Boron-doped Homoepitaxial Diamond Films Research Articles

Multi-microscopy characterization of threading dislocations in CVD-grown diamond films

We present site-coincident imaging of a high dislocation density boron-doped chemical vapor deposition-grown homoepitaxial (001) diamond film using electron channeling contrast imaging (ECCI), cathodoluminescence, transmission electron microscopy (TEM), and scanning secondary ion mass spectroscopy (SIMS). With growing interest in large area heteroepitaxial substrates, we show that ECCI is a promising technique to accurately evaluate dislocations in diamond films. We find the electron backscatter yield is sufficient to distinguish individual threading dislocations in ECCI despite the low atomic number of carbon, and we have generally good agreement between dislocations observed in ECCI and TEM of the same region. Importantly, relying on luminescence in the 430 nm dislocation-related A-band alone results in an underestimation of the defect density as we find only 20%–40% of the threading dislocations luminesce. We further show that dislocations do not perturb the spatial uniformity of boron doping measured by SIMS, even when the dislocations are clustered with high density, and we can tentatively rule out strong segregation effects at the dislocations. Our results establish the complementary use of microscopy and microanalysis methods to rapidly characterize and understand the impact of dislocations in diamond thin films.

Raman scattering in boron-doped single-crystal diamond used to fabricate Schottky diode detectors

Thanks to its exceptional physical and electronic properties, diamond is an attractive material for electronic devices working at high temperature and in harsh chemical environment. Its use as a semiconducting material for electronics is related to the possibility of doping it in order to control its conductivity. Semiconducting p-type diamond films can be grown when boron is introduced into the film. In this work, boron-doped (B-doped) homoepitaxial diamond films are grown by Microwave Plasma Enhanced Chemical Vapor Deposition. Raman and electrical characterizations are carried out on the films as a function of boron doping level. As the boron content increases, we observe systematic modifications in the Raman spectra of single-crystal diamonds. A significant change in the lineshape of the first-order Raman peak, as well as a wide and structured signal at lower wavenumbers, appears simultaneously in samples grown with higher boron content. A single crystal diamond Schottky diode based on a metal/intrinsic/p-type diamond junction is analysed.

RAMAN SCATTERING IN HEAVILY BORON-DOPED SINGLE-CRYSTAL DIAMOND

A series of boron-doped homoepitaxial diamond films grown by Microwave Plasma Enhanced Chemical Vapor Deposition at the University of Rome Tor Vergata have been investigated with Raman spectroscopy. As the boron content increases, we observed systematic modifications in the Raman spectra of single-crystal diamonds. A significant change in the lineshape of the first-order Raman peak as well as a wide and structured signal at lower wavenumbers appeared simultaneously in samples grown at higher boron content.

Development of ionizing radiation detectors based on synthetic diamond material for the nuclear power industry

Results of the experimental study of boron-doped single-crystal synthetic diamond substrates and thin homoepitaxial diamond films grown on them with the predetermined properties are discussed. The homogeneity of the substrates and films is examined. Diamond detectors based on the grown diamond structures have been produced. The sensitivity of these detectors to UV photons, X and γ rays, α particles, and fast neutrons under stationary and pulsed irradiation has been determined.

Front Cover (Phys. Status Solidi A 9/2009)

Homoepitaxial diamond films are more and more frequently proposed as the basic component of electronic devices, but like in other semiconductors, defects and unwanted impurities may hamper optimal operation. The article by Pierre Muret et al. (p. 2016) deals with these aspects in boron-doped homoepitaxial diamond films. A new understanding of the role and structure of impurities comes both from ab-initio calculations of the density of states and total energy of possible defect configurations, and from the electrical response of elementary devices, mainly Schottky diodes, whose transient regimes of current, charge or capacitance are influenced by local defects. The Fourier transforms of capacitance transients in a borondoped diamond Schottky diode, shown on the front cover, are representative of the experimental turning back to equilibrium of the hole trap population in the frequency domain at several temperatures. The small image displays the local structure of the most stable BH2 defect. The results obtained in this work demonstrate that defects incorporating hydrogen atoms are the most probable, but that their concentration profiles decrease rapidly inward the diamond film.

