Electronic properties of the organic semiconductor hetero-interface CuPc/C60

We present a study of the electronic properties of the interface between the well-established molecular organic semiconductor copper phthalocyanine (CuPc) and the fullerite C₆₀ using photoelectron spectroscopy and the Kelvin-probe (KP) method. Upon deposition of CuPc on C₆₀, we found interfacial shifts of the vacuum level indicating the formation of a dipole layer, while band bending is found to be negligible. The interface dipole of 0.5 eV measured with KP is close to the difference between the work functions of bulk CuPc and C₆₀. No evidence for a chemical interaction at the interface is concluded from the absence of additional features in the core-level spectra at the earliest stages of deposition. The energy-level alignment diagram at the CuPc/C₆₀ interface is derived.     

Comparative photoemission study of the electronic properties of L-CVD SnO2 thin films

In this work we present the results of comparative XPS and PYS studies of electronic properties of the space charge layer of the L-CVD SnO₂ thin films after air exposure and subsequent UHV annealing at 400 °C, with a special emphasis on the interface Fermi level position.From the centre of gravity of binding energy of the main XPS Sn 3d_5/2 line the interface Fermi level position E_F − E_v in the band gap has been determined. It was in a good correlation with the value estimated from the offset of valence band region of the XPS spectrum, as well as from the photoemission yield spectroscopy (PYS) measurements. Moreover, from the valence band region of the XPS spectrum and PYS spectrum two different types of filled electronic band gap states of the L-CVD SnO₂ thin films have been derived, located at 6 and 3 eV with respect to the Fermi level.     

Investigation of the Shockley surface state on clean and air-exposed Au (1 1 1)

Scanning tunneling microscopy (STM) and spectroscopy (STS) carried out in vacuum and air were used to study the electronic structure of the Au (1 1 1) surface in the range of 0.0-0.7 eV below the Fermi level. The STS experiment carried out in UHV showed the existence of the Shockley surface state (SS) located 0.48 eV below the Fermi level. STS carried out in air showed strong local maximum located 0.35 eV below the Fermi level. This maximum was ascribed to the SS shifted toward lower energy due to carbon and oxygen overlayer. To confirm that the SS could exist on the sample exposed to air we did ultraviolet photoemission spectroscopy (UPS) experiment on air-treated and clean Au (1 1 1). Our results suggest that the SS position initially measured at 0.38 eV below the Fermi level was shifted to 0.27 eV after air treatment. Additionally, the level of contamination was measured using X-ray photoelectron spectroscopy (XPS).     

A study of interface characteristics in HfAlO/p-Si by deep level transient spectroscopy

Deep level transient spectroscopy (DLTS) and high-frequency capacitance-voltage (HF-CV) measurement are used for the investigation of HfAlO/p-Si interface. The so-called “slow” interface states detected by HF-CV are obtained to be 2.68 × 10¹¹ cm⁻². Combined conventional DLTS with insufficient-filling DLTS (IF-DLTS), the true energy level position of interfacial traps is found to be 0.33 eV above the valance band maximum of silicon, and the density of such “fast” interfacial traps is 1.91 × 10¹² cm⁻² eV⁻¹. The variation of energy level position of such traps with different annealing temperatures indicates the origin of these traps may be the oxide-related traps very close to the HfAlO/Si interface. The interfacial traps’ passivation and depassivation effect of postannealing in forming gas are shown by comparing samples annealed at different temperatures.     

Electronic surface states on the MOVPE-prepared InGa-terminated InGaAs(1 0 0) (4 × 2)/c(8 × 2) surface

In this paper, the InGa-terminated InGaAs(1 0 0) (4 × 2)/c(8 × 2) surface was studied in detail, which turned out to be the most suitable to develop an InGaAs/GaAsSb interface that is as sharp as possible. In ultra high vacuum the InGaAs surface was investigated with low-energy electron diffraction, scanning tunneling microscopy and UV photoelectron spectroscopy employing synchrotron radiation as light source. Scanning the Γ−Δ−X direction by varying the photon energy between 8.5 eV and 50 eV, two surface states in the photoelectron spectra were observed in addition to the valence band peaks.     

Band bending and band alignment at HfO2/HfSixOy/Si interfaces

Band bending and band alignment at HfO₂/SiO₂/Si and HfO₂/Hf/SiO₂/Si interfaces were investigated using X-ray photoelectron spectroscopy. After Hf-metal pre-deposition, a 0.55 eV band bending in Si and a 1.80 eV binding energy decrease for Hf 4f and O 1s of HfO₂ were observed. This was attributed to the introduction of negative space charges at interface by Hf pre-deposition. Band bending decrease and synchronous binding energy increases of O 1s and Hf 4f for HfO₂ were observed during initial Ar⁺ sputtering of the Hf pre-deposited sample. This was interpreted through the neutralization of negative space charges by sputtering-induced oxygen vacancies.     

