Transfer Doping Research Articles

Surface transfer doping of hydrogen-terminated diamond by C60F48: Energy level scheme and doping efficiency

Surface sensitive C1s core level photoelectron spectroscopy was used to examine the electronic properties of C(60)F(48) molecules on the C(100):H surface. An upward band bending of 0.74 eV in response to surface transfer doping by fluorofullerene molecules is measured. Two distinct molecular charge states of C(60)F(48) are identified and their relative concentration determined as a function of coverage. One corresponds to ionized molecules that participate in surface charge transfer and the other to neutral molecules that do not. The position of the lowest unoccupied molecular orbital of neutral C(60)F(48) which is the relevant acceptor level for transfer doping lies initially 0.6 eV below the valence band maximum and shifts upwards in the course of transfer doping by up to 0.43 eV due to a doping induced surface dipole. This upward shift in conjunction with the band bending determines the occupation of the acceptor level and limits the ultimately achievable hole concentration with C(60)F(48) as a surface acceptor to values close to 10(13) cm(-2) as reported in the literature.

Chemically modified multilayer graphene with metal interlayer as an efficient current spreading electrode for InGaN/GaN blue light-emitting diodes

This paper describes a detailed systematic study based on the fabrication and performance of InGaN/GaN blue light-emitting diodes (LEDs) with multilayer graphene film as a current spreading electrode. Two facile approaches to improve the electrical coupling between graphene and p-GaN layer are demonstrated. Using chemical charge transfer doping, the work function (Φ) of graphene is tuned over a wide range from 4.21 to 4.93 eV with substantial improvements in sheet resistance (Rs). Compared with pristine graphene, the chemically modified graphene on p-GaN yields several appealing characteristics such as low specific contact resistance (ρc) and minimized barrier height. In addition, insertion of a thin gold interlayer between graphene and p-GaN profoundly enhances the contact properties at the interface. Combining these two approaches in a single LED, the current spreading and thus the device forward voltage (Vf) are considerably improved comparable to that of an LED fabricated with an indium tin oxide electrode. The importance of pre-metal deposition oxygen plasma treatment and rapid thermal annealing in improving the contact characteristics is also addressed.

Surface Ferromagnetic p-Type ZnO Nanowires through Charge Transfer Doping

We report first-principles theoretical investigation of p-type charge transfer doping of zinc oxide (ZnO) nanowires by molecular adsorption. We find that spontaneous dissociative adsorption of fluorine molecules introduces half-emptying of otherwise fully filled oxygen-derived surface states. The resulting surface Fermi level is so close to the valence band maximum of the ZnO nanowire that the nanowire undergoes significant p-type charge transfer doping. Those half-filled surface states are fully spin-polarized and lead to surface ferromagnetism that is stable at room temperature. We also analyze the kinetic control regime of the surface transfer doping and find that it may result in nonequilibrium steady states. The present results suggest that postgrowth engineering of surface states has high potential in manipulating ZnO nanostructures useful for both electronics and spintronics.

Fermi-Level Tuning of EpitaxialSb2Te3Thin Films on Graphene by Regulating Intrinsic Defects and Substrate Transfer Doping

High-quality Sb2Te3 films are obtained by molecular beam epitaxy on a graphene substrate and investigated by in situ scanning tunneling microscopy and spectroscopy. Intrinsic defects responsible for the natural p-type conductivity of Sb2Te3 are identified to be the Sb vacancies and Sb(Te) antisites in agreement with first-principles calculations. By minimizing defect densities, coupled with a transfer doping by the graphene substrate, the Fermi level of Sb2Te3 thin films can be tuned over the entire range of the bulk band gap. This establishes the necessary condition to explore topological insulator behaviors near the Dirac point.

Surface transfer doping of diamond with a molecular heterojunction

Surface conductivity and C1s core level measurements were employed to show that surface transfer doping of hydrogen-terminated diamond C(100) can be achieved with a molecular heterojunction formed with C60F48 and an intralayer of zinc-tetraphenylporphyrin. Measurement of the shift in the diamond Fermi energy shows that the zinc-tetraphenylporphyrin (ZnTPP) layer modifies the C60F48–diamond interaction, modulating the extent of charge transfer between the diamond and the fluorofullerene. In contrast to the case of C60F48 acceptors, the presence of a ZnTPP layer prevents the formation of air-induced surface conductivity, showing that the intralayer acts to selectively separate these two doping channels.