Recovery of negative electron affinity by annealing on (111) oxidized diamond surfaces

We discovered that oxidised (111) p-type boron-doped homoepitaxial diamond film surfaces grown by chemical vapor deposition (CVD) recover negative electron affinity (NEA) after annealing the samples in vacuum or under an Ar atmosphere. On the other hand, (001) p-type boron-doped homoepitaxial CVD diamond surfaces do not show NEA recovery. The oxidised (111) surface possesses positive electron affinity of + 0.6 eV, whereas the subsequently annealed surface has a negative electron affinity of − 0.8 eV, indicating a partial hydrogen termination condition that is supported by XPS results. These distinct behaviours should be taken into consideration for investigating the surface electronic properties of diamond.

Photoelectron emission from heavily B-doped homoepitaxial diamond films

We discuss the energy band structure near the valence band maximum based on photoemission yield spectroscopy experiments using a hydrogen-terminated heavily boron-doped homoepitaxial diamond film with concentration of 3 × 10 20 cm − 3 . The experimental results showed a metallic photoemission behavior with a negative electron affinity surface. Based on the fitting as metallic photoemission behavior with a Fowler plot, the Fermi level should be at 5.35 eV below the conduction band minimum, which means that the Fermi level lies at 0.12 eV (5.47–5.35 eV) above the valence band maximum. Thus the film shows metallic conduction by the Mott transition, but not as degenerate semiconductor.

High-quality diamond films grown at high deposition rates using high-power-density MWPCVD method with conventional quartz-type chamber

Undoped and boron-doped homoepitaxial diamond films with high quality have been successfully grown on high-pressure/high-temperature-synthesized type-Ib single-crystalline diamond (1 0 0) substrates. In the growth process, a conventional microwave-plasma (MWP) chemical-vapor-deposition (CVD) system with an easily-exchangeable 36-mm-inner-diameter quartz-tube growth chamber was employed under a condition of high MW power densities while a rather high methane concentration (4%) and high substrate temperatures (>1000 °C) were used. The growth conditions applied to the undoped and B-doped diamond thin films were separately optimized by controlling the MW plasma density and substrate temperatures. The homoepitaxial films thus grown yielded strong exciton-related luminescence even at room temperature, meaning that their crystalline quality was good and roughly comparable with that of homoepitaxial films deposited using a high-power MWPCVD system with a stainless steel chamber having a rather large diameter. This indicates that by using such a conventional deposition system with inexpensive and easily-exchangeable exclusive-use quartz-tube chambers, various growth experiments can be performed under different process conditions without any severe interference among the different experiments.

Electron-beam-induced dissociation of B–D complexes in diamond

The diffusion of deuterium in boron-doped homoepitaxial diamond films leads to the passivation of boron acceptors via the formation of B–D complexes. In this letter, the stability of B–D pairs is investigated under the stress of a low-energy (10keV) electron-beam irradiation at low temperature (∼100K). The dissociation of the complexes is evidenced by cathodoluminescence spectroscopy and is shown to result in the reactivation of most acceptors. The dissociation yield per incident electron is found to be strongly dependent on the e-beam current, which suggests a dissociation involving a vibrational excitation of the complexes by hot electrons.

Highly efficient doping of boron into high-quality homoepitaxial diamond films

High-quality boron-doped homoepitaxial diamond (100) films were grown using high-power microwave plasma chemical vapor deposition (MPCVD). Results showed that the incorporation efficiency of boron from gas phase was increased by two orders of magnitude compared to those of conventional growth conditions using high-density plasma. Boron-doped crystal thus grown with the low B / C ratio of 50 ppm in the gas phase had reasonably low resistivity of 2 Ω cm at 290 K, whereas Hall mobility of 830 cm 2 V s − 1 at this temperature was higher than those reported previous for such resistivity. The highly boron-doped film showed strong free exciton recombination emissions with a bound exciton component in the cathodoluminescence spectra taken at room temperature, reflecting the considerably high density of the substitutional boron in the film despite its low density of electronic defects. The diamond growth rate in the high-power MPCVD was 3.5 μm h − 1 . The highest room-temperature Hall mobility achieved in this study was 1620 cm 2 V s − 1 . These results indicate that the resultant high-rate growth with high-power MPCVD is advantageous for depositing high-quality and conductive diamond films.