Li adsorption on a Fullerene–Single wall carbon nanotube hybrid system: Density functional theory approach

In this study, we investigate Li adsorption mechanisms on the C₆₀-SWCNT hybrid system using density functional theory. It is found that the Li adsorption energy of the C₆₀-SWCNT hybrid system is increased in comparison to that of the pure SWCNT. The Li adsorption energy ranges from −1.917 eV to −2.642 eV for the single-Li adsorbed system and from −2.351 eV to −2.636 eV for the double-Li adsorbed system. It is also found that the adsorption energy becomes similar at most positions throughout the structure. In addition, the Li adsorption energy of 31-Li system is calculated to be −1.863 eV, which is significantly lower than the Li–Li binding energy (−1.030 eV). These results infer that Li atoms will be adsorbed on the space 1) between C₆₀ and C₆₀; 2) between SWCNT and C₆₀; 3) the rest of the space (e.g. between SWCNTs), rather than form Li clusters. As more Li atoms are adsorbed onto the C₆₀-SWCNT hybrid system due to such improved Li adsorption capability, the metallic character of the system is enhanced, which is confirmed via the band structure and electronic density of states.     

Comparative study of the GaAs(1 0 0) surface cleaned by atomic hydrogen

In attempt to correlate electronic properties and chemical composition of atomic hydrogen cleaned GaAs(1 0 0) surface, high-resolution photoemission yield spectroscopy (PYS) combined with Auger electron spectroscopy (AES) and mass spectrometry has been used. Our room temperature investigation clearly shows that the variations of surface composition and the electronic properties of a space charge layer as a function of atomic hydrogen dose display three successive interaction stages. There exists a contamination etching stage which is observed up to around 250 L of atomic hydrogen dose followed by a transition stage and a degradation stage which is observed beyond 700 L of exposure. In the first stage, a linear shift in the surface Fermi level is observed towards the conduction band by 0.14 eV, in agreement to the observed restoration of the surface stoichiometry and contamination removal. The next stage is characterized by a drop in ionization energy and work function, which quantitatively agrees with the observed Ga-enrichment as well as the tail of the electronic states attributed to the breaking As-dimers. As a result of the strong hydrogenation, the interface Fermi level E_F − E_v has been pinned at the value of 0.75 eV what corresponds to the degradation stage of the GaAs(1 0 0) surface that exhibits metallic density of states associated with Ga_As antisites defects. The results are discussed quantitatively in terms of the surface molecule approach and compared to those obtained by other groups.     

Effect of surface preparation on the properties of Au/p-Cd1−xZnxTe

The effect of bromine methanol (BM) etching and NH₄F/H₂O₂ passivation on the Schottky barrier height between Au contact and semi-insulated (SI) p-Cd_1−xZn_xTe (x ≈ 0.09-0.18) was studied through current-voltage (I-V) and capacitance-voltage (C-V) measurements. Near-infrared (NIR) spectroscopy technique was utilized to determine the Zn concentration. X-ray photoelectron spectroscopy (XPS) for surface composition analysis showed that BM etched sample surface left a Te⁰-rich layer, however, which was oxidized to TeO₂ and the surface [Te]/([Cd] + [Zn]) ratio restored near-stoichiometry after NH₄F/H₂O₂ passivation. According to I-V measurement, barrier height was 0.80 ± 0.02-0.85 ± 0.02 eV for Au/p-Cd_1−xZn_xTe with BM etching, however, it increased to 0.89 ± 0.02-0.93 ± 0.02 eV with NH₄F/H₂O₂ passivation. Correspondingly, it was about 1.34 ± 0.02-1.43 ± 0.02 eV and 1.41 ± 0.02-1.51 ± 0.02 eV by C-V method.     

Electrical characterization and carrier transportation in Hf-silicate dielectrics using ALD gate stacks for 90 nm node MOSFETs

Metal-oxide-semiconductor capacitors (MOSCs) and metal-oxide-semiconductor field-effect transistors (MOSFETs) incorporating hafnium silicate (Hf-silicate) dielectrics were fabricated by using atomic layer deposition (ALD). The electrical properties of these Hf-silicate thin films with various postnitridation annealing (PNA) temperatures were then examined to find the best nitridation condition. It is found that the best conditions to achieve the lowest gate leakage current and best equivalent oxide thickness (EOT) are when PNA is performed at 800 °C in NH₃ ambient for 60 s. To understand the obtained film, carrier transportation mechanisms, the temperature dependence of the leakage current was measured from 300 K to 500 K for both gate injection and substrate injection. The result reveals that the leakage mechanisms involve Schottky emission at high temperature and low electrical field and Poole-Frenkle emission at low temperature and high electrical field. The barrier heights of poly-Si/Hf-silicate and Hf-silicate/Si interfaces extracted from Schottky emission are 1.1 eV and 1.04 eV, respectively. The interface traps per unit area, the mean density of interface traps per area and energy and the mean capture cross-section are determined about 8.1 × 10¹⁰ cm⁻², 2.7 × 10¹¹ cm⁻² eV⁻¹ and 6.4 × 10⁻¹⁵ cm⁻² using charge pumping method.     

