Monolayer to multilayer nanostructural growth transition in N-type oligothiophenes on Au(111) and implications for organic field-effect transistor performance

The evolution in growth morphology and molecular orientation of n-type semiconducting alpha,omega-diperfluorohexyl-quaterthiophene (DFH-4T) on Au(111) is investigated by scanning tunneling microscopy and scanning tunneling spectroscopy as the film thickness is increased from one monolayer to multilayers. Monolayer-thick DFH-4T films are amorphous and morphologically featureless with a large pit density, whereas multilayer films exhibit drastically different terraced structures consisting of overlapping platelets. Large changes in DFH-4T molecular orientation are observed on transitioning from two to four monolayers. Parallel electrical characterization of top-versus-bottom contact configuration DFH-4T FETs with Au source/drain electrodes reveals greatly different mobilities (mu(TOP) = 1.1 +/- 0.2 10(-2) cm(2)V(-1)s(-1) versus mu(BOTTOM) = 2.3 +/- 0.5 10(-5) cm(2)V(-1)s(-1)) and contact resistances (R(C-TOP) = 4-12 MOmegacm vs R(C-BOTTOM) > 1 GOmegacm). This study provides important information on the organic semiconductor-source\drain electrode interfaces and explains why top-contact OFET devices typically have superior performance. By direct visualization, it demonstrates that the DFH-4T film growth transition from monolayer to multilayer on Au is accompanied by dramatic morphology and molecular orientation changes, starting from an amorphous, pitted, and disordered monolayer, to crystalline and smooth bi/tetralayers but with the molecules reoriented by 90 degrees . These chemisorption-derived inhomogenities at the contact-molecule interface and the large monolayer --> multilayer --> bulk microstructural changes are in accord with the large bottom-contact device resistance and poor OFET performance.     

Electrical isolation of dislocations in Ge layers on Si(001) substrates through CMOS-compatible suspended structures

Abstract

Suspended crystalline Ge semiconductor structures are created on a Si(001) substrate by a combination of epitaxial growth and simple patterning from the front surface using anisotropic underetching. Geometric definition of the surface Ge layer gives access to a range of crystalline planes that have different etch resistance. The structures are aligned to avoid etch-resistive planes in making the suspended regions and to take advantage of these planes to retain the underlying Si to support the structures. The technique is demonstrated by forming suspended microwires, spiderwebs and van der Pauw cross structures. We finally report on the low-temperature electrical isolation of the undoped Ge layers. This novel isolation method increases the Ge resistivity to 280 Ω cm at 10 K, over two orders of magnitude above that of a bulk Ge on Si(001) layer, by removing material containing the underlying misfit dislocation network that otherwise provides the main source of electrical conduction.     

Electrical isolation of dislocations in Ge layers on Si(001) substrates through CMOS-compatible suspended structures

Suspended crystalline Ge semiconductor structures are created on a Si(001) substrate by a combination of epitaxial growth and simple patterning from the front surface using anisotropic underetching. Geometric definition of the surface Ge layer gives access to a range of crystalline planes that have different etch resistance. The structures are aligned to avoid etch-resistive planes in making the suspended regions and to take advantage of these planes to retain the underlying Si to support the structures. The technique is demonstrated by forming suspended microwires, spiderwebs and van der Pauw cross structures. We finally report on the low-temperature electrical isolation of the undoped Ge layers. This novel isolation method increases the Ge resistivity to 280 Ω cm at 10 K, over two orders of magnitude above that of a bulk Ge on Si(001) layer, by removing material containing the underlying misfit dislocation network that otherwise provides the main source of electrical conduction.     

Electrical characterization of Ge microcrystallites by atomic force microscopy using a conducting probe

We have studied the nucleation and growth of Ge microcrystallities on Si(100) or evaporated Cr substrates from an rf glow discharge decomposition of GeH₄ highly-diluted with H₂, where the crystallinity, the surface microroughness and the local electric transport of the films have been measured as a function of the film thickness. For the film growth thicker than ∼65 nm, Raman scattering spectra show that the evolution of the microcrystalline phase tends to be saturated. In the thickness range of 7-65 nm, the nucleation and/or microcrystalline grain formation with progressive film growth and corresponding significant difference in the electrical conductivity in the direction of the film thickness between the grains and their boundaries have been demonstrated from topographic and current images taken simultaneously by an atomic force microscope with a conducting probe.     

