B-doped Layers Research Articles

Back-Surface-Field Effects at the Heterojunctions between Boron-Doped p-Type Hydrogenated Amorphous Silicon and Crystalline Silicon in Thin-Film Crystalline Silicon Solar Cells

We have found that the back-surface recombination velocity, Sb, of minority carriers in crystalline silicon (c-Si) thin film solar cells can be reduced to less than 103 cm/s when a boron (B)-doped p-type hydrogenated amorphous silicon (a-Si:H) layer is deposited on the back surface of a p-type c-Si substrate at 200°C, while the value of Sb is 106 cm/s when a B-doped p-type layer is epitaxially grown on the c-Si substrate. We have clarified, from internal photoemission (IPE) and attenuated-total-reflection Fourier transform infrared (ATR-FTIR) spectroscopy measurements, that the band lineup of the heterojunction between B-doped p-type a-Si:H and c-Si and the hydrogen passivation of defects in B-doped a-Si:H are possible reasons for the observed low value of Sb.

Boron lattice location in room temperature ion implanted Si crystal

The B lattice location in presence of a Si-self-interstitial (I Si) supersaturation, controlled by energetic proton bombardment, has been studied by means of ion channelling and massive Monte Carlo simulations. B-doped layers of Si crystals with a B concentration of 1 × 10 20 B/cm 3 were grown by Molecular Beam Epitaxy. Point defect engineering techniques, with light energetic ion implants, have been applied to generate an I Si uniform injection in the electrically active layer. The displacement of B atoms out of substitutional lattice sites was induced by 650 keV proton irradiations at room temperature (R.T.) and the resultant defect configuration was investigated by ion channelling and Nuclear Reaction Analysis (NRA) techniques. Angular scans were measured both through 〈1 0 0〉 and 〈1 1 0〉 axes along the (1 0 0) plane using the 11B(p,α) 8Be nuclear reaction at 650 keV proton energy. Monte Carlo simulated angular scans were calculated considering a variety of theoretical defect configurations, supported by literature, and compared with experimental data. Our experimental scans can be fitted by a linear combination of small (0.3 Å) and large B displacements (1.25 Å) along the 〈1 0 0〉 direction, compatible with the B-dumbbell oriented along 〈1 0 0〉 as proposed by ab initio calculations.

Microcrystalline B-doped window layers prepared near amorphous to microcrystalline transition by HWCVD and its application in amorphous silicon solar cells

The electronic and structural properties of p-type microcrystalline silicon films prepared near the microcrystalline to amorphous (μc-amorphous) transition by hot-wire chemical vapor deposition are studied. Silane is used as a source gas while H 2 as diluent and trimethylboron (TMB) and boron trifluoride (BF 3) as doping gases. Increasing TMB concentration from 0.01% to 5% favors the amorphous growth whereas for BF 3 the crystalline fraction remains constant. The dark conductivity ( σ d) of μc-Si:H p-layers remains approximately constant for TMB = 1–5% at constant crystalline fraction X c. This dark conductivity behavior is attributed to the decrease in doping efficiency with increasing TMB concentration. The best initial efficiency obtained for a 400 nm amorphous pin solar cell with optimized μc-Si:H p-layer is 7.7% ( V oc = 874 mV, J sc = 12.91 mA/cm 2, FF = 68%).

Quantitative Measurements of Two-Dimensional Ultrashallow B Profiles in Si by Selective Chemical Etching

A 2D delineation of ultrashallow B-doped layers was performed by means of selective etch combined with transmission electron microscopy imaging. Specifically, the possibility to extract the active dopant distribution from the profile of the etched region was explored. The etch process was simulated in order to obtain the etch profile evolution and reproduce the experimental data. Good agreement between experimental results and simulated profiles was achieved. Therefore, this experimental technique supported by simulation allows quantitative correlation of the etch depth to the active dopant distribution and the expected etch depth to be calculated for a given dopant profile.

