Boron-doped Films Research Articles

Dopant effect on in situ doped metal-induced lateral crystallization of amorphous silicon films

Dopant (boron or phosphorus) effect on metal-induced lateral crystallization (MILC) of amorphous silicon (a-Si) films has been systematically studied by using scanning electron microscopy (SEM) and X-ray diffraction (XRD). The MILC was realized by depositing a-Si film onto an oxidized Si wafer and then annealing in a N 2 atmosphere. The doped amorphous films have been developed by plasma-enhanced chemical vapor deposition (PECVD). Various dopant gases and annealing times were investigated to study dopant effects on MILC films. It has been found that the un-doped poly-Si film has a MILC length larger than that of samples doped with boron or phosphorus. The phosphorus-doped films exhibit larger grain size as compared with un-doped and boron-doped films. Larger MILC length is obtained by increasing annealing time, but the MILC rate is decreased. In addition, nickel-silicides were found in the doped samples by XRD. Physical mechanisms for these experimental results are proposed.

Transport mechanism in lightly doped hydrogenated microcrystalline silicon thin films

Boron-doped microcrystalline silicon films have been deposited in a plasma-enhanced chemical vapor deposition system using silane diluted in hydrogen, and diborane (B2H6) as a dopant gas. The temperature dependence of the dark conductivity has been measured from 120to420K in all samples. In the high-temperature range above room temperature, the carrier transport is found to be thermally activated, with a single activation energy that changes with the B2H6 compensation degree. In the low-temperature range (300–120K), variable range hopping (VRH) was established as a predominant electronic transport mechanism for all samples, with the exception of the sample with a diborane concentration of 12.5ppm. A model for Mott’s VRH, referred to as the “diffusional model,” which yields a relation between the conductivity and the localized density of gap states, is presented. Using classical equations from the percolation theory and the diffusional model, the density of states near the Fermi level, as well as the hopping parameters, are calculated. A correlation between the hopping parameters for both models is deduced. A numerical factor that improves the value of each parameter is calculated.

Current image tunneling spectroscopy of boron-doped nanodiamonds

The electron field emission properties of the nanodiamond films were examined using scanning tunneling microscopic (STM) technique. Current image tunneling spectroscopic measurements reveal the direct dependence of electron tunneling/field emission behavior of the films on the proportion of grain boundaries present. Local tunneling current-voltage (It–V) measurements show that incorporation of boron species insignificantly alters the occupied state, but markedly modifies the empty state of the diamond films, viz. it induces the presence of impurity states for the films heavily doped with borons, resulting in smaller emission energy gap for the samples. Such a characteristic improves both the local electron field emission behavior of the diamond films measured by STM and the average electron field emission properties measured by conventional parallel plate setup. These results infer clearly that the presence of impurity states due to boron doping is a prime factor improving the field emission properties for these boron-doped nanodiamond films.

A Wide Band Gap Boron-doped Microcrystalline Silicon Film Obtained with VHF Glow Discharge Method

AbstractA wide bandgap microcrystalline silicon film for the window layer of microcrystalline silicon thin film solar cells was obtained with very high frequency (VHF) glow discharge technology. The material was deposited on corning 7059 substrate at about 170 When H2/SiH4was more than 100, Raman spectra showed that this material was highly crystallized, and no peak correlation with amorphous silicon was observed. This material showed strong n type before any intentional doping. We considered that the unintentional doping of oxygen and unpurified gases. The doping performance of this material was investigated by introducing B2H6into the reacting gas. As increasing the rate of B2H6/SiH4from zero to 0.5%, the conductivity changed from 10-1S.cm-1(n type) to 10-8S.cm-1dramatically and than backed to 10-1S.cm-1(p type), which indicated that this material had excellent doping ability. Raman spectra also showed that the microstructure of these materials did not change obviously in this doping range. We gained the p-uc-Si:H film with thickness less than 30nm, and the conductivity was more than 10-2S.cm-1, and crystalline volume fraction no less than 40%, the Egoptcould be wider than 2.10eV. Using this p window layer in microcrystalline silicon solar cells with no ZnO rear reflection, the conversion efficiency was 8.30% (Voc=0.531V, Jsc=24.66mA/cm2, FF=63.41% ).