Influence of charge transfer doping on the morphologies of C60islands on hydrogenated diamond C(100)-(2×1)

The adsorption and island formation of C${}_{60}$ fullerenes on the hydrogenated C(100)-($2\ifmmode\times\else\texttimes\fi{}1$):H diamond surface is studied using high-resolution noncontact atomic force microscopy in ultrahigh vacuum. At room temperature, C${}_{60}$ fullerene molecules assemble into monolayer islands, exhibiting a hexagonally close-packed internal structure. Dewetting is observed when raising the substrate temperature above approximately 505 K, resulting in two-layer high islands. In contrast to the monolayer islands, these double-layer islands form extended wetting layers. This peculiar behavior is explained by an increased molecule-substrate binding energy in the case of double-layer islands, which originates from charge transfer doping. Only upon further increasing the substrate temperature to approximately 615 K, the wetting layer desorbs, corresponding to a binding energy of the charge transfer-stabilized film of 1.7 eV.

Local charge transfer doping in suspended graphene nanojunctions

We report electronic transport measurements in nanoscale graphene transistors with gold and platinum electrodes whose channel lengths are shorter than 100 nm and compare them with transistors with channel lengths from 1 μm to 50 μm. We find a large positive gate voltage shift in charge neutrality point (NP) for transistors made with platinum electrodes but negligible shift for devices made with gold electrodes. This is consistent with the transfer of electrons from graphene into the platinum electrodes. As the channel length increases, the disparity between the measured NP using gold and platinum electrodes disappears.

Towards industrial applications of graphene electrodes

Since the first isolation of graphene in 2004 by mechanical exfoliation from graphite, many people have tried to synthesize large-scale graphene using various chemical methods. In particular, there has been a great number of advances in the synthesis of graphene using chemical vapor deposition (CVD) on metal substrates such as Ni and Cu. Recently, a method to synthesize ultra-large-scale (∼30 inch) graphene films using roll-to-roll transfer and chemical doping processes was developed that shows excellent electrical and physical properties suitable for practical applications on a large scale. Considering the outstanding scalability/processibility of roll-to-roll and CVD methods as well as the extraordinary flexibility/conductivity of graphene films, we expect that transparent graphene electrodes can replace indium tin oxide in the near future.

Surface charge transfer doping of germanium nanowires by MoO3 deposition

We report on the surface transfer p-type doping of germanium nanowires (GeNWs) via MoO3 thin film deposition. The GeNWs studied were prepared by a conventional thermal evaporation approach. Electrical property analysis shows that the conductance, hole mobility and concentration were all prominently enhanced after MoO3 thin film coating. Such a remarkable surface doping effect can be attributed to the surface charge transfer at the GeNWs/MoO3 interface, which is verified by in situ XPS analysis of GeNWs as a function of increasing MoO3 coverage. Further hole mobility and concentration evolution study of MoO3/GeNWs reveals that the GeNWs embedded in the MoO3 layer can retain their electrical property after storage in air for 3 months. The generality of the above results suggests that the charge transfer doping via surface deposition has great potential in GeNW-based nanoelectronic devices and may also be applicable to the modulation of other semiconductor nanostructures.

Molecular interactions on epitaxial graphene

This paper reviews our recent work on molecular interactions on epitaxial graphene (EG) grown on 6H-SiC(0001). A major challenge in graphene-based devices is opening the energy band gap and doping. Molecular functionalization of graphene is one approach to modifying its electronic properties. Such surface transfer doping relies on charge separation at interfaces, and represents a valuable tool for the controlled and non-destructive doping of graphene, thereby facilitating the development of hybrid organic-graphene nanoelectronics. We have also demonstrated molecular self-assembly of bimolecular systems on EG and graphite.

Raman Doping Profiles of Polyelectrolyte SWNTs in Solution

We present a resonance Raman study of electrochemical charge transfer doping on polyelectrolyte single-walled carbon nanotubes (SWNTs) in solution. Changes in the intensity of the radial breathing modes of well-identified SWNTs are measured as a function of the electrochemical potential. The intensity is maximum when the nanotubes are neutral. Unexpectedly, the Raman signal decreases as soon as charges are transferred to the nanotubes, leading to intensity profiles that are triangular for metallic and trapezoidal for semiconducting nanotubes. A key result is that the width in energy of the plateaus for the semiconducting nanotubes is roughly equal to the optical gap (rather than the free carrier gap). While these experiments can be used to estimate the energy levels of individual nanotubes, strong dynamical screening appears to dominate in individual SWNT polyelectrolytes so that only screened energy levels are being probed.

