Search

A significant increase of the drain current is verified if defined numbers and types of dislocations are present in the channel of MOSFETs. For pMOSFETs, analysed here, an enhancement by a factor of eight exists if mixed dislocations are placed in the channel. The drain current increase is caused by higher concentration and higher mobility of holes on dislocations. It is shown that cores of mixed dislocations possess uniaxial compressive strain components in the order of e ≅ –0.1 which are significantly higher than in the strain field surrounding a dislocation. The exceptional high uniaxial strain results in dramatic alterations of the silicon band structure. Upward shifts of the upper valence bands appear forming a quantum wire. The generation of the quantum wire forces hole confinement along dislocations and generates a one-dimensional hole gas (1DHG). Confinement and energy quantization are assumed to be most important for the increased carrier transport along dislocations. (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Dislocations are elementary crystal defects. Their dimensions of about 1 nm2 (cross-section area) and lengths of a few micrometers characterize the defects itself as (native) nanostructures embedded in a perfect crystalline matrix. The structure and properties of dislocations in silicon have been studied for more than five decades by numerous analytical technics. Introducing dislocations into electronic devices result in significant effects on device parameters. A detrimental effect was proved if dislocations are generated in an uncontrolled manner by high-temperature treatments during processing. Therefore they are avoided. A beneficial effect, however, is obtained if dislocations are generated far from device active areas. This effect is used, for example, as internal gettering of impurities. Furthermore, measurements on individual dislocations exhibit several extraordinary optical and electronic properties. For instance, an increase of the electrical conductivity of about four orders of magnitude compared to that of bulk silicon was measured along dislocations. If these distinguished properties are usable, MOSFETs with dimensions below 10 nm and high ION currents are feasible. Their realization, however, requires the defined and reproducible generation of dislocation arrays. The present paper describes a technique to generate defined dislocation arrays, summarizes new results about the properties of individual dislocations, and demonstrates the effect of dislocations on the performance of devices. A smart technique to realize two-dimensional arrays of dislocations is semiconductor wafer direct bonding. The technique allows one not only to control the type of the dislocations in the array, but also to adjust the distance between defects making it possible to analyze individual dislocations. In addition, the realization of dislocation arrays in thin SOI layers eliminates the interaction with bulk defects. Measurements on a few or individual dislocations proved their extraordinary properties caused by the nanowire behavior. Fig. 1 shows the electroluminescence spectrum of a pn-LED with dislocations at room temperature. The D1 to D3 lines in the spectrum represent the strong radiative recombination by dislocations, while band-to-band (BB) excitation is suppressed by using thin SOI layers. Previous investigations showed that the appearance of the D-lines depends on the dislocation type making it possible to configure mono- or polychromatic light emitters. Moreover, the wavelengths of the D-lines between 1300 nm and 1550 nm, their reasonable efficiency, and CMOS-compatible fabrication qualify dislocation networks as promising candidates for silicon-based, on chip light emitters for optical data communication. The incorporation of dislocations in the channel of SOI MOSFETs, Tunnel-FETs, and pn-diodes enables a detailed characterization of electronic properties of the defects. Because dislocations are parallel to - directions, they are parallel to the channel orientation and source and drain form direct contacts. An increase of the drain current (ID) by a factor of 50 is obtained if dislocations are present in the channel of nMOSFETs (Fig. 2). This suggests a preferred electron transport along dislocations. Decreasing the number of dislocations result in an exceptional increase of ID. A current density of J = 3.8∙1012 A/cm2 is measured for a single dislocation, which is consistent with a resistivity of r = 1∙10-8 Wcm. This corresponds to supermetallic properties of dislocations. Reasons of the exceptional behavior are discussed in combination with 2- and 3-dimensional device simulations using classical and quantum mechanical simulation paths. Further details about the electronic properties of dislocations are obtained by low-temperature measurements. The existence of a 1-dimensional electron gas on dislocations is proved. This, for instance, is the reason for Coulomb blockades characterizing single electron transitions. Concepts of the use of the exceptional electronic properties of dislocations in nanometer-scaled devices are figured.