Si(111)衬底上6H-SiC薄膜的低压化学气相外延生长与表征

在Si(111)衬底上,采用SiH4-C3H8-H2气体反应体系,通过低压化学气相沉积(LPCVD)工艺外延出结晶质量良好的SiC薄膜.低温光致发光谱表明该薄膜属于6H-SiC多型体.X射线衍射图表明该薄膜具有高度的择优取向性.扫描电子显微镜图表明该薄膜由片状SiC晶粒组成.拉曼光谱和透射电子衍射谱的结果进一步表明该薄膜具有较高的结晶质量.对Si(111)衬底上6H-SiC薄膜的生长机制进行了初步探讨.     

Epitaxial growth of cadmium sulphide on (111) germanium substrates

Single-crystal expitaxial layers of CdS on (111) Ge substrates, 8 to 60 μm thick, have been grown from the vapour phase in a closed-tube system. Hydrogen was used as a transport agent. The experimental conditions (source and deposit temperatures, and initial pressure of hydrogen) have been defined where the growth of single-crystal expitaxial layers is feasible. Observations on the morphology of the layers are reported, which suggest that at least two different growth mechanisms should be active in the system. Finally, the composition of the gaseous phase was calculated by assuming a non-reactivity of the Ge substrate with the vapour phase.     

Dissolution kinetics of Si into Ge (111) substrate on the nanoscale

In this paper we present experiments and simulations on the dissolution of Si into single crystalline Ge(111) substrates. The interface shift during the dissolution was tracked by X-ray Photoelectron Spectroscopy. It was obtained that the interface remained sharp and shifted according to anomalous kinetics similarly to our previous measurement in the Si/amorphous-Ge system. The interface shift, x, can be described by a power function of time x ∝ t^k_c with a kinetic exponent, k_c, of 0.85 ± 0.1, larger than the one measured for the amorphous system (0.7 ± 0.1). Both exponents, however, are different from the k_c = 0.5 Fickian (parabolic) value and it is interpreted as a nanoscale diffusional anomaly caused by the strong composition dependence of the diffusion coefficients.     

Electrical and structural characterization of AlGaN/GaN field-effect transistors with recessed gate

Performance of AlGaN/GaN heterostructure field-effect transistors (HFETs) with recessed gate was investigated and compared with non-recessed counterparts. Optimal dry etch conditions by plasma assisted Ar sputtering were found for ∼6 nm gate recess of a 20 nm thick AlGaN barrier layer. A decrease of the residual strain after the gate recessing (from −0.9 GPa to −0.68 GPa) was evaluated from the photoluminescence measurement. The saturation drain current at the gate voltage V_G = 1 V decreased from 1.05 A/mm to 0.85 A/mm after the recessing. The gate voltage for a maximal transconductance (240−250 mS/mm) has shifted from −3 V for non-recessed HFETs to −0.2 V for recessed counterparts. Similarly, the threshold voltage increased after the gate recessing. A decrease of the sheet charge density from 1 × 10¹³ cm⁻² to 4 × 10¹² cm⁻² at V_G = 0 V has been evaluated from the capacitance measurements. The RF measurements yielded a slight increase of the cut-off frequencies after the gate recessing. All these indicate that the gate recessing is a useful tool to optimize the AlGaN/GaN HFET performance for high-frequency applications as well as for the preparation of normally-off devices.     

Controllable electrodeposition of ordered carbon nanowalls on Cu(111) substrates

We report the growth of amorphous carbon nanowalls with molten salt electrolytes and a carbonate carbon source at 600 °C on home-made Cu(111) foil as the growth substrate (and cathode). The nanometer thick nanowalls grow preferentially along symmetric slip lines on the Cu(111) surface and their ordered arrangement appears to also be dictated by the electrosynthesis parameters. Computational chemistry suggests that nucleation of carbon growth is favored at the slip lines (atomic steps) of the Cu(111) surface. The electrodeposited carbon structures can be varied by tuning the potential on the electrodes and temperature of the molten salt. The macro, micro, and nanoscale structure of the nanowalls was studied and is reported.     