Correlation between Residual Stress and Boron Concentration in Boron-Doped Silicon Films

This paper represents the correlation between the residual stress and the boron concentration in boron-doped silicon films. Residual stress measurement structures of different thicknesses are simultaneously fabricated for the quantitative determination of residual stress profiles. All structures are diffused by various boron diffusion processes and consecutively etched by an improved etch process on the same silicon wafer. The obtained residual stress profiles illustrate that as the drive-in process temperature or the time increases, the average residual stress and the maximum residual stress decrease. For cases of several drive-in process conditions, correlation coefficients between the boron concentration and the residual stress are calculated and the lattice contraction of a boron-doped silicon wafer is measured by high-resolution X-ray diffractometry(HRXRD). The calculated correlation coefficients illustrate that the residual stress profile is closely correlated with the boron concentration after drive-in.

Synthetic Semiconductor Diamond Electrodes: Electrochemical Characteristics of Homoepitaxial Boron-doped Films Grown at the (111), (110), and (100) Faces of Diamond Crystals

Electrode behavior of homoepitaxial (single-crystal) boron-doped diamond films deposited onto differently orientated faces of dielectric diamond single crystals is studied by the electrochemical impedance and potentiodynamic curve methods. It is shown that the acceptor concentration determined from the slope of Mott–Schottky plots decreases, in the epitaxial films grown under the same conditions, in the series: (111) > (110) > (100). This is explained by different intensity of boron incorporation, from gas phase, into differently orientated faces of the diamond crystals during their growth. The rate of electrode reactions in the Fe(CN)6 3–/4– and Ru(NH3)6 2+/3+ redox systems decreases in the above series, which obeys the earlier found interrelationship between the electrochemical kinetics at diamond electrodes and their doping level.

Roles of hydrogen dilution on the microstructural and optoelectronic properties of B-doped nanocrystalline Si:H thin films

We report on the roles of hydrogen dilution on the microstructural and optoelectronic properties of boron-doped nanocrystalline silicon thin films grown by plasma-enhanced chemical vapor deposition on glass substrates, through x-ray diffraction, scanning electron microscope, Raman scattering, optical transmission, temperature-dependent dark conductivity, and elastic recoil detection analysis (ERDA) measurements. The grain size, crystallinity, absorption coefficient, refractive index, and conductivity are found to decrease basically with increasing hydrogen dilution ratio ΔH. The Tauc’s optical band gap is found to increase with ΔH due to the quantum size effect. By the aid of hydrogen ion bombardment effect [J. Appl. Phys. 93, 1262 (2003)] as well as the hydrogen-induced annealing effect, the clear increase of hydrogen content with ΔH and the different hydrogen distribution from ERDA help us to elucidate the correlation of hydrogen dilution with the crystallinity and grain size. The argument has been further supported by a simple absorption model and the yielded refractive index and absorption coefficient.

Aging effects in microcrystalline silicon films studied by transient photoconductivity

Measurements of dark conductivity, steady-state and transient photoconductivity in undoped and boron-doped microcrystalline silicon films exposed to room air are reported. Two aging processes are identified: (i) an increase in dark conductivity and mobility-lifetime product occurring over several days, that may be reversed by heating to 160 °C under vacuum, and subsequently re-cycled, (ii) an irreversible change in the density of states occurring over a period of several months. It is proposed that the reversible effect is associated with charge transport in a region of electron accumulation induced by weakly-adsorbed water, whereas the irreversible effect, identified from its transient photocurrent signature, is associated with a true change in the density of states within the transport path, possibly as a result of slow chemical reactions at silicon grain boundaries.