Hydrogen passivation effects on carbon dangling bond defects accompanying a nearby hydrogen atom in p-type CVD diamond

Hydrogen (deuterium) passivation effects on carbon dangling bond defects accompanying a nearby hydrogen atom (H1′) in boron-doped CVD homoepitaxial diamond films were investigated by electron paramagnetic resonance (EPR). Deuterium was incorporated into the films using microwave deuterium plasma at 673 K for 22 h. Incorporation of deuterium was confirmed by the passivation of boron acceptors. From the comparison of the EPR signal intensity before and after the deuterium plasma exposure, it was revealed that H1′ could not be passivated by the present condition. From these, the high stability of H1′ was indicated.

Cathodoluminescence measurements on heavily boron doped homoepitaxial diamond films and their interfaces with their Ib substrates

AbstractHeavily boron doped 1.8 to 2.4 µm thick homoepitaxial diamond films with 1.5 × 1021 cm–3 ≤ [B] ≤ 1.75 × 1021 cm–3 have been deposited directly on their (100) Ib substrates at 830 °C. Their cathodoluminescence spectra probe the controlled thicknesses from 0.28 to 2.8 µm, therefore the bulk of the films as well as their interfaces with their substrates. The bulk of these films exhibit a band with shoulders ascribed to BETO (5.036 eV), FETO (5.094 eV) and BENP (5.184 eV) excitons whose energies are downward shifted by about 180 meV in comparison with monocrystalline diamond with low [B] < 5 × 1018 cm–3. This large shift allows the appearance of narrow peaks around 5.216, 5.271 and 5.357 eV ascribed to BETO, FETO and BENP from interfacial layers with low [B]. From their BETO to FETO ratio, their concentration of boron on isolated substitutional sites is significantly lower than their total low [B] content measured by SIMS. A tentative model is proposed to explain the characteristics of these 40 to 160 quasihomogeneous interfacial layers. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Electron-Beam-Induced Dissociation of Boron-Deuterium Pairs in Diamond

ABSTRACTThe diffusion of deuterium in boron-doped homoepitaxial diamond films leads to the passivation of boron acceptors via the formation of B-D pairs. In this letter, the stability of these complexes is investigated under the stress of a low-energy (10keV) electron-beam irradiation at low temperature (∼100K). The dissociation of the complexes is evidenced by cathodoluminescence spectroscopy and is shown to result in the reactivation of most acceptors. The dissociation yield per incident electron is found to be strongly dependent on the e-beam current, which suggests a dissociation involving a vibrational excitation of the complexes by hot electrons.

Electrical and optical characterization of boron-doped (111) homoepitaxial diamond films

Deposition condition dependence of electrical and optical properties of B-doped (111) diamond films grown by the microwave plasma chemical vapor deposition (CVD) technique using a CH 4/H 2 gas mixture was investigated mainly by Hall effect and cathodoluminescence (CL) measurements. It was found that Hall mobility ( μ H) of hole and peak intensity due to free-exciton recombination (FE TO) and boron acceptor bound exciton recombination (BE TO) increased and the compensation ratio decreased as the CH 4/H 2 ratio (growth rate) increased from 0.05% to 0.25%, while the B incorporation efficiency was independent of the CH 4/H 2 ratio. These results are contrary to those of (100) diamond film growth and indicate that the growth and doping mechanisms of (111) diamond films are different from those of (100) diamond films. The difference in the doping mechanism between (111) and (100) is discussed.

Ohmic contacts on p-type homoepitaxial diamond and their thermal stability

We have fabricated metals (Au/Pt/Ti) Ohmic contacts on mesa structures of boron-doped homoepitaxial diamond films by metal deposition followed by various thermal annealing processes. Specific contact resistance was determined by characterizing the current–voltage relations from transmission line model (TLM) measurement. The lowest specific contact resistance of 1.3 × 10−5 Ω cm2 at room temperature was obtained after 873 K annealing. The temperature dependence of the specific contact resistance of the (Au/Pt/Ti)/diamond contacts has confirmed the thermal stability of the contacts.

Excitonic recombinations and energy levels of highly boron doped homoepitaxial diamond films before and after hydrogenation

The incorporation of ≈10 19 B-cm −3 has several effects in the cathodoluminescence spectra of homoepitaxial MPCVD film: (1) the TO free exciton and all the boron-bound excitons' peaks broaden and shift downward in energy (“electronic” effect). Plasma post-hydrogenation (P-H) destroys all of them. (2) It induces a low concentration of several species of defects (“structural” effect): (i) a broad band around 4.99 eV with a shoulder around 4.86 eV, small bands on a broad 4.68 eV band, peaks and bands between 2.9 and 4.5 eV, the “2.8 eV” A band. They originate from B-induced species of defects only existing when [ B]≥≈10 19 cm −3. Most of them disappear after P-H. (ii) broad bands around 3.65, 4.1 and 4.68 eV. They originate from B-induced species of defects already existing when [ B]<10 17 cm −3. P-H prevents the formation of free exciton, doesn't induce any new defect giving radiative recombination above 2.6 eV, heals some structural defects of diamond.