Improved electron injection of OLEDs with a thin PBD layer at Alq3/Cs2CO3 interface

In this study, the effect of one oxadiazole derivative (PBD) using as an electron injection layer (EIL) at Alq₃/Cs₂CO₃ interface has been investigated. The present of PBD EIL was showed an interesting enhanced electron injection for OLEDs although the nominal electron injection barrier for PBD based OLEDs is much larger, because PBD owns an obvious higher intrinsic the Lowest Unoccupied Molecular Orbital level (2.3 eV) than that of Alq₃ (3.0 eV). For example, the current density of OLEDs at 8 V was increased from 54 mA/cm² to 168 mA/cm² when inserting a thin PBD layer (5 nm) at Alq₃/Cs₂CO₃ interface. Here the increased current is suggested associating with the changed electronic structure of PBD when it contacts with Cs₂CO₃.     

Molecular materials derived from MPc (M=Fe, Pb, Co) and 1,8-dihydroxiantraquinone thin films: Formation, electrical and optical properties

Semiconductor-like thin films were grown using metallic phthalocyanines (MPc) (M=Fe, Pb, Co) and 1,8 dihydroxiantraquinone as initial compounds. The morphology of the deposited films was studied by using scanning electron microscopy and atomic force microscopy. The powder and thin-film samples of the synthesized materials, deposited by vacuum thermal evaporation, showed the same intra-molecular bonds as in IR spectroscopy studies, which suggests that the evaporation process does not alter these bonds. The optical band gap values of C₆₀H₂₈N₈O₈Fe, C₆₀H₂₈N₈O₈Pb and C₆₀H₂₈N₈O₈Co calculated from the absorption coefficient were found to be 1.60, 1.89 and 1.75 eV, respectively, arising from non-direct transitions. The effect of temperature on conductivity was also measured in these samples. It was found that the temperature-dependent electric current in all cases showed a semiconductor behavior with conductivities in the order of 10⁻⁶ Ω⁻¹ cm⁻¹ where the highest value corresponded to the cobalt material. The linear dependence observed in the films implies only one type of conduction mechanism in all cases, with mean activation energies of the order of 1.55, 1.77 and 1.50 eV for iron, lead and cobalt-based thin films, respectively.     

Electronic structure of solid C48N12 aza-fullerene

I report electronic structures and the cohesive energy for face-centered-cubic (fcc) solid C₄₈N₁₂ using generalized-gradient density-functional theory. The full vibrational spectrum of the C₄₈N₁₂ cluster is calculated within the harmonic approximation at the B3LYP/6-31G* level of theory. The results show that fcc is energetically preferred and a more stable crystal form than body-centered-cubic (bcc). C₄₈N₁₂ clusters are found to condense by a weak (0.29 eV) van der Waals force. The band gap of fcc C₄₈N₁₂ is calculated to be 1.3 eV at the GGA-PW91 level, whereas the HOMO-LUMO gap is calculated to be 2.74 eV using B3LYP/6-31G*.     

Experimental investigation of the CMN matrix element in the band anticrossing model for GaAsN and GaInAsN layers

The temperature dependence of bandgap energy of GaAs_0.98N_0.02 and Ga_0.95In_0.05As_0.98N_0.02 compounds has been analysed in order to determine the C_MN matrix element of the band anticrossing model. We have found that the element equals 2.48 and 2.60 eV for GaAs_0.98N_0.02 and Ga_0.95In_0.05As_0.98N_0.02 layers, respectively. When the same value has been assumed for annealed layers, an increase in the energy of the resonant nitrogen level E_N has been obtained. Two possible mechanisms leading to the increase in this energy are discussed in this work.     

The interface structure and band alignment at alumina/Cu(Al) alloy interfaces—Influence of the crystallinity of alumina films

Both epitaxial and amorphous ultra-thin alumina films were grown on a Cu-9 at.%Al(1 1 1) substrate by selective oxidation of Al in the alloy in ultra high vacuum. The crystallinity of the alumina films was controlled by oxidation temperature. The photoelectron spectra of Al 2p, O 1s and valence band were measured in-situ during oxidation. The influence of the crystallinity on the interface structure between the alumina films and the substrate was discussed by analyzing the Al 2p spectra composed of multiple peaks. The energy difference between the Fermi level of the substrate and the valence band maximum of the alumina films (band offset) was derived from the valence band spectra. The energy band alignment at the interface between each of the two alumina films and the substrate was revealed by combining the binding energy values of the core levels with the band offset values. The influence of the alumina crystallinity on the band alignment was discussed.     

