm 2 V À 1 s À 1 was observed in films comprising annealed ZnO nanocrystals coated with Al 2 O 3 .

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“…Zinc oxide nanocrystals were synthesized in the two-stage plasma reactor 13,14 illustrated in Supplementary Fig. 1.…”

Section: Resultsmentioning

confidence: 99%

High electron mobility in thin films formed via supersonic impact deposition of nanocrystals synthesized in nonthermal plasmas

et al. 2014

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“…The detailed description of synthesis can be found elsewhere [30,33]. The doping concentration is controlled by changing the flow rate of phosphine (PH 3 ) while maintaining constant flow rates for Ar and SiH 4 .…”

Section: Experimental Methodsmentioning

confidence: 99%

“…Freestanding Si NCs were synthesized in a nonthermal radio-frequency plasma reactor as reported previously [30]. We investigated six Si NC films with different P concentrations.…”

Section: Electron Transport In Films Of Heavily Doped Silicon Nanocrymentioning

confidence: 99%

Metal–insulator transition in films of doped semiconductor nanocrystals

Chen¹,

Reich²,

Kramer³

et al. 2015

Nature Mater

107

134

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To fully deploy the potential of semiconductor nanocrystal films as low-cost electronic materials, a better understanding of the amount of dopants required to make their conductivity metallic is needed. In bulk semiconductors, the critical concentration of electrons at the metal-insulator transition is described by the Mott criterion. Here, we theoretically derive the critical concentration nc for films of heavily doped nanocrystals devoid of ligands at their surface and in direct contact with each other. In the accompanying experiments, we investigate the conduction mechanism in films of phosphorus-doped, ligand-free silicon nanocrystals. At the largest electron concentration achieved in our samples, which is half the predicted nc, we find that the localization length of hopping electrons is close to three times the nanocrystals diameter, indicating that the film approaches the metal-insulator transition.Semiconductor nanocrystals (NCs) have shown great potential in optoelectronics applications such as solar cells [1], light emitting diodes [2], and field-effect transistors [3,4] by virtue of their size-tunable optical and electrical properties [5] and low-cost solution-based processing techniques [6,7]. These applications require conducting NC films and the introduction of extra carriers through doping can enhance the electrical conduction. Several strategies for NC doping have been developed. Remote doping, the use of suitable ligands as donors in the vicinity of NC surface, increased the conductivity of PbSe NC films by 12 orders of magnitude [8]. Electrochemical doping, which tunes the carrier concentration accurately and reversibly, resulted in conducting NC films [9,10]. Lately, stoichiometric control has emerged as a strategy to dope lead chalcogenide NCs [11]. Finally, electronic impurity doping of NCs, originally impeded by synthetic challenges [12], was recently achieved in InAs [13] and CdSe [14] NCs.While many experimental studies have been directed towards increasing the conductivity of NC films, there is still no clear consensus on the fundamental question: what is the condition for the metal-insulator transition (MIT) in NC films [15][16][17]? In a bulk semiconductor, the critical electron concentration n M for the MIT depends on the Bohr radius a B according to the well-known Mott criterion [18] n M a 3 B 0.02, where a B = ε 2 /m * e 2 is the effective Bohr radius (in Gaussian units), ε is the dielectric constant of the semiconductor, and m * is the effective electron mass. It is obvious that a dense film of undoped semiconductor NCs is an insulator, while a film of touching metallic NCs with the same geometry is a conductor. Therefore, the MIT has to occur in semiconductor NC films at some criti-FIG. 1. The origin of the metal-to-insulator transition in semiconductor nanocrystal films. The figure shows the cross section of two nanocrystals in contact through facets with radius ρ. The blue spherical cloud represents an electron wave packet which moves through the contact. Such a compact wave pac...

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“…[15][16][17][18][19][20][21][22][23] Experimentally, free-standing Si QDs have been synthesized in either liquid phase or gas phase. [24][25][26][27][28] Gas-phase doping of P and B in these free-standing Si QDs with at least partial hydrogen coverage of their surface has also been achieved by introducing dopant precursors (diborane and phosphine) into the plasma. 29 Although the doping in bulk Si have been widely studied before, as the size of the semiconductor approaches nano dimensions, the doping properties could be very different from those in the bulk system.…”

Section: Introductionmentioning

confidence: 99%

Chemical trend of the formation energies of the group-III and group-V dopants in Si quantum dots

Wei

2013

Phys. Rev. B

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Doping behavior in quantum dots (QDs) differs from that in the bulk. Despite many efforts, the doping properties are still not fully understood. Using first-principles methods, we have calculated the formation energies of various group-III acceptors and group-V donors doping at all non-equivalent sites in a Si QD (Si147H100). To analyze the trend of the formation energy, we decompose it into two terms: the unrelaxed formation energy (chemical energy) and the relaxation energy. We find that the unrelaxed formation energy generally increases as the dopant moves from the center of the QD to the surface. The variation of the unrelaxed formation energy in the surface region is explained by the variation of the local potential of the QD and the size effect. The relaxation energy gain increases as the size mismatch between the dopant and Si atom increases. Generally, the relaxation effect becomes more significant as the dopant moves toward the surface of the QD. The trend of the formation energy is determined by the two terms discussed above. If the size mismatch between the dopant and Si atom is small, the trend of the formation energy generally follows that of the unrelaxed formation energy, increasing as the dopant moves from the center to the surface; thus, these dopants have a better chance of staying in the core region. On the other hand, if the size mismatch is large, the relaxation effect dominates and the formation energy decreases, which indicates these dopants cannot enter the core region under equilibrium growth conditions.

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High-Yield Plasma Synthesis of Luminescent Silicon Nanocrystals

Cited by 690 publications

References 34 publications

High electron mobility in thin films formed via supersonic impact deposition of nanocrystals synthesized in nonthermal plasmas

High electron mobility in thin films formed via supersonic impact deposition of nanocrystals synthesized in nonthermal plasmas

Metal–insulator transition in films of doped semiconductor nanocrystals

Chemical trend of the formation energies of the group-III and group-V dopants in Si quantum dots

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