Quantum Size Effects in Amorphous Diamond-like Carbon Superlattices

Superlattices are periodic structures where the constituents alternate between low-and high-bandgap materials; the resulting quantum confinement tailors the resulting device properties and increases their operating speed. Amorphous carbon is an excellent candidate for both the well and barrier layers of the superlattices, leading to a fast and reliable device manufacturing process. We show theoretically and experimentally that, using low energy-loss spatially resolved spectroscopy, we can characterize the component layers of a superlattice. We measure quantum confinement of the electron wave function in the superlattice's wells and calculate the effective tunneling mass for amorphous carbon superlattices as m * = 0.067m e . This effective mass makes diamondlike carbon films as feasible candidate for electronic devices. © 2006 American Institute of Physics. ͓DOI: 10.1063/1.2188593͔ Diamond-like carbon films have optical band gaps from 1.2-4.0 eV controllable through the deposition parameters, such as the plasma power. 1 They are an attractive option for the semiconductor industry due to the inexpensive, fast and reliable manufacturing method. When the deposition parameter is varied cyclically during deposition, we obtain a bandgap modulation in an essentially homogenous material system, 2,3 leading to higher device speeds and controllable electronic properties. The confining potential of a band-gapmodulated artificial structure on the electron wave function leads to the quantization of the particle momentum and energy, controlled by the well's width and depth and the number of alternating barrier and well layers. 4,5 This can lead to the tailoring of devices for specific applications, such as frequency generators for the mobile phone industry. The design and testing of manufacturing processes for superlattices require not only the ability to characterize the morphology of the individual layers, but also their individual electronic properties. This places electron energy loss spectroscopy ͑EELS͒ in a transmission electron microscope ͑TEM͒ in the unique position of providing all this information. Here, we use an alternative method of acquiring spectral information on a subnanometer scale, and then set the theoretical basis for interpreting the measured collective excitation on the subnanometer scale. We then extract the tunneling effective mass by modeling the changes measured in the collective excitations using the "particle-in-a-box" quantum confinement model.In an electron microscope, it is possible to image the barrier and well layers constituent of a superlattice, even when their respective physical properties are very similar, at large defoci ͓Fig. 1͑a͔͒. 6 Energy loss spectroscopic profiling provides spatially resolved energy-loss spectra across linear features, under parallel illumination. 7 This method utilizes the manner in which a Gatan imaging filter forms images and energy-loss spectra, by collecting two-dimensional data sets with one axis, the energy loss and the other axis the spatial dimension norma...

Section: Figmentioning

confidence: 94%

“…2,3 The nominal dimensions of the sample are contained in its name, where B140W80 signifies a barrier width of 140 Å and a well width of 80 Å, respectively. Cross-sectional samples for TEM were prepared by mechanical thinning and polishing, followed by ion beam polishing.…”

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confidence: 99%

Subnanometer-resolved measurement of the tunneling effective mass using bulk plasmons

Stolojan

Moreau

Goringe

et al. 2006

“…It can even contribute to the tunneling current at room temperatures showing negative differential resistance ͑NDR͒ features. 12 Also, if we consider variations of the thickness of carbon films, then the thinnest part of the films can be considered for highest T t . Therefore, for b ϳ 1 V and d Ͻ 10 nm, up to 10 −6 of T t can be achieved.…”

Section: ͑1͒mentioning

confidence: 99%

“…8 Hence, direct tunneling signatures in carbon MIS structures at low bias or strong charge inversion at high bias has not been reported. 5,[9][10][11][12][13][14][15] In practice, the well-known negative differential conductance in oxide based MIS tunnel diodes or transistors has been suppressed due to the contribution of a large density of interface trap states 16,17 and limits high frequency operation. We have recently demonstrated negative differential resistance in a-C quantum well structures.…”