Temperature dependence of the current-voltage characteristics of the Al/Rhodamine-101/p-Si(1 0 0) contacts

The current-voltage (I-V) characteristics of Al/Rhodamine-101/p-Si/Al contacts have been measured at temperatures ranging from 280 to 400 K at 20 K intervals. A barrier height (BH) value of 0.817 eV for the Al/Rh101/p-Si/Al contact was obtained at the room temperature that is significantly larger than the value of 0.58 eV of the conventional Al/p-Si Schottky diode. While the barrier height Φ_b0 decreases the ideality factors (n) become larger with lowering temperature. The high values of n depending on the sample temperature may be ascribed to decrease of the exponentially increase rate in current due to space-charge injection into Rh101 thin film at higher voltage. Therefore, at all temperatures, it has been seen that the I-V characteristics show three different regions, the ohmic behavior at low voltages, and the space charge limited current with an exponential distribution of traps at high voltages.     

STUDY OF THE Al/GRAPHITE INTERFACE

Thin Al films with a thickness of 20－30nm were prepared by ultra-high vacuum deposition of Al onto a graphite surface parallel to a (0001) basal plane. The samples were annealed up to 1070K. X-ray photoelectron spectroscopy analysis has shown that for temperatures just higher than 770K, a little carbide occurs in the Al film and only an Al－C phase is present at the Al/graphite interface. After annealing at 970K, the Al₄C₃ phase can be observed, and the binding energy of the Al2p electrons increases continuously from 72.7 to 74.2eV with increasing temperature up to 1070K. Auger electron spectroscopy depth profiles are measured to investigate the phases existing in the Al film as well as at the Al/graphite interface. It is found that the Al₄C₃ phase at the interface is the final product of a series of Al carbides from the interfacial reaction between Al and graphite.     

FePc与TiO2(110)及C60界面电子结构研究

基于C₆₀受体和有机分子给体的太阳能电池是目前非常重要的一个研究热点, 利用同步辐射真空紫外光电子能谱(SRUPS) 技术研究了酞菁铁(FePc)与TiO₂(110)及C₆₀的界面电子结构, 以及FePc与C₆₀分子混合薄膜的电子结构. SRUPS价带谱显示, FePc沉积在化学计量比与还原态两种不同的TiO₂(110)表面时, FePc分子的HOMO能级均随FePc厚度的变化发生了移动, 而在化学计量比的TiO₂(110)表面位移较大, 同时发生界面能带弯曲, 说明存在从有机层向衬底的电子转移. 在FePc/C₆₀和C₆₀/FePc界面形成过程中, FePc与C₆₀分子的最高占据分子轨道(HOMO)位移大小基本相同. 由界面能级排列发现, 在FePc与C₆₀的混合薄膜中, FePc分子的HOMO与C₆₀分子的最高占据分子轨道能级差较大, 这有利于提高器件开路电压, 改善器件性能.     

Investigation of the effect of CdCl2 processing on vacuum deposited CdS/CdTe thin film solar cells by DLTS

Thin film CdS/CdTe solar cells have been prepared by conventional vacuum deposition technique. Deep level transient spectroscopy (DLTS), temperature and frequency dependent capacitance-voltage (C-V) measurements were utilised to investigate the performance limiting defect states in the CdTe layer subjected to the post deposition treatments such as CdCl₂-dipping and/or annealing in air. Five hole traps, all of which have been previously reported in the literature, were identified in as-grown CdTe at 0.19, 0.20, 0.22, 0.30 and 0.40 eV above the valence band. A single hole trap level has been evidenced at 0.45 eV after both post deposition heat and CdCl₂ treatments.     

Electrical characterization of rare-earth implanted GaN

Deep-level transient spectroscopy (DLTS) measurements were used to characterize the electrical properties of MOCVD grown, europium- (Eu) and xenon- (Xe) implanted GaN films on sapphire substrates. Implantation energy was 80 keV with a fluence of 1×10¹⁴ cm⁻² along a channeled crystallographic direction. Defect levels were observed at E_C−0.19 eV for both Eu- and Xe-implantation which were predicted to be a rare-earth related donor level by theoretical calculations. Other defect levels are observed with energy levels located at 0.22, 0.68, 0.49, 0.60, 0.77 eV and 0.48, 0.64, 0.45, 0.72 eV below the conduction band for Eu and Xe implantation, respectively. Some of these levels have similar defect signatures and can be related to other implantation related defects introduced in erbium, praseodymium and helium implantations.