Experimental evidence explaining the extremely low-threshold electron emission from diamond reported in 1996 has been obtained for the first time. Direct observation using combined ultraviolet photoelectron spectroscopy/field emission spectroscopy (UPS/FES) proved that the origin of field-induced electron emission from heavily nitrogen (N)-doped chemical vapour deposited (CVD) diamond was at conduction band minimum (CBM) utilising negative electron affinity (NEA). The significance of the result is that not only does it prove the utilisation of NEA as the dominant factor for the extremely low-threshold electron emission from heavily N-doped CVD diamond, but also strongly implies that such low-threshold emission is possible from other types of diamond, and even other materials having NEA surface. The low-threshold voltage, along with the stable intensity and remarkably narrow energy width, suggests that this type of electron emission can be applied to develop a next generation vacuum nano-electronic devices with long lifetime and high energy resolution.
We report large negative electron affinity (NEA) on diamond (100) using magnesium adsorption on a previously oxygen-terminated surface. The measured NEA is up to $(-2.01\pm0.05)$ eV, the largest reported negative electron affinity to date. Despite the expected close relationship between the surface chemistry of Mg and Li species on oxygen-terminated diamond, we observe differences in the adsorption properties between the two. Most importantly, a high-temperature annealing step is not required to activate the Mg-adsorbed surface to a state of negative electron affinity. Diamond surfaces prepare…
We report results of quantum efficiency (QE) measurements carried out on a 150 nm thick nitrogen-incorporated ultrananocrystalline diamond terminated with hydrogen; abbreviated as (N)UNCD:H. (N)UNCD:H demonstrated a QE of $\sim$10$^{-3}$ ($\sim$0.1%) at 254 nm. Moreover, (N)UNCD:H was sensitive in visible light with a QE of $\sim$5$\times$10$^{-8}$ at 405 nm and $\sim$5$\times$10$^{-9}$ at 436 nm. After growth and prior to QE measurements, samples were exposed to air for about 2 hours for transfer and loading. Such design takes advantage of a key combination: 1) H-termination inducing negative…
Nonlinear current voltage characteristics and in some cases also step like characteristics were observed in diamond like carbon films. We suggest that the transport mechanism is tunneling between conducting nanoparticles in the films and the observation of step like structures is a manifestation of Coulomb blockade found for the first time in these films. We point out that the origin of similar current voltage characteristics observed in other carbon structures can be also Coulomb blockade phenomenon.
We observe the electron spin resonance of conduction electrons in boron doped (6400 ppm) superconducting diamond (Tc =3.8 K). We clearly identify the benchmarks of conduction electron spin resonance (CESR): the nearly temperature independent ESR signal intensity and its magnitude which is in good agreement with that expected from the density of states through the Pauli spin-susceptibility. The temperature dependent CESR linewidth weakly increases with increasing temperature which can be understood in the framework of the Elliott-Yafet theory of spin-relaxation. An anomalous and yet unexplained…
We present density-functional theory (DFT) and quantum Monte Carlo (QMC) calculations designed to resolve experimental and theoretical controversies over the optical properties of H-terminated C nanoparticles (diamondoids). The QMC results follow the trends of well-converged plane-wave DFT calculations for the size dependence of the optical gap, but they predict gaps that are 1-2 eV higher. They confirm that quantum confinement effects disappear in diamondoids larger than 1 nm, which have gaps below that of bulk diamond. Our QMC calculations predict a small exciton binding energy and a negativ…
We present an ab-initio calculation of the electronic affinity of the hypothetical C-46 clathrate by studying its bare and hydrogenated (100) surfaces. We show that such a system shares with the diamond phase a small electronic affinity. Further, contrary to the diamond phase, the possibility of doping endohedrally these cage-like systems allows to significantly raise the position of the Fermi level, resulting in a true metal with a small work function. This is illustrated in the case of the Li8@C-46 doped compound. Such a class of materials might be of much interest for the design of electron…
The energetic evaluations of graphite-to-diamond transition by electron irradiation are performed. The heat conduction problem is solved for the diamond synthesis when a pulse-periodic source of energy is located within a graphite cylinder; time dependences of temperature and pressure are found. It is shown, that the temperatures and pressures implemented in graphite are sufficient for graphite-to-diamond transition under electron bombardment.