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confidence: 99%

Demonstration of an amorphous carbon tunnel diode

Bhattacharyya

Silva

2007

Self Cite

Negative differential conductance in metal/amorphous nitrogenated carbon ͑a-CN x ͒ / Si structures is demonstrated at room temperature. These metal-insulator-semiconductor tunnel diodes are fabricated by optimizing the tunnel barrier at the a-CN x / Si junction through the control of the band gap and nitrogen doping level in carbon where this a-C layer acts as a semi-insulator. A small electron tunneling effective mass of about 0.06 times the free electron mass, a coherence length of ϳ10 nm in these thin a-CN x layers and a low interface trap density suggest fast device operation similar to classical tunnel diodes. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2454512͔ Fabrication of tunnel junctions for fast devices has been attempted using a number of different graphitic carbon nanostructures including nanotubes, fullerenes, nanostructured amorphous carbon ͑a-C͒, and nanocrystalline diamond for large area electronics.1-5 However, it is somewhat surprising that the use of amorphous diamondlike carbon ͑DLC͒ films having high breakdown voltage and a low dielectric constant together with the high thermal stability for metal/insulator/ semiconductor ͑MIS͒ diodes and MIS transistors has not been fully explored. 5,6 Previously, by employing insulating DLC films or metallic graphitic films, heterojunctions have been created on crystalline silicon.6,7 However, charge accumulation at the interface or its leakage to the metal electrode without accumulation resulted due to the high defect density of states present in the films. 8 Hence, direct tunneling signatures in carbon MIS structures at low bias or strong charge inversion at high bias has not been reported. 5,[9][10][11][12][13][14][15] In practice, the well-known negative differential conductance in oxide based MIS tunnel diodes or transistors has been suppressed due to the contribution of a large density of interface trap states 16,17 and limits high frequency operation. We have recently demonstrated negative differential resistance in a-C quantum well structures. 18 In this letter, we explore the properties of a-CN x / Si devices grown at room temperature by carefully controlling the band gap, density of states at the Fermi level ͑E F ͒, and height of the tunnel barrier. In these a-CN x / Si heterostructures, we show features similar to those observed in state of the art direct tunnel devices with negative differential conductance. 16The samples were prepared by ablation of a high purity graphite target ͑99.99%͒ using a 248 nm pulsed UV excimer laser ͑Lambda Physik LPX 210i͒. By choosing different laser fluences ͑4 and 16 J / cm 2 ͒ and partial pressures of nitrogen ͑5 and 100 mT͒ in the deposition chamber, films A and B were fabricated. This gives a large variation of sp 3 bonded carbon and nitrogen doping levels in the samples, which helps control the band gap ͑E g ͒ in the range of 1 -2.5 eV, respectively. These samples, up to 20 nm thick, were deposited at room temperature on n-type Si ͑111͒ wafers ͑resistiv-ity ϳ0.01 ⍀ cm͒ to make a-CN x / Si heterost...

“…14,15 Higher values of 1 -10 cm 2 V −1 s −1 have also been observed under special conditions of nanostructuring in the a-CN x films 16 and under superlattice conditions. 17 The small variation in the data as a function of substrate temperature is expected with the evolution of the bonding, especially the C-N sp 2 lone pair states and band gap. The variation is small, and we believe this could be due to the little variation observed in the unpaired defects measured by ESR.…”

mentioning

confidence: 90%

Highly photoconductive amorphous carbon nitride films prepared by cyclic nitrogen radical sputtering

Katsuno

Nitta

Habuchi

et al. 2004

We report on the growth of amorphous carbon nitride films ͑a-CN x ͒ showing the highest conductivity to date. The films were prepared using a layer-by-layer method ͑a-CN x :LL͒, by the cyclical nitrogen radical sputtering of a graphite radical, alternated with a brief hydrogen etch. The photosensitivity S of these films is 10 5 , defined as the ratio of the photoconductivity p to the dark conductivity d and is the highest value reported thus far. We believe that the carriers generated by the monochromatic light (photon energy 6.2 eV) in the a-CN x : LL films are primarily electrons, with the photoconductivity shown to increase with substrate deposition temperature. The family of tetrahedral amorphous carbon films has shown promise toward device applications based on luminescence. In particular, hydrogenated tetrahedral amorphous carbon has shown relatively good photoconducting properties, 1-3 which can be further improved by the addition of impurities such as nitrogen. 1,[4][5][6] The resulting amorphous carbon nitride films ͑a-C:N x ͒ show interesting properties, including electroluminescence, 7 photoluminescence, 8 low dielectric constant 9 and detection capability for heavy-ion particle. 10 The layer-by-layer amorphous carbon nitride films ͑a-CN x :LL͒ prepared in this study show a two to three orders of magnitude improvement in photoconductivity reported for any form of a-C films. We investigated the density , sp 3 fraction and nitrogen concentration [obtained using an electron-energy-loss spectroscopy (EELS)], the optical energy gap E 04 , the Urbach energy E U [calculated from photothermal deflection spectroscopy (PDS)], and defect density [from electron spin resonance (ESR)] as a function of substrate temperature T S during sputtering, in order to examine the thermal evolution of the growth and structure affecting the photoconductivity of these films.The a-CN x : LL films were prepared by a cyclic process of a deposition step of thin a-CN x film by a nitrogen radical sputtering, immediately followed by a hydrogen treatment step that helps to reconstruct the bulk material; this process is repeated until the required thickness of film is grown. The hydrogen treatment on the a-CN x films helps to reduce the defect density and remove weak C dangling bonds without the inclusion of significant atomic hydrogen. 4 The a-CN x films were prepared using sputtering in a 13.56 MHz rf system, at a power of 85 W, a substrate temperature T S that varies from room temperature (RT) to 400°C, and a N 2 sputter gas pressure of 0.12 Torr (gas purity of 99.999%). For the hydrogen treatment step, we used a glow discharge of H 2 at 0.50 Torr (gas purity 99.999%) at a rf power of 85 W at T S . The a-CN x films initially grown are reduced with structural reconstruction during this phase. Typical film thicknesses per layer for a single-sputter growth and etching cycle are estimated to be 15 and 5 nm, respectively, for a growth time of 3 min, followed by H 2 etching for 40 s. We ran seven cycles, resulting in a film thickness of 70 ...