Raman scattering and photoluminescence analysis of B-doped CdS thin films

Boron-doped CdS thin films were chemically deposited onto glass substrates. X-Ray diffraction (XRD), photoluminescence (PL), and Raman techniques were used to evaluate the quality of B-doped CdS films. XRD results have confirmed that B-doped CdS films have a hexagonal structure with a preferential orientation of the (002) plane. The grain size of B-doped CdS films slightly decreases, but no change of the microstructure was observed. The PL spectra for all samples consist of two prominent broad bands approximately 2.3 eV (green emission) and 1.6 eV (red emission) and the higher doping concentrations gradually decreased the green emission and red emission. Raman analysis has shown that undoped films have structure superior to those of B-doped CdS films.

Preferred growth of nanocrystalline silicon in boron-doped nc-Si:H Films

Preferred growth of nanocrystalline silicon (nc-Si) was first found in boron-doped hydrogenated nanocrystalline (nc-Si:H) films prepared using plasma-enhanced chemical vapor deposition system. The films were characterized by high-resolution transmission electron microscope, X-ray diffraction (XRD) spectrum and Raman Scattering spectrum. The results showed that the diffraction peaks in XRD spectrum were at 2 θ≈47° and the exponent of crystalline plane of nc-Si in the film was (2 2 0). A considerable reason was electric field derived from dc bias made the bonds of Si–Si array according to a certain orient. The size and crystalline volume fraction of nc-Si in boron-doped films were intensively depended on the deposited parameters: diborane (B 2H 6) doping ratio in silane (SiH 4), silane dilution ratio in hydrogen (H 2), rf power density, substrate's temperature and reactive pressure, respectively. But preferred growth of nc-Si in the boron-doped nc-Si:H films cannot be obtained by changing these parameters.

Effect of oxidation treatments on the redistribution of the boron in the thin films of polycrystalline silicon Si-LPCVD used in VLSI

Results of a quantitative characterisation SIMS at high levels of boron-doping concentration (2 × 10 20 cm −3 ) of in situ boron-doped LPCVD-polysilicon thin films before and after thermal-oxidation treatments are presented. Measurements of the precedent characterisation methods (SIMS) are performed on submicron layers (300 nm) deposited at the extreme temperatures T d = 520 °C and T d = 605 °C. The thermal-oxidation experiments are carried out under dry oxygen (O 2 ) at three oxidation temperatures T ox = 840 °C, 945 °C and 1050 °C for several durations. After these thermal-oxidation processes, remarkable changes in the behavior of doping profile are observed. This behavior seems to be typically characteristic of the in situ heavily boron-doped films. In addition to the presence of some saturation-thermal-dependence phenomenon, we find that these results well correlate and support the presence of another phenomenon called “Differential of the Oxidation Rate (DOR)” which is evidenced as typical of the in situ doped films.

Increase of temperature and crystallinity during electrical switching in microcrystalline silicon

ABSTRACTWe investigate electrical stressing and switching in hydrogenated microcrystalline silicon (mc-Si:H) by thermal, and optical and electrical measurements of Cr/mc-Si:H/metal thin-film structures. Boron-doped microcrystalline Si films of 30-50 nm thick are deposited by hot-wire chemical vapor deposition (HWCVD) on Cr-coated glass at 160°C and contacted with Ag or Al. Switching in devices of size 5 to 30 mm is stimulated by a current-ramp from 10 nA to 50 mA. We find that the voltage across the mc-Si:H devices initially increases logarithmically with current, then saturates at 2∼3 V, and finally drops to a low value of 1 to 1.5 V. This drop indicates a permanent decrease of device resistance to below 1 kW. During current stressing, the surface temperature increases with the bias current, and the surface reflectivity changes. After switching, a small increase in crystalline fraction can be observed by micro-Raman scattering measurements. The observations suggest electrothermal processes which cause changes in microstructure of the mc-Si bulk during current stress.