Incorporation of boron in SiGe(C) epitaxial layers grown by reduced pressure chemical vapor deposition

In this paper the strain and electrical properties of epitaxial in situ B-doped (10 18–10 21 cm −3) SiGeC layers (23, 28% Ge and 0, 0.5% C) has been investigated. The growth rate was shown to have a significant increase at 3×10 −2 mTorr diborane partial pressure. This point coincides with an enhancement in boron incorporation, which was explained by the strain compensation effect of boron in the highly strained SiGeC layers. In these samples, the total Ge and C content was shown to remain constant with increasing diborane partial pressure. The substitutional/active dopant concentration in SiGe layers was obtained by high-resolution X-ray diffraction by measuring the strain compensation effect of boron. The interaction between C and B in SiGe matrix was also investigated. This was compared with the active dopant concentration obtained from Hall measurements in order to achieve a Hall scattering factor of 0.3–0.7 for dopant concentrations between 3×10 18 and 5×10 21 cm −3. The resistivity values of these layers were in the range 2×10 −2–4×10 −4 Ω cm. Finally, it was shown that boron atoms in SiGeC layers locate preferably at substitutional sites in contrary to carbon atoms at both substitutional and interstitial sites.

Fabrication of a submicron source-drain gap for p–i–p field effect transistors using epitaxial diamond layers

A p–i–p diamond field effect transistors (FETs), which consist of an epitaxial channel with submicron length, are fabricated. Two different fabrication processes to make p–i–p FETs are demonstrated, depending on how heavily B-doped diamond layers for source and drain are formed: ion implantation and selective deposition. The p–i–p FET is operated by the modulation of space charge limited current and featured a transconductance up to 9.1 mS/mm.

Observation of electron field emission patterns from B-doped diamond films

To develop electron beam sources of carbon materials, field emission patterns were observed in three different setups. The first was a diode-type, in which a carbon specimen was facing to a positively biased fluorescence plate. The second was a triode-type, in which a positively biased grid was placed between them. In the third setup, a commercial electron gun was modified so that it could accommodate a carbon specimen and a grid. A fluorescence plate was placed in a vacuum chamber outside the gun. As the carbon specimen for electron emission source, B-doped diamond films, a single crystal diamond with a B-doped layer, an undoped diamond film and a glass-like carbon both with a fibrous structure at the surface, and a sponge carbon were used. It was found that electron emission from edges was dominant for 1×1 cm diamond films and carbon specimens in the diode-type setup. In the triode-type setup, the edges of the specimens were masked with a Kapton® tape. The electron emission occurred only from some spots on the specimen. In the electron gun setup, it was confirmed that an electron beam was generated, and a fairly uniform circle was seen on the fluorescence plate under defocused situation, while the circle became smaller by adjusting the current of the focusing lens. Although more uniform emission from the electron source materials seemed to be necessary for practical applications, it was demonstrated that an electron beam could be generated even in such a simple setup.

N-type diamond with high room temperature electrical conductivity by deuteration of boron doped diamond layers

The conversion of the conductivity of B-doped homoepitaxially grown diamond layers from p- to n-type upon deuteration and its reconversion to p-type following annealing is extensively studied. Several B doped samples have been converted to n-type when exposed to a deuterium plasma at approximately 500 °C. The n-type features are related to D uptake of the samples. The donors, thus formed, have an ionization energy of approximately 0.34 eV below the conduction band edge and a high RT (room temperature) mobility (up to 430 cm 2/V s), as determined by Hall effect measurements as a function of temperature. In the B-doped layers that do convert to n-type upon deuteration, the D to B concentration ratio is found to be in the range of one to two. Higher D uptake (D to B ratio ≫2) must be connected to B-doped layers containing a large amount of growth defects, which trap the D. The exact nature of the donor is yet unknown. However, the reversibility of the effect and its relationship with the D concentrations suggest that deuterium is involved in the formation of some complex with B or with some other defects.

Low temperature InP/Si wafer bonding

An oxide-free, covalently bonded interface of InP/silicon wafer pairs has been realized at low temperature by B2H6 plasma treatment of bonding surfaces in the reactive ion etch mode followed by a HF dip and room temperature bonding in air. The bonding energy reaches InP fracture surface energy of 630 mJ/m2 at 200 °C. A total B-doped amorphous layer of about 15 Å with peak concentration of ∼2×1020 cm−3 was detected at the bonding interface. The release of hydrogen at low temperature from B–H complexes and subsequent absorption of the atomic hydrogen by the amorphous layer at the bonding interface is most likely responsible for the enhanced bonding energy.

