Dark Conductivity Activation Energy Research Articles

Improved PECVD processed hydrogenated germanium films through temperature induced densification

Amorphous and nano-crystalline germanium is of potential interest for a wide range of electronic, optical, opto-electronic and photovoltaic applications. In this work the influence of deposition temperature on hydrogenated germanium (Ge:H) films was characterized, using over 200 Ge:H and over 70 SiGe:H films. The demonstrated temperature-induced densification of Ge:H films resulted in more stable films with a lower bandgap energy and dark conductivity and higher activation energy.

Influence of the Dopant Gas Precursor in P-Type Nanocrystalline Silicon Layers on the Performance of Front Junction Heterojunction Solar Cells

Silicon heterojunction solar cells can employ p-type hydrogenated nanocrystalline silicon nc-Si:H(p) on their front side, since these can provide better transparency and contact resistance compared to hydrogenated p-type amorphous silicon layers. We investigate here the influence of trimethyl boron (TMB) and BF3 as dopant source on the layer properties and its performance in solar cells. Both gases enable high efficiencies but yield a different crystallinity and effective doping. A high BF3 flow lowers the series resistance through a low activation energy of dark lateral conductivity and maintains a high crystallinity. This allows fill factors up to 83%, however with the apparition of a parasitic absorption in the UV. A low TMB flow enables simultaneously a high crystallinity and a low activation energy. As an illustration of this layer potential, a 23.9%-certified efficiency is achieved with a 2 × 2 cm2 screen-printed device. We finally suggest that similar transport versus transparency trade-offs can be reached for both dopant types for front junction application, while high BF3 flow allowing lower series resistance might be of interest when placed on the rear side.

Detection of dislocation‐related midgap levels in pulsed laser deposited GaN by photo‐induced current transient spectroscopy

Deep levels in unintentionally doped GaN epilayers grown on sapphire (0001) by ultra high vacuum pulsed laser deposition at 600 and 700 °C have been studied using photo‐induced current transient spectroscopy (PICTS). The GaN epitaxial layers were deposited by laser ablating a hydride vapor phase epitaxy grown bulk GaN polycrystalline target in the ambient of r.f. nitrogen radicals. The activation energy of dark conductivity for these layers lies in the range 0.17–0.25 eV. An acceptor‐like deep trap level is detected in the PICTS spectrum with activation energy of 1.32 eV. For the 700 °C grown GaN layer, an additional well‐pronounced acceptor‐like defect level near mid‐gap at 1.52 eV has been observed. From the analysis of trap capture kinetics, both defect levels are identified to be extended ‘bandlike’ electronic states associated with extended defects such as dislocations present in GaN layers.

Improvement of the electrical properties of nanocrystalline silicon films by the KrF pulsed excimer laser irradiation method

The effects of laser fluence on the microstructure and electrical properties of nanocrystalline silicon films were investigated on samples prepared by a plasma-enhanced chemical vapor deposition technique. It was found that the content of hydrogen in the films could be reduced gradually by increasing the laser fluence, accompanied by the formation of nc-Si. The evolution of hydrogen is responsible for a change in the optical band gap and dark conductivity. By controlling the laser fluence at 1.0 J cm−2, a dark conductivity as high as 5.9 × 10−3 S cm−1 could be obtained. Based on measurements of the temperature-dependent conductivity, the carrier transport processes are discussed. The effusion of hydrogen and the increase of crystallinity are thought to be contributed to the high dark conductivity and low conductivity activation energy.

Growth and characterization of Cr-doped semi-insulating GaN templates prepared by radio-frequency plasma-assisted molecular beam epitaxy

Cr-doped semi-insulating GaN templates were grown using radio-frequency plasma-assisted molecular beam epitaxy (RF-MBE). The samples were characterized by high resolution x-ray diffraction (HRXRD), atomic force microscopy (AFM) and transmission electron microscopy (TEM). The Cr incorporation was strongly dependent on the growth condition, which was primarily influenced by Cr cell temperature. The activation energy of dark conductivity was about 0.48eV which corresponds to the depth of the dominant electron traps pinning the Fermi level. The experimental results indicated that Cr doping did not affect the crystalline orientation of the GaN film but introduced more threading dislocations, and step-graded GaN/AlxGa1−xN superlattices (SLs) played an important role in hindering the penetration of the threading dislocations.