Silicon deposition from BCl3/SiH4 mixtures: Effect of very high boron concentration on microstructure

The microstructures of in situ boron-doped silicon films deposited by low-pressure chemical vapor deposition using SiH4 and BCl3 source gases on SiO2 substrates have been characterized. As-deposited films have a decreasing sheet resistance with increasing boron concentration up to 3.1±0.6×1021/cm3, and are polycrystalline. As the boron concentration exceeds 3.1±0.6×1021/cm3, the sheet resistance increases dramatically. It is shown that as the boron concentration in the silicon film exceeds 3.1±0.6×1021/cm3, the films transition from a completely polycrystalline phase to a combination of polycrystalline and amorphous phases as deposited. The amorphous phase has a higher boron concentration than the polycrystalline phase, as shown by selective Auger electron spectroscopy and secondary ion mass spectrometry. The relative fraction of amorphous phase can be controlled by boron concentration.

A photovoltaic cell from p-type boron-doped amorphous carbon film

Abstract Boron-doped amorphous carbon (a-C(B)) films were prepared on n-type silicon using pulsed laser deposition technique of a graphite target. The a-C(B) films have been proved to be p-type by the formation of a heterojunction between the a-C(B) film and n-Si. The device of a-C(B)/n-Si structure yielded an open-circuit voltage (Voc) of 0.27 V and a short-circuit current density (Jsc) of 2.2 mA/cm2 under illumination (AM1.5 100 mW/cm2). According to calculation, the energy conversion efficiency and fill factor were found to be about 0.3% and 0.53, respectively.

Electrical Properties of Phosphorus-Doped and Boron-Doped Nanocrystalline Germanium Thin-Films for p-i-n Devices

AbstractNanocrystalline germanium thin-films deposited on glass by plasma-enhanced chemical vapor deposition from germane and hydrogen were doped with phosphorus and boron. We report some electrical transport and structural properties of Ge films as a function of dopant species and doping levels. The dark conductivities of the phosphorus- and boron-doped films are approximately three to four orders of magnitude higher than the intrinsic nanocrystalline germanium. In the solid phase, phosphorus comprised about 1 atomic percent in the Ge bulk over the range of source gas ratios used, and the conductivity remained fairly constant, indicating saturated conditions. Boron comprised about 10 atomic percent in Ge at the highest dark conductivity, while increased doping turned the films amorphous. To test the doped layers for device applications, an all-nanocrystalline germanium p-i-n diode was constructed and showed rectification when measured in the dark at room temperature.

Temperature dependence of surface band bending andfield emission for boron-doped diamond anddiamond-like films

In this experiment we have measured the temperature dependenceof field emission characteristics for boron-doped diamond anddiamond-like films at various temperatures and low fields. Thethreshold voltage increases and emission current densitydecreases as the temperature decreases. These fieldemission characteristics can be successfully explained by the semiconductor thermionic emission theory. This measurementapparently reveals that the electron affinity is not changedwhile bending down of the surface band is enhanced astemperature decreases, resulting in reduction of the emissioncurrent.

Preparation of p-n Junction Diode by B-Doped DiamondFilm Grown on Si-Doped c-BN

A heterojunction diode has been fabricated by boron-doped p-type diamondthin film grown epitaxially on a silicon-doped n-type cubic boronnitride bulk crystal using the conventional hot filament chemical vapourdeposition method. The ohmic electrode of Ti (50 nm)/Mo (100 nm)/Au(300 nm) for the p-type diamond film and the bulk crystal of the c-BN weredeposited by the rf planar magnetron method. Then the device wasannealed at 410°C in air for 1 h in order to form ohmic metal alloy. Thecurrent-voltage characteristics of the heterojunction diode were measured and theresult indicated that the rectification ratio reached 105, and theturn-on voltage and the highest current were 7 V and 0.35 mA,respectively.