Structural control over self-assembled crystals of pi--conjugated poly(3,3'-didodecyl-quaterthiophene) for organic field-effect transistor applications

Liquid-crystalline (LC) poly(3,3'-didodecyl-quaterthiophene) (PQT-12) was investigated to optimize its side alkyl ordering and pi-conjugated structure for high performance organic field-effect transistor (OFET) applications. Initially, low-crystal films spun-cast on OTS-treated SiO2 substrates were further crystallized via either thermal or solvent treatments. At temperatures (125-138 degrees C) driving a LC state of PQT-12, mobile chains were better migrated into preformed crystals. The resulting films showed highly crystal nanofibrils, in which pi-conjugated polymer backbones (with an conjugated backbone spacing, d(010) of 3.80 A) and the orientation of self-assembled side-chains were tilted with respect to the film surface, respectively. However, via melting or solvent exposure providing isotropic states, film crystallization generated less ordered crystals with randomly oriented side-chains, increasing d(010) up to 4.16 A. As a result, the usage of LC characteristic allowed us to consistently achieve the desirable crystal structure of PQT-12 and to robustly obtain high field-effect mobility for FET applications.     

Electrical Resistance of W/Si–Ge (0–75 wt % Ge) Contacts

The electrical resistance of the contact between tungsten and silicon–germanium alloys of different compositions was measured as a function of temperature. The results indicate that the room-temperature contact resistance increases with increasing Ge content, annealing temperature, and annealing time. With increasing measuring temperature, the contact resistance drops to a level of the experimental error. Annealing at 1070 K tends to break down both W/p-Si–Ge and W/n-Si–Ge contacts.     

Ordering of Ge islands on Si(001) substrates patterned by nanoindentation

Spatial organization of Ge islands, grown by physical vapor deposition, on prepatterned Si(001) substrates has been investigated. The substrates were patterned prior to Ge deposition by nanoindentation. Characterization of Ge dots is performed by atomic force microscopy and scanning electron microscopy. The nanoindents act as trapping sites, allowing ripening of Ge islands at those locations during subsequent deposition and diffusion of Ge on the surface. The results show that island ordering is intrinsically linked to the nucleation and growth at indented sites and it strongly depends on pattern parameters.     

Separation of silicon wafers by the smart-cut method

Great efforts have been made for many years to develop methods of achieving thin monocrystalline layers of semiconductor material. The Smart-Cut^® process is presented here, a generic process enabling practically any type of monocrystalline layer to be achieved on any type of support. The Smart-Cut^® process is based on proton implantation and wafer bonding. Proton implantation enables delamination of a thin layer from a thick substrate to be achieved whereas the wafer bonding technique enables different multilayer structures to be achieved by transferring the delaminated layer onto a second substrate. The physical mechanisms involved in the delamination process are discussed based on the study of proton-induced microcavity formation during implantation and growth during annealing. It is shown that this industrially economic process is particularly well suited to achieving very high-quality SOI material. Other examples of industrially developed applications of the process are also given.     

SiO2/Si(111)表面Ge量子点的生长研究

Si衬底用化学方法清洗后,表面大约残余1.0 nm厚SiO2薄膜.利用原子力显微镜(AFM)和反射高能电子衍射(RHEED)来研究温度和Ge蒸发厚度对在SiO2薄膜表面生长的Ge量子点的影响.实验结果表明,当衬底温度超过500 ℃时,SiO2开始与Ge原子发生化学反应,并形成与Si(111)表面直接外延的Ge量子点.在650 ℃时,只有Ge的厚度达到0.5nm时,Ge量子点才开始形成.     

Electrical transport of bottom-up grown single-crystal Si(1-x)Ge(x) nanowire

In this work, we fabricated an Si(1-x)Ge(x) nanowire (NW) metal-oxide-semiconductor field-effect transistor (MOSFET) by using bottom-up grown single-crystal Si(1-x)Ge(x) NWs integrated with HfO(2) gate dielectric, TaN/Ta gate electrode and Pd Schottky source/drain electrodes, and investigated the electrical transport properties of Si(1-x)Ge(x) NWs. It is found that both undoped and phosphorus-doped Si(1-x)Ge(x) NW MOSFETs exhibit p-MOS operation while enhanced performance of higher I(on)～100?nA and I(on)/I(off)～10(5) are achieved from phosphorus-doped Si(1-x)Ge(x) NWs, which can be attributed to the reduction of the effective Schottky barrier height (SBH). Further improvement in gate control with a subthreshold slope of 142?mV?dec(-1) was obtained by reducing HfO(2) gate dielectric thickness. A comprehensive study on SBH between the Si(1-x)Ge(x) NW channel and Pd source/drain shows that a doped Si(1-x)Ge(x) NW has a lower effective SBH due to a thinner depletion width at the junction and the gate oxide thickness has negligible effect on effective SBH.     