Thin Film Amorphous Silicon Nanoscale Photodetectors

We present nanoscale photodetectors based on two types of thin film hydrogenated amorphous silicon (a-Si:H), one of which is made of intrinsic a-Si:H and the other of n-type doped a-Si:H. 80 nm thick nanophotoconductors with different lengths (2-20 μm) and widths (60 nm-1 μm) are fabricated. The electronic properties of both types of photoconductores are characterized through the following techniques: the temperature dependence of the dark conductivity and determination of the activation energy of dark conductivity; the dependence of the photocurrent density with the photogeneration rate; and the spectral response of the photoconductor. This work demonstrates that a-Si:H photoconductors can be scaled down to nanoscale while maintaining the optoelectronic properties characteristic of their large-area (mm-size and above) counterparts.

Thermal annealing dependence of some physical properties of Bi-substituted Sn–Sb–Se glassy thin films

Bulk glasses of the Sn10SbBixSe70 (0 x ≤ 8) system were prepared by the conventional melt quenching technique. Thin films were prepared by the thermal evaporation technique on glass substrates. Appearance of some crystalline phases is observed from the X-ray diffractograms after heat treatment below the glass transition temperature for 1 h. Scanning electron microscopy studies also show the presence of microcrystalline phases in the amorphous matrix after annealing for 1 h. The effect of Bi concentration and heat treatment on the optical gap and activation energy for dark conductivity were also investigated for the pristine as well as annealed films. The results are discussed on the basis of models related to the presence of defect states in chalcogenide materials.

Hydrogen-induced-intrinsic transport in undoped microcrystalline silicon

It is demonstrated that microcrystalline silicon (μc-Si:H) of intrinsic character can be produced by post-growth atomic hydrogen treatment. Undoped μc-Si:H films with a dark-conductivity activation energy (Ea) of about 0.20 eV were grown by plasma-enhanced chemical vapor deposition, and then subsequently exposed to an atomic hydrogen plasma. The hydrogen treatment is shown to result in a gradual increase in the Ea with increasing treatment time, followed by saturation at about 0.57 eV, a value observed for truly intrinsic μc-Si:H films. In the saturated state, the dark conductivity is on the order of 10−7 S/cm. The dark conductivity prefactor is found to follow the Meyer–Neldel rule. It is proposed that charge transport takes place in amorphous-like tissue surrounding the crystalline grains. The results are attributed to the Fermi level shift due to a change in the gap state distribution.

Vanadium-doped photorefractive titanosillenite crystal

Holographic recording in a vanadium-doped B12TiO20 (BTO) photorefractive crystal puts into evidence a large hole–electron competition showing a fast and a slow hologram components. From the fast component evolution, some material parameters for the electron-donor photoactive centers are computed. The wavelength-resolved photoconductivity is shown to be strongly modified by V-doping compared to undoped and doped BTO with other elements. The increase of photoconductivity by green light preexposure is almost negligible here if compared with undoped BTO. Activation energy for dark conductivity measured for BTO:V is similar to that for undoped BTO, as measured close to room temperature, but sensibly lower than the value reported in the literature for a much higher temperature range. Optical absorption and EPR spectra do confirm already published results and suggestions about the possible role of vanadium in the sillenite structure.

The properties of a-Si:H films produced under various silicon substrate temperatures

a-Si:H films are produced under various substrate temperatures and at different compositions of a gaseous mixture. Their IR spectra, dark conductivity, photoconductivity, photosensitivity, and conductivity activation energy are studied. It has been shown that one can produce specimens marked by low conductivity activation energy and high photosensitivity by varying the substrate temperature and gaseous mixture composition. These specimens are more stable as compared to a-Si:H films produced under a low substrate temperature.

Structural and optical properties of Cr-doped semi-insulating GaN epilayers

The properties of Cr-doped GaN epilayers grown by rf-plasma-assisted molecular beam epitaxy were studied. The deep acceptor nature of Cr was used to grow semi-insulating GaN epilayers on sapphire substrates for electronic device applications. The room-temperature (RT) sheet resistivity of the epilayers reached 1010 Ω/square. The activation energy of dark conductivity was about 0.48 eV. Step-graded AlxGa1−xN/GaN (x=0.3−0.2) superlattices (SLs) were designed to filter dislocations. Transmission electron microscopy images showed that the SLs can dramatically reduce dislocation density. Al0.35Ga0.65N/GaN heterostructure grown on Cr-doped semi-insulating GaN epilayer exhibited a RT mobility of 960 cm2/V s and sheet carrier density of 2.1×1013 cm−2.