A Confocal Raman Imaging Study of an Optically Transparent Boron-Doped Diamond Electrode

Confocal micro-Raman imaging was used to investigate the structural and, to some extent, the chemical inhomogeneities that exist in a high-quality, free-standing, polished, optically transparent boron-doped chemical-vapor-deposition (CVD) diamond disk. A number of boron-related Raman lines centered at about 610, 925, 1045, 1375, and 1470 cm-1 were evidenced and found to vary from one region of the disk to another. These lines have previously been reported in the literature for other boron-doped crystals or films obtained using completely different growth conditions. Even if their origin is still somewhat uncertain, their presence enabled the construction of spatial maps of the boron signal intensity over the surface. It was confirmed that boron is incorporated into CVD diamond with significant inhomogeneities. In most cases, the boron signal intensity tracked the structure of the grains. The high quality of the disk also allowed a detailed analysis of the grain boundaries to be made. However, a unique description of these grain boundaries in terms of their microstructure and chemical composition could not be developed. There is some evidence that nondiamond phases are incorporated into the grain boundaries, but their Raman signature is quite distinct from that usually reported. Another remarkable feature of this disk is that it is nearly photoluminescence-free. Photoluminescence, as excited by the 514.5 or 457.9 nm lines of an argon ion laser, was only detected within some of the nondiamond amorphous carbon-rich grain boundaries. The low photoluminescence background was also a common feature of a heavily boron-doped, polycrystalline film deposited on silicon, another sample that was also examined in this work for comparison. Despite its high quality, the optically transparent disk is far from being stress-free. Localized stress variations were observed. The strained regions are not systematically associated with the incorporation of nondiamond carbon. Grain boundaries and, most probably, planar defects have been identified as the stress sources. However, it remains difficult to explain the large compressive stress observed for some of the crystals.

Characterization of sub-micrometre silicon films (Si-LPCVD) heavily in situ boron-doped and submitted to treatments of dry oxidation

We present the results of a comprehensive investigation concerning changes in the structural and electrical properties of heavily (2 × 1020 cm−3) in situ boron-doped polysilicon thin films by the low pressure chemical vapour deposition method before and after thermal oxidation treatments. Secondary ion mass spectrometry and transmission electron microscopy, with their associated diffraction diagrams, and four-point probe resistivity measurements are performed on sub-micrometre layers (≈300 nm) deposited at two interesting temperatures Td1 = 520 °C and Td2 = 605 °C. The thermal oxidation experiments are carried out under dry oxygen (O2) at three oxidation temperatures, Tox, of 840, 945 and 1050 °C for several durations. After these thermal oxidation processes, remarkable changes in the behaviour of the doping profile, grain growth and electrical conductivity have been observed. This behaviour seems to be typically characteristic of the in situ heavily boron-doped films. In addition, to the presence of some saturation thermal dependence phenomena, we find that these results correlate well with, and support the presence of, another phenomena called the ‘differential of oxidation rate’ which is evidenced as typical of the in situ doped films.

Synchrotron radiation X-ray analysis of boron-doped diamond films grown by hot-filament assisted chemical vapor deposition

This paper presents a synchrotron X-ray radiation analysis of boron-doped diamond films grown by hot-filament assisted chemical vapor deposition (HFCVD). The diamond films were grown at different doping levels with the introduction of boron to the gas mixture by bubbling hydrogen in a B 2O 3 solution in methanol. The B/C ratio in methanol varied from 2000 to 20 000 ppm and the gas flow rates were controlled so that boron incorporation to the film varied in the range from 10 18 to 10 21 boron/cm 3. All other process parameters were kept unchanged to allow comparison only of the influence of the doping level. The film analyses were performed at the X-ray diffraction beamline of the Laboratório Nacional de Luz Sincrotron – LNLS, Brazil. The Debye–Scherrer configuration was used in this study. A high intensity monochromatic beam at λ=1.46 Å was used and an excellent signal to noise ratio was obtained for 2θ varying from 20° to 150°. The difractogram for the undoped diamond film show intense peaks from the (111), (220), (311), (400) and (331) crystallographic planes. For the boron-doped films a set of new diffraction lines appear and their intensities increase considerably with the doping level. The set of diffraction peaks of similar intensities are related to a hexagonal structure and were assigned with high confidence to tungsten carbide. This reveals that the boron-doping process in HFCVD facilitates the incorporation of tungsten carbide from the filament in the diamond film. The FWHM analysis of the diamond diffraction peaks shows a dependence of film crystallinity with doping level with a definite maximum at approximately 2.3×10 19 boron/cm 3.