Mechanism of surface proton transfer doping in pentacene based organic thin‐film transistors

AbstractA number of studies show that chemical modification of the semiconductor–dielectric interface can be used to control the threshold voltage (Vth) of organic thin film transistors (OTFTs). A promising chemical functionality to achieve that are acidic groups, which – at the semiconductor–dielectric interface – have been used to realize chemically responsive OTFTs and easy to fabricate inverter structures. Especially for pentacene based OTFTs, the underlying chemical and physical mechanisms behind the acid‐induced Vth shifts are not yet fully understood. Their clarification is the topic of the present paper. To distinguish between space‐charge and dipole‐induced effects, we study the impact of the thickness of the gate oxide on the device characteristics achieving maximum Vth‐shifts around 100 V. To elucidate the role of the acid, we compare the doping of pentacene by acidic interfacial layers with the impact of hydrochloric acid vapour and investigate the consequences of exposing devices to ammonia. Complementary experiments using 6,13‐bis(triisopropylsilylethynyl)pentacene (TIPS‐pentacene) as active layer hint toward the central (6 and 13) carbon atoms being subject to the electrophilic attack by the acidic protons. They also prove that the observed Vth shifts in pentacene devices are indeed a consequence of the interaction between the acidic groups and the active material. The experimental device characterization is supported by drift‐diffusion based device modelling, by quantum chemically simulations, as well as by contact angle, atomic force microscopy (AFM) and X‐ray reflectivity (XRR). The combination of the obtained results leads us to suggest proton transfer doping at the semiconductor–dielectric interface as the process responsible for the observed shift of Vth.

Erratum: Transfer doping of a metallic carbon nanotube and graphene on metal surfaces [Phys. Rev. B83, 155435 (2011)

In Table I, the values of the equilibrium separation ${d}_{\mathrm{eq}}$ for the graphene on Al, Ag, Cu, and Au surfaces are erroneously taken from those obtained by Khomyakov et al. [Phys. Rev. B 79, 195425 (2009)]. Our results for these surfaces, which are 3.49, 3.35, 3.31, and 3.35 \AA{}, respectively, are slightly larger than those of Khomyakov et al.; this is probably because of the differences in the computational details. The discussions related to ${d}_{\mathrm{eq}}$ for graphene on metal are based on our results and remain unchanged. All the other results, discussions, and conclusions in this paper are also unchanged.

Work function, band bending, and electron affinity in surface conducting (100) diamond

AbstractThe change in electron affinity and band bending as a result of atmosphere and fullerene (C60F48) induced surface conductivity on H‐terminated (100) diamond is determined by simultaneously measuring the work function (ϕ) and C1s core level shift for both intrinsic and boron doped diamond. Changes in work function as a result of surface transfer doping are shown to not only reflect variations in the position of the surface Fermi level as reported by earlier investigations but also changes in the electron affinity, with the latter accounting for up to 0.63 eV of the highest observed change in ϕ of 1.45 eV.

Temperature enhanced gas sensing properties of diamond films

Nanocrystalline diamond (NCD) film was used as a functional part of gas sensor. The gas sensing properties of H-terminated nanocrystalline diamond films were examined to oxidizing gases (i.e., COCl2 and humid air). Pronounced increase in the surface conductivity (3 orders of magnitude) was found after sensor exposure to phosgene gas and was explained by the surface transfer doping effect. We also present a possible way how to achieve sensor selectivity, i.e. how to distinguish between phosgene and humid air (the mostly present background gas in a common environment).

Fluorination of the diamond surface by photoinduced dissociation ofC60F48

The effect of synchrotron beam irradiation on ${\mathrm{C}}_{60}$F${}_{48}$ adlayers absorbed on the hydrogen-terminated C(100) diamond surface has been examined using core level photoelectron spectroscopy. Irradiation causes the dissociation of fluorine from the ${\mathrm{C}}_{60}$F${}_{48}$ and the formation of a fluorine-terminated surface with a (2\ifmmode\times\else\texttimes\fi{}1) surface reconstruction. This is accompanied by a loss in the ${\mathrm{C}}_{60}$F${}_{48}$-induced surface transfer doping of the diamond substrate. The simplicity of this process makes it highly applicable to monofluoride surface patterning for nanoelectronic and surface functionalization applications of diamond.