Optical properties of nanocrystalline silicon deposited by PECVD

Nanocrystalline silicon (nc-Si) thin films were deposited by 13.56 MHz plasma enhanced chemical vapor deposition (PECVD) using silane plasma highly diluted in hydrogen to induce microstructural changes from amorphous to nanocrystalline. Raman spectroscopy measurements showed that nc-Si films can be obtained at hydrogen dilutions greater than 98%. High hydrogen dilution in conjunction with high reactor pressure lead to lower absorption of photon energies less than 1.1 eV, and, consequently, to lower defect density inside nc-Si thin films. The nc-Si dark conductivity, dark conductivity activation energy, and photosensitivity were in the range 10−6 S/cm, 0.43 eV, and 10-20, respectively. Photocurrent spectra showed absorption peaks at photon energies around 2.5, 3.2, and 4.5 eV. Measured optical bandgaps were in the range of 2.3–2.5 eV.

Heterojunction solar cells with n-type nanocrystalline silicon emitters on p-type c-Si wafers

Hydrogenated nanocrystalline silicon (nc-Si:H) n-layers have been used to prepare heterojunction solar cells on flat p-type crystalline silicon (c-Si) wafers. The nc-Si:H n-layers were deposited by radio-frequency (RF) plasma enhanced chemical vapor deposition (PECVD), and characterized using Raman spectroscopy, optical transmittance and activation energy of dark-conductivity. The nc-Si:H n-layers obtained comprise fine grained nanocrystallites embedded in amorphous matrix, which have a wider bandgap and a smaller activation energy. Heterojunction solar cells incorporated with the nc-Si n-layer were fabricated using configuration of Ag (100nm)/lT0 (80nm)/n-nc-Si:H (15nm)/buffer a-Si:H/p-c-Si (300μm)/Al (200nm), where a very thin intrinsic a-Si:H buffer layer was used to passivate the p-c-Si surface, followed by a hydrogen plasma treatment prior to the deposition of the thin nanocrystalline layer. The results show that heterojunction solar cells subjected to these surface treatments exhibit a remarkable increase in the efficiency, up to 14.1% on an area of 2.43cm2.

Nanostructure in the p-layer and its impacts on amorphous silicon solar cells

The open circuit voltage (Voc) of n–i–p type hydrogenated amorphous silicon (a-Si:H) solar cells has been examined by means of experimental and numerical modeling. The i- and p-layer limitations on Voc are separated and the emphasis is to identify the impact of different kinds of p-layers. Hydrogenated protocrystalline, nanocrystalline and microcrystalline silicon p-layers were prepared and characterized using Raman spectroscopy, high resolution transmission electron microscopy (HRTEM), optical transmittance and activation energy of dark-conductivity. The n–i–p a-Si:H solar cells incorporated with these p-layers were comparatively investigated, which demonstrated a wide variation of Voc from 1.042V to 0.369V, under identical i- and n-layer conditions. It is found that the nanocrystalline silicon (nc-Si:H) p-layer with a certain nanocrystalline volume fraction leads to a higher Voc. The optimum p-layer material for n–i–p type a-Si:H solar cells is not found at the onset of the transition between the amorphous to mixed phases, nor is it associated with a microcrystalline material with a large grain size and a high volume fraction of crystalline phase.

N and p-type doping of PECVD a-SiC:H obtained under “silane starving plasma” condition with and without hydrogen dilution

N and p-type doping of highly structural and chemically ordered a-Si 0.5C 0.5:H films are studied. Ion implantation is utilized as doping technique and the films are obtained by rf plasma chemical vapor deposition (PECVD) in the so-called “silane starving plasma” condition, which plays an important role to improve the structural properties of a-SiC:H films. Since hydrogen plasma improves even more the structural properties of the material, the doping efficiency is studied in two types of a-Si 0.5C 0.5:H samples grown with and without hydrogen dilution of the precursor gaseous mixture. For n-type doping, phosphorus was implanted in concentrations varying between 10 18 and 10 21 cm −3, according to previous theoretical simulations. For p-type doping boron and, for the first time in a-SiC:H technology, boron difluoride (BF 2) were implanted, both with a fixed concentration of 10 20 cm −3. The electrical characterization for phosphorus doping shows that the conductivity of the films increases with increasing impurity concentration and, in the best case a room temperature dark conductivity of ∼10 −4 (Ω cm) −1 and an dark conductivity activation energy of 0.1 eV were obtained. For p-type doping the results indicate that BF 2 + is much more efficient than boron, which is related to the production of a shallow doped layer, in accordance to observations in c-Si. In this way a p-type dark conductivity of ∼10 −1 (Ω cm) −1 and energy of 20 meV were obtained.