Electrical properties of B-related acceptor in B-doped homoepitaxial diamond layers grown by microwave plasma CVD

Electrical properties of B-related acceptor associated with B doping have been studied by current–voltage ( I– V) measurements, frequency-dependent capacitance–voltage ( C– V) measurements, Hall effect measurements and secondary ion mass spectroscopy measurements in B-doped homoepitaxial diamond layers grown by microwave plasma CVD. Lateral dot-and-plane (with ring-shaped gap) Schottky barrier diodes using Au/Ni and Au/Pt/Ti were fabricated. These diodes have been considered to be nearly ideal Schottky junctions, in which the ideality factor n was 1.04 at 300 K. Frequency-dependent measurements on these Schottky barrier diodes have shown that capacitance is reduced at a high-frequency, most probably due to the inability of a deep center to maintain an equilibrium ionization state under a high-frequency modulation. We have shown the relation between doped B concentration, net acceptor concentration and hole concentration at 300 K. B electrical activity, which means ratio of net acceptor concentration to B concentration, was found to be almost unity in the B concentration range from 5×10 16 to 6×10 17 cm −3. It was considered that the background (or compensating) donor concentration is quite low in comparison with the net acceptor concentration in B-doped homoepitaxial diamond layer in this work.

Defect-dopant interaction in n- and p-type diamond and its influence on electrical properties

The electrical properties of semiconducting diamond are governed by the material quality, i.e. by the presence of defects. Here we report on the results of a comparative study on the dependence of the electrical properties of n- and p-type doped diamond layers on the density of defects present in the layer. Three main points are addressed: (1) the nature and conductivity-type of the defects, (2) the influence of the defects on the electrical characteristics of the layer (compensation, passivation of the dopants) and (3) the comparison between the defect/dopant interaction in the p- vs. n-type samples. For that end, defects are introduced by implanting ever-increasing doses of hydrogen through homoepitaxially B- and P doped diamond layers, leaving in the layer well-known amounts of point defects. Following each implantation dose, the electrical characteristics of the sample are evaluated by using resistivity and Hall effect measurements over a wide temperature range. For the first time, we demonstrate by means of electrical measurements the amphoteric nature of the defects introduced in the B- and P-doped diamond films, i.e. the fact that defects may exist in different charge states, depending on the conductivity type of the doped layers; the defects being in a positive charge state in B-doped layers and in a negative state in P-doped diamond; hence both impurities are compensated by the irradiation induced defects.

Using point-defect engineering to increase stability of highly doped ultrashallow junctions formed by molecular-beam-epitaxy growth

Stability of p+/n junctions remains a critical issue for device performance. We report that the technique of point-defect engineering (PDE) can substantially increase the stability of ultrashallow junctions formed by molecular-beam epitaxy. It is shown that an as-grown 15 nm, 2×1020/cm3 B-doped Si layer becomes unstable during 10 min thermal anneal above 650 °C. The thermal stability can be increased by performing a 5×1015/cm2 1 MeV Si ion implantation. The B profile with the MeV Si implant does not show significant diffusion during annealing up to 750 °C, and the final junction depth after an 800 °C/10 min anneal is about half that of an annealed unimplanted sample. Although with Mev implantation the as-implanted B profile becomes slightly deeper due to recoil implantation, and some of the B has been electrically deactivated by the MeV implantation, PDE is advantageous for postgrowth thermal processes above 700 °C. The mechanism causing the instability is discussed.

Hydrogen-induced splitting in silicon over a buried layer heavily doped with boron

Formation of interior hydrogen-passivated surfaces in hydrogen-implanted single-crystal Si containing a buried layer heavily doped with boron is investigated. With the use of the infrared absorption spectroscopy, it is shown that, upon annealing, the composition of hydrogen-containing defects in Si samples containing a buried heavily doped layer is the same as in Si samples that do not have such a layer. However, the presence of a heavily doped layer enhances the blistering and exfoliation of a thin silicon film from the Si sample, and the activation energies of the relevant processes change. Thus, the process of development of cavities in such layers changes upon thermal annealing. The depth at which hydrogen-passivated surfaces are formed corresponds to the projected range of H ions in Si, which also corresponds to the depth at which the B-doped layer is located. When a thin exfoliated film is transferred onto an insulator to form a silicon-on-insulator structure, the surface roughness of the film decreases by a factor of 2–5.