Dislocations are basic crystal defects and represent one-dimensional native nanostructures embedded in a perfect crystalline matrix. Their structure is predefined by crystal symmetry. Two- dimensional, self-organized arrays of such nanostructures are realized reproducibly using specific preparation conditions (semiconductor wafer direct bonding). This technique allows separating dislocations up to a few hundred nanometers which enables electrical measurements of only a few, or, in the ideal case, of an individual dislocation. Electrical properties of dislocations in silicon were measured using MOSFETs as test structures. It is shown that an increase of the drain current results for nMOSFETs which is caused by a high concentration of electrons on dislocations in p-type material. The number of electrons on a dislocation is estimated from device simulations. This leads to the conclusion that metallic-like conduction exists along dislocations in this material caused by a one-dimensional carrier confinement. On the other hand, measurements of pMOSFETs prepared in n-type silicon proved the dominant transport of holes along dislocations. The experimentally measured increase of the drain current, however, is here not only caused by an higher hole concentration on these defects but also by an increasing hole mobility along dislocations. All the data proved for the first time the ambipolar behavior of dislocations in silicon. Dislocations in p-type Si form efficient one-dimensional channels for electrons, while dislocations in n-type material cause one- dimensional channels for holes.

A significant increase of the drain current appears if defined arrangements of dislocations are present in the channel of MOSFETs. Furthermore, analyses of the electronic properties of individual defects refer to a supermetallic behavior of dislocations. The reason is the extremely high strain in the dislocation core exceeding values of ϵ ≅ 0.1. Such high strain causes substantial changes of the band structure and means that dislocations represent quantum wires. Quantum mechanical device simulations based on this conclusion demonstrated the transport of carriers on dislocations. The effect of gate voltage and strain in the dislocation core was analyzed in detail.

Two-dimensional arrangements of well-controlled numbers and types of dislocations were realized in thin SOI device layers by semiconductor wafer bonding. Remarkable improvements of nMOSFET performance are obtained if the defects are positioned on well-defined locations in the channel. An increase of the drain current (ID) was proved by one order of magnitude at room temperature. The increase of ID depends on the dislocation type. Mixed dislocations result in the highest increase of the drain current. Such MOSFETs operate also at cryogenic temperatures with excellent parameters (subthreshold swing of 21 mV/dec, etc.). Coulomb blockades related to dislocations are observed making it possible to realize single electron transistors for future cryogenic applications.

Low-temperature measurements proved the existence of a two-dimensional electron gas at defined dislocation arrays in silicon. As a consequence, single-electron transitions (Coulomb blockades) are observed. It is shown that the high strain at dislocation cores modifies the band structure and results in the formation of quantum wells along dislocation lines. This causes quantization of energy levels inducing the formation of Coulomb blockades.

A detailed knowledge of the electronic properties of individual dislocations is necessary for next generation nanodevices. Dislocations are fundamental crystal defects controlling the growth of different nanostructures (nanowires) or appear during device processing. We present a method to record electric properties of single dislocations in thin silicon layers. Results of measurements on single screw dislocations are shown for the first time. Assuming a cross-section area of the dislocation core of about 1 nm2, the current density through a single dislocation is J = 3.8 × 1012 A/cm2 corresponding to a resistivity of ρ ≅ 1 × 10−8 Ω cm. This is about eight orders of magnitude lower than the surrounding silicon matrix. The reason of the supermetallic behavior is the high strain in the cores of the dissociated dislocations modifying the local band structure resulting in high conductive carrier channels along defect cores.

Dislocations exhibit a number of exceptional electronic properties resulting in a significant increase in the drain current of MOSFETs if defined numbers of these defects are placed in the channel. Measurements on individual dislocations in Si refer to a supermetallic conductivity. A model of the electronic structure of dislocations is proposed based on experimental measurements and tight-binding simulations. It is shown that the high strain level on the dislocation core—exceeding 10 % or more—causes locally dramatic changes in the band structure and results in the formation of a quantum well along the dislocation line. This explains experimental findings (two-dimensional electron gas, single-electron transitions). The energy quantization within the quantum well is most important for supermetallic conductivity.