Nanocrystalline Silicon Films Deposited by RF PECVD for Bottom-gate Thin-film Transistors

AbstractThin-film transistors (TFTs) in active-matrix organic light emitting diode (AMOLED) displays are required to supply high and stable driving current to OLEDs. Top-gate TFTs with nanocrystalline silicon (nc-Si) active layer have shown promise to render high mobility and stable driving current. However, to be compatible with current production facilities, bottom-gate TFTs are demanded. Currently, bottom-gate nc-Si TFTs show insufficient field effect mobility and exhibit driving current instability due to presence of amorphous incubation layer at the interface with gate dielectric. Our research is motivated by the need to eliminate the incubation layer. In order to do so, we studied nc-Si deposition process to find the RF PECVD deposition regimes which lead to minimum incubation layer.We have deposited a set of undoped nc-Si films by 13.56 MHz PECVD at 250°C by varying RF power, reactor pressure, silane and hydrogen flow rates. Raman spectroscopy, constant-photocurrent method (CPM) and optical absorption have been used to measure film crystallinity, defect density and optical bandgap, respectively. Carrier transport in the films has been studied using dark conductivity, photoconductivity and conductivity activation energy measurements.Our results reveal that silane and hydrogen flow rates are the most contributing factors to film characteristics. The results also indicate that the reactor pressure does not have a significant effect on the film crystallinity. However, CPM data confirm that to obtain lower defect density, medium or high deposition pressures are preferred. We obtained films with dark conductivity and Raman crystallinity in the order of 10-6-10-7S/cm and 60-80 %, respectively. Furthermore, we have deposited nc-Si films as thin as 20nm with 60% crystallinity, which is crucial for bottom-gate TFTs. Finally, four different sets of bottom-gate TFTs have been fabricated by changing gate dielectric compositions and changing [SiH4]/([SiH4] + [H2]) gas flow ratio. The device performance, relationship to the film structure and deposition process, and future improvements will be discussed in details.

Micro-crystalline silicon-germanium thin films prepared by the multi-target RF sputtering system

AbstractMicro-crystalline silicon-germanium (μc-SiGe) films were prepared by the multi-target RF sputtering system using Ar and Ar-H2 mixture gases. The crystallization temperature of Si0.3Ge0.7 films is reduced from 600 °C to 400 °C by the introduction of H2 into the sputtering gases. The dark conductivity of 1.7x10-7 S/cm and one order of magnitude of photosensitivity are obtained with the H2/Ar flow ratio of 2. The activation energy of dark conductivity is 0.42 eV, which is a half of the energy gaps of Si0.3Ge0.7, and show that the films have intrinsic nature. Besides, the absorption coefficients are similar to those of single crystalline Si0.3Ge0.7. The results suggest that the H2 introduction is effective both to reduce the dangling bond defects and to decrease the crystallization temperature of the μc-SiGe films.

Relation between the thermal activation energy of conduction and the first excited singlet state energy—a case of photo-conducting organic materials

The thermal activation energy of dark conductivity has been measured for a new series of cationic cyanine dyes (seven in numbers) derived from different heterocyclic nuclei. The thin layers of the dyes were obtained by vacuum deposition on a surface type raster cell in a vacuum of the order of 10 −4 Torr. The results concerning the nature of charge transport have been discussed. A relationship between the thermal activation energy (Δ E D) and the first excited singlet state energy of the molecules ( 1E) has also been attempted.

Long-range disorder and the Staebler-Wronski effect inN-type amorphous silicon

Experimental measurements of the Q function (a comparison of the dark conductivity and thermopower activation energies) as a function of light exposure are reported for a variety of n-type-doped hydrogenated amorphous silicon films. The samples studied represent a wide range of deposition conditions, including variations of the deposition temperature, the rf power during deposition, and the doping ratio of phosphorus to silicon. The Q function of these films is measured before and after light-induced defect creation. No significant change in the Q function was observed after light-induced defect creation for any of these samples, consistent with no light-induced change in the long-range disorder at the conduction-band mobility edge. Measurements of the non-Gaussian statistics which characterize the conductance fluctuations $(1/f$ noise) are also found to remain unchanged following light exposure.

Relative importance of deposition rate for surface potential in amorphous silicon/SiO2 interface

Hydrogenated amorphous silicon (a-Si:H)-based metal oxide semiconductor (MOS) structures were prepared with various deposition conditions. A-Si:H films were deposited by reactive sputtering method with different substrate temperatures, hydrogen partial pressures and argon-helium gas mixture. The surface potential at the interface a-Si:H/oxide was deduced from MOS current density-voltage characteristic and a-Si:H film dark conductivity activation energy. The correlation between the deposition rate, the surface potential and their respective variation with deposition parameters indicates the clear evidence of the deposition rate influence on a-Si:H/insulator interface quality; this is explained in terms of film growth mechanism. We have concluded that the interface states are due to the silicon dangling bonds and are mostly negatively charged.