Electrical properties of boron-doped diamond films synthesized by MPCVD on an iridium substrate

Boron-doped diamond films were synthesized on an iridium substrate by microwave plasma-assisted chemical vapor deposition, using trimethylboron as the dopant source. The Ir substrate was bias-treated by the constant-current mode to permit the formation of oriented diamond nuclei. In order to isolate the B-doped diamond layer electrically from the Ir substrate, the non-doped diamond particles that were formed were grown in the 〈1 0 0〉 and then 〈1 1 1〉 directions and finally, a B-doped layer was synthesized to give a B/C ratio of 100–400 ppm. Surface morphology, Hall mobility and hole concentration were investigated for the resulting B-doped diamond films. The Hall mobility was closely related to the surface morphology of the diamond films. After optimizing the synthesis conditions, a Hall mobility of 340 cm −2 V −1 s −1 and hole concentration of 2×10 10 at 250 K was obtained for a heteroepitaxial B-doped diamond film synthesized at a B/C ratio of 200 ppm. These values are smaller than previously reported values for homoepitaxial B-doped diamond film.

Ultraviolet electroluminescence at room temperature from a pn junction of heteroepitaxial diamond film by CVD

The ultraviolet emission from a pn junction of heteroepitaxial diamond film was investigated. Diamond films were deposited on Si(100) by microwave plasma chemical vapor deposition. B-doped and P-doped layers were formed by trimethylboron and phosphine as impurity source gasses. The properties of p- and n-type layers were characterized by SEM, SIMS, Raman spectroscopy and Hall measurements. The experimental results showed that the current–voltage ( I– V) characteristics of the pn junction exhibited good rectifying properties. A sharp emission peak at 235 nm was observed at 22 V for 9 mA at room temperature. Broad A-band emission in the visible region also appeared simultaneously. The results obtained are discussed in detail.

Selective Epitaxy of Si1 − xGex Layers for Complementary Metal Oxide Semiconductor Applications

The selective growth of structures on patterned substrates aimed for channel layer applications in a metal-oxide-semiconductor field effect transistor structure was investigated. By optimizing the growth parameters the surface roughness of these structures was reduced. Furthermore, selective epitaxy of high B- or P-doped SiGe layers for source/drain applications was also studied. Abrupt dopant profiles with a good epitaxial quality and low sheet resistances, e.g., 195 and 260 Ω/□ for 420 Å thick, B-doped and P-doped layers, respectively, were obtained. In this study, secondary ion mass spectroscopy, high-resolution reciprocal lattice mapping, atomic force microscopy, and cross-sectional transmission electron microscopy were used as the main characterization tools. © 2003 The Electrochemical Society. All rights reserved.