High-quality boron-doped homoepitaxial diamond grown by high-power microwave-plasma chemical-vapor deposition

Boron-doped homoepitaxial diamond (100) films with substantially flat surface have been deposited by a high-power microwave-plasma chemical-vapor-deposition (MPCVD) method. Hall mobilities of 1500cm2∕Vs at 290K and 2220cm2∕Vs at 235K were attained for such a specimen with an acceptor density of 1.4×1018cm−3 grown at a growth rate of 3.5μm∕h. These mobilities are comparable with those of the highest quality homoepitaxial diamond grown by a standard low-power MPCVD method, where the growth rate and carrier concentration were, respectively, ∼1∕30 and 1∕5 of the corresponding values attained in the present case. This fact verifies that the high-power MPCVD is suitable for deposition of a high quality and high carrier-concentration p-type diamond at a reasonably high growth rate.

EPR study of hydrogen-related defects in boron-doped p-type CVD homoepitaxial diamond films

Boron-doped p-type single crystalline chemical vapor deposition (CVD) homoepitaxial diamond films were investigated by electron paramagnetic resonance (EPR). Carbon dangling bond defects, which were accompanied by a nearby hydrogen atom, were observed in boron-doped p-type CVD diamond films on a IIa substrate similar to those observed in undoped diamond. This result suggested that the energy level position of the defects is located below the Fermi energy of boron-doped diamond, at around 0.3 eV above the valence-band top. The reason why the Fermi energy could be changed by the incorporation of boron atoms at low density (10 16–10 17/cm 3) in the film in spite of the existence of the large defect density of EPR centers (∼10 18/cm 3) is thought to be that the singly occupied electron states of defects are located near the band edge. As for the thermal annealing effect of the defects, it was revealed that the concentration of the defects and the mobility of the p-type film did not change after annealing up to 1200 °C which is much higher than the temperature of boron–hydrogen pair dissociation.

Schottky junction properties on high quality boron-doped homoepitaxial diamond thin films

Experiments of annealing effect of boron-doped homoepitaxial diamond film on Schottky junction properties have been conducted to investigate the origin of the “pinning” states in oxidized surface of diamond. Metals of Al, Ni, Au, and Pt are evaporated onto boron-doped diamond films with post thermal annealing less than 800 °C or higher than 900 °C. The current–voltage (I–V) characteristics of Schottky junctions for low-temperature annealing show excellent rectification behavior, indicating the existence of the “pinning” states. After high-temperature annealing, the I–V curves show worse rectification or ohmic properties depending on the metal work function, suggesting the decrement of the density of “pinning” state. Capacitance–voltage measurements for the Schottky diodes show the strong thermal annealing effects on surface region property of the diamond film only after high temperature annealing. A possible explanation is the oxygen-related “pinning” states existing in the oxidized-surface of diamond are annealed out partially during high temperature annealing.

Charge-sensitive deep level transient spectroscopy of boron-doped and gamma-irradiated mono- and polycrystalline diamond

Electrically active defects in the boron-doped chemical-vapor deposited polycrystalline diamond films and single-crystal diamonds synthesized by high-pressure high-temperature technique were investigated. Isothermal charge-sensitive deep level transient spectroscopy was applied to study the density, activation energy and capture cross-section of native and boron-induced defects in diamond samples. It was shown that defects in the boron-doped polycrystalline diamond samples have a continuous energy spectrum with two peaks that correspond to discrete energy levels in the forbidden band of diamond, while in boron-doped homoepitaxial diamond films and single-crystal diamonds only one boron-induced deep level was displayed. The activation energy of the boron-induced levels decreases and the defect density increases with increasing boron concentration. It was found that gamma irradiation changes the parameters of boron-induced levels and increases the density of point defects with a continuous energy spectrum. The difference in activation energy of the boron-induced level into (1 1 1) and (1 0 0) faces of single-crystal diamond was observed as well.