Potassium-Induced Charge Transfer Effects in Oligo(-phenylene ethynylene)-Based Molecular Layers

Charge transport behavior in molecular monolayers based on oligo-(phenylene ethynylene)s (OPEs) shows distinct variations with substitutions on the OPE framework. Explanations of this dependence often center on the impact of the substituted group’s electron affinity on charge transfer effects. To examine these effects, we used ultraviolet photoemission to follow changes in the electronic structure of OPE-based thiol-linked self-assembled monolayers induced by charge transfer doping with potassium. The impact of OPE (C6H5–C≡C–C6H4–C≡C–C6H5–SH) versus −NO2-substituted OPE (NO2–OPE, C6H5–C≡C–C6H3NO2–C≡C–C6H5–SH) on the electronic structure modifications was compared for a range of K additions. Addition of less than 0.5 K/molecule to self-assembled monolayers produced rigid shifts of molecular levels by about 0.5 eV for both monolayers, with larger shifts at lower K addition for NO2–OPE. Continued addition of K leads to increased shifts for OPE, while saturation of the effect was observed for NO2–OPE. Incorporation of K beyond about 2 K atoms per OPE or NO2–OPE leads to an additional structure, broadening, and changes in the relative spacing between molecular levels probably indicating distortion in the monolayer and also of the molecular backbone. The results show clear influence of −NO2 substitution on K-induced charge transfer effects. The coupled impact of K-induced charge transfer and electrostatic effects from K counterions on molecular level energies is discussed and compared to recent theoretical work.

Transfer doping of a metallic carbon nanotube and graphene on metal surfaces

In this paper we show the results of systematic investigations for the electronic-structure modifications of an armchair (10,10) single-walled carbon nanotube (SWNT) and graphene adsorbed on metal surfaces. The first-principles calculations based on the density-functional theory (DFT) were used to investigate transfer doping of these nanomaterials adsorbed on the fcc (111) surfaces of Al, noble metals (Ag, Cu, and Au), and transition metals (Rh, Pd, Ir, and Pt). We confirmed that the SWNT weakly interacts with the surfaces of Al and the noble metals (physisorption), while it strongly bonds to the transition-metal surfaces (chemisorption). The graphene adsorption on these metal surfaces is reinvestigated and found to be similar except for the Ir and Pt substrates, for which the interaction is weak as in the case of Al and the noble metal substrates. A phenomenological model is also developed on the basis of the rigid-band picture appropriate to physisorption. This model provides a relation between the Fermi-level shift and work-function difference and can conveniently be used in interpreting the DFT results for the transfer doping of both a SWNT and graphene on metal surfaces. We also identify the effect of hybridization between the graphene \ensuremath{\pi} orbitals and metallic valence states on the Fermi-level shift and find that the hybridization induces an extra downward shift of the linear-dispersion bands near the Dirac point relative to the other bands.

Laser transfer doping for contacting n‐type crystalline Si solar cells

AbstractWe demonstrate laser transfer doping (LTD) for producing locally doped regions in surface passivated Silicon wafers. A pulsed laser transfers phosphorus‐doped amorphous silicon from a carrier glass to a surface passivated silicon wafer. We employ this process for producing local high‐low junctions for the formation of n+–n back surface field layers that are contacted with Al. The contact's surface recombination velocities and the contact resistances depend on the laser processing and annealing conditions. They range from Scont = 20 to 1500 cm s−1 and from Rcont = 1 to 1000 mΩ cm2, respectively. The combination of acceptable contact resistance and low recombination velocities makes the LTD technique suitable for the formation of the rear contacts of n‐type high‐efficiency solar cells. The simplicity of the process makes it a candidate for industrial production.

Ambipolar and Unipolar PbSe Nanowire Field-Effect Transistors

Wet-chemical methods, under rigorous air-free conditions, were used to synthesize single-crystalline 10 nm diameter PbSe nanowires (NWs), and electric-field, directed assembly was employed to align NW arrays to form the semiconducting channels of field-effect transistors (FETs). Electrical measurements revealed as-aligned NWs in bottom, gold, contact FETs are predominantly p-type ambipolar, consistent with the presentation of small barriers to electron and hole injection for this low band gap semiconductor. Exposing the NW FET to UV-ozone p-doped the NWs, illustrating the sensitivity of PbSe to oxygen, but controlled oxidation allowed the fabrication of unipolar p-type FETs. Selectively exposing the contact region of as-aligned NW FETs to low to moderate concentrations of hydrazine, commonly used to n-dope nanocrystal and NW devices, switched the predominantly p- to n-type ambipolar behavior as if the entire NW channel was exposed. At these hydrazine concentrations, charge transfer doping the metal-semiconductor interface dominates the FET characteristics. Only upon exposing the NW FETs to high hydrazine concentrations did charge transfer doping of the NW channel overcome the large intrinsic, thermally generated carrier concentration of this low band gap material, modulating the NW carrier concentration and forming unipolar n-type FETs. Pulling low vacuum removed surface hydrazine returning the predominantly p-type ambipolar FET behavior. Doping and dedoping with hydrazine were repeatedly reversible. By applying surface modification to n- and p-dope PbSe NW FETs, we fabricated the first PbSe NW inverters, demonstrating the promise of these nanostructured materials in integrated circuits.