Diffusion of hydrogen in undoped, p-type and n-type doped diamonds

Hydrogen is a key impurity in diamond since it is unintentionally incorporated in all chemical vapor deposition (CVD) grown diamond layers. Its presence in the material can grossly affect its electrical and optical properties. Theoretically, hydrogen has been predicted to be present in diamond in one of the three charge states, H +, H 0 and H −. Moreover it may form complexes with impurities, native defects or with other hydrogen atoms. This paper is comprised of two parts: (a) a review of previous results of studies investigating different aspects of the diffusion of hydrogen (deuterium) in various kinds of diamonds. The diamonds studied are: undoped type IIa diamonds, undoped CVD diamond layers containing growth defects only, p-type B-doped homoepitaxially CVD grown diamond layers or B ion implanted type IIa diamonds and n-type P doped homoepitaxially CVD grown diamond or N-doped type Ib natural diamonds. Hydrogen is introduced in diamond by exposing the diamond surface to hydrogen plasma or by using hydrogen ion implantation. The following issues are discussed: (1) the influence of the interaction between H and the dopants and defects on the hydrogen diffusion. (2) The kinetic of (B, H), (P, H) and (N, H) pair formation and dissociation. (3) The modification of the optical and electrical properties as a result of hydrogen incorporation and annealing. It is found that, under certain conditions, H diffuses into the B containing layer and it passivates B acceptors. In contrast, no H diffusion could be observed in n-type diamonds, up to 1000 °C. (b) Recent results of our group regarding other aspects related to the diffusion of H in diamond are presented. These include results on: (i) the influence of ion implantation related defects on the diffusion of deuterium. For this study type IIa samples implanted with B or non-dopant ions are used. (ii) The determination of the charge state of H or H/defects complex as a function of diamond type. For that, annealing under bias is applied to deuterated diamond layers. We show that the presence of implantation defects retards the deuterium diffusion in a B-implantation doped diamond, demonstrating that D strongly interacts with defects, thus inhibiting diffusion. The new-formed complexes deteriorate the electrical properties of the diamonds and are very stable up to high temperatures. We confirm that, as expected, in highly B-doped CVD diamond layers, H diffuses as a positive ion. In lightly B-doped homoepitaxial layers, however, D is incorporated in complexes which seem to be negatively charged.

Formation and thermal stability of ultra-shallow p+ junctions in Si and Si1−x Gex formed by molecular beam epitaxy

Ultra-shallow junction layers are required for deep-submicron complementary metal-oxide-semiconductor transistors and quantum devices. The use of low temperature Si molecular beam epitaxy (MBE) has been investigated for the formation of 10 nm ultra-shallow doped layers. Uniformly B-doped Si layers, B delta-doped Si, and uniformly B-doped Si1−x Gex (x=0, 0.2, or 0.4) layers have been studied. Regardless of the growth technique (uniform versus delta-doped) or Ge concentration, the sheet resistance of the optimally-doped, as-grown samples were equivalent to idealized, box profiles having a carrier concentration equal to the B solid solubility limit in Si, 6×1020/cm3, which is a substantial improvement over the best reported, ion implanted shallow layers. The B in the as-grown, optimally-doped MBE layers is fully electrically active and does not require a post-growth anneal. The thermal budget of the layers was established so that the layers can be employed in a device fabrication process. The MBE-grown shallow-doped layers were not affected by 10 min thermal processing up to 700 °C. While substantial B redistribution occurred as a result of the 800 °C 10 min anneal, the sheet resistances of the MBE-grown layers are still less than ion implanted samples having the same junction depth. In addition, it was observed that the Ge alloy layers impeded the diffusion of B at 800 °C.

Characterization of a diamond p-n junction using electron-beam-induced current and cathodoluminescence

We have characterized a diamond p-n junction by means of electron-beam-induced current (EBIC) and cathodoluminescence (CL). The diamond p-n junction was fabricated by growing a B-doped p-type layer and a P-doped n-type layer on the {111} diamond substrate by microwave plasma enhanced chemical vapor deposition. The cross section of p-n junction was revealed by the mesa etching. The substrate, B- and P-doped layers were distinguished by CL spectra. The EBIC profiles across the p-n junction were recorded under various reverse bias conditions. These data confirmed that the p-n junction was actually formed at the interface between B- and P-doped layers. The energy-band profile suggests that the carrier concentration of P-doped layer is more than ten times higher than that of B-doped layer. The observed results strongly support the fact that the UV luminescence is emitted from the p-n junction region in diamond light-emitting diode.

The Effect of SiGe Barriers on the Thermal Stability of Highly B-Doped Si Surface Layers

ABSTRACTWe have investigated the use of low-temperature (320 °C) molecular-beam epitaxy (MBE) to form highly conductive, p+, ultra-shallow layers in Si. Although the as-grown B-doped Si is electrically active, in a practical application the doped layers may be exposed to high temperature during post-growth device processing. To minimize the B diffusion, we investigated the use of SiGe diffusion barrier layers. In this work we demonstrate there is less B redistribution with the SiGe diffusion barriers. The use of SiGe diffusion barriers may prove to be critical in the activation of B implants for the formation of ultra-shallow junctions.