Optical properties of GaN epilayers grown on Si (111) and Si (001) substrates

We report the observation of bright photoluminescence (PL) emission from two types of GaN epilayers grown by molecular beam epitaxy (MBE). Wurtzite phase GaN/Si (111) epilayers are grown by gas source MBE process, whereas cubic phase GaN epilayers are grown on (001) Si covered by thin SiC film in the process of Si annealing in propane prior to the GaN growth. PL emissions are identified based on the results of detailed PL and time-resolved PL investigations. For the wurtzite phase GaN we observe an efficient up in the energy transfer from bound to free excitons. This process is explained by a large difference in the PL decay times for two types (free and bound (donor, acceptor)) of excitonic PL emissions. For cubic phase GaN we confirm recent suggestion that acceptors have smaller thermal ionization energies than those in the wurtzite phase GaN.     

Gas-source molecular beam epitaxy of Si(111) on Si(110) substrates by insertion of 3C-SiC(111) interlayer for hybrid orientation technology

A method to realize a novel hybrid orientations of Si surfaces, Si(111) on Si(110), has been developed by use of a Si(111)/3C-SiC(111)/Si(110) trilayer structure. This technology allows us to use the Si(111) portion for the n-type and the Si(110) portion for the p-type channels, providing a solution to the current drive imbalance between the two channels confronted in Si(100)-based complementary metal oxide semiconductor (CMOS) technology. The central idea is to use a rotated heteroepitaxy of 3C-SiC(111) on Si(110) substrate, which occurs when a 3C-SiC film is grown under certain growth conditions. Monomethylsilane (SiH₃-CH₃) gas-source molecular beam epitaxy (GSMBE) is used for this 3C-SiC interlayer formation while disilane (Si₂H₆) is used for the top Si(111) layer formation. Though the film quality of the Si epilayer leaves a lot of room for betterment, the present results may suffice to prove its potential as a new technology to be used in the next generation CMOS devices.     

Structure and optical spectrum of GaN nanorods produced on Si(111) substrates

A novel method is applied to prepare nanorods. In this method, nanorods have been successfully synthesized on Si(111) substrates through annealing sputtered Ga₂O₃/Nb films under flowing ammonia at 950 °C in a quartz tube. The as-synthesized nanorods are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), and photoluminescence (PL) spectra. The results show that the nanorod is single-crystalline GaN. It has a diameter of about 200 nm and lengths typically up to several micrometers. Photoluminescence spectrum under excitation at 325 nm only exhibits a UV light emission peak is located at about 368.5 nm. Finally, the growth mechanism of nanorods is also briefly discussed.     

The study of the interactions between graphene and Ge(001)/Si(001)

The interaction between graphene and germanium surfaces was investigated using a combination of microscopic and macroscopic experimental techniques and complementary theoretical calculations.Density functional theory (DFT) calculations for different reconstructions of the Ge(001) surface showed that the interactions between graphene and the Ge(001) surface introduce additional peaks in the density of states,superimposed on the graphene valence and conduction energy bands.The growth of graphene induces nanofaceting of the Ge(001) surface,which exhibits well-organized hill and valley structures.The graphene regions covered by hills are of high quality and exhibit an almost linear dispersion relation,which indicates weak graphene-germanium interactions.On the other hand,the graphene component occupying valley regions is significantly perturbed by the interaction with germanium.It was also found that the stronger graphene-germanium interaction observed in the valley regions is connected with a lower local electrical conductivity.Annealing of graphene/Ge(001)/Si(001) was performed to obtain a more uniform surface.This process results in a surface characterized by negligible hill and valley structures;however,the graphene properties unexpectedly deteriorated with increasing uniformity of the Ge(001) surface.To sum up,it was shown that the mechanism responsible for the formation of local conductivity inhomogeneities in graphene covering the Ge(001) surface is related to the different strength of graphene-germanium interactions.The present results indicate that,in order to obtain high-quality graphene,the experimental efforts should focus on limiting the interactions between germanium and graphene,which can be achieved by adjusting the growth conditions.     

Growth of GaAs nanowhisker arrays by magnetron sputtering on Si(111) substrates

The growth of GaAs nanowhisker (NW) arrays on Si(111) substrates by magnetron sputtering is demonstrated. The characteristic NW length is proportional to the effective thickness of a deposited layer and inversely proportional to the transverse whisker size at the top. The results are explained in terms of the diffusion model of NW growth.