Renormalization of the electronic band gap due to electron-phonon coupling in the tetrahedral semiconductors diamond, silicon and cubic silicon carbide is studied from first principles. There is a marked difference between the coupling of the vibrational state to the valence band maximum and to the conduction band minimum. The strength of phonon coupling to the valence band maximum is similar between the three systems and is dominated by vibrations that change the bond length. The coupling strength to the conduction band minimum differs significantly in diamond, silicon carbide and silicon. In…
We present the electrical transport characteristics of a batch of nano-crystalline diamond films of varying nitrogen concentrations and explain the conduction mechanism by the disordered quasi-superlattice model applied to semiconductor heterostructures. Synthesized by the hot filament chemical vapour deposition technique, the degree of structural disorder in the films, confirmed from Raman spectroscopy, is found to be controllable, resulting in the transition of conduction mechanism from localized and activated to the metallic conduction regime. Hence through high field magneto-resistance mea…
Coating metal nanotips with a negative electron affinity material like hydrogen-terminated diamond bears promise for a high brightness photocathode. We report a recipe on the fabrication of diamond coated tungsten tips. A tungsten wire is etched electrochemically to a nanometer sharp tip, dip-seeded in diamond suspension and subsequently overgrown with a diamond film by plasma-enhanced chemical vapor deposition. With dip-seeding only, the seeding density declines towards the tip apex due to seed migration during solvent evaporation. The migration of seeds can be counteracted by nitrogen gas fl…
We used high order harmonics of a femtosecond titanium-doped sapphire system (pulse duration 25 fs) to realise Ultraviolet Photoelectron Spectroscopy (UPS) measurements on diamond. The UPS spectra were measured for harmonics in the range 13 to 27. We also made ab initio calculations of the electronic lifetime of conduction electrons in the energy range produced in the UPS experiment. Such calculations show that the lifetime suddenly diminishes when the conduction electron energy reaches the plasmon energy, whereas the UPS spectra show evidence in this range of a strong relaxation mechanism wit…
Ultra-wide bandgap and the absence of shallow dopants are the major challenges in realizing diamond based electronics. However, the surface functionalization offers an excellent alternative to tune electronic structure of diamonds. Herein, we report on tuning the surface electronic properties of hydrogenated polycrystalline diamond films through oxygen functionalization. The hydrogenated diamond (HD) surface transforms from hydrophobic to hydrophilic nature and the sheet resistance increases from ~ 8 kohms/sq. to over 10 Gohms/sq. with progressive ozonation. The conductive atomic force microsc…
Measurement of zeta potential of Ga and N-face gallium nitride has been carried out as function of pH. Both the faces show negative zeta potential in the pH range 5.5-9. The Ga face has an isoelectric point at pH 5.5. The N-face shows higher negative zeta potential due to larger concentration of adsorbed oxygen. Zeta potential data clearly showed that H-terminated diamond seed solution at pH 8 will be optimal for the self assembly of a monolayer of diamond nanoparticles on the GaN surface. Subsequent growth of thin diamond films on GaN seeded with H-terminated diamond seeds produced fully coal…
The electron-phonon interaction contribution to the electronic energies is included in density functional total energy calculations with ab initio pseudopotentials via the Allen formalism [Phys. Rev. B 18, 5217 (1978)] to obtain temperature dependent electronic structure of diamond and silicon. This method allows us to obtain the thermally-averaged ab initio electronic structure in a straightforward and computationally inexpensive way. Our investigations on the finite temperature electronic structure of diamond and silicon lead to a new criterion, temperature transferability, which is required…
The interfacial band configuration of the high-k dielectric Al$_2$O$_3$ deposited at 120°C by atomic layer deposition (ALD) on boron- and phosphorus-doped hydrogen-terminated (111) diamond was investigated. Performing X-ray photoelectron spectroscopy measurements of core level binding energies and valence band maxima values, the valence band offsets of both heterojunctions are concluded to be ΔE$_V$ = 1.8 eV and ΔE$_V$ = 2.7 eV for the Al$_2$O$_3$/H(111)p and the Al$_2$O$_3$/H(111)n respectively. The ALD Al$_2$O$_3$ bandgap energy was measured from the O 1s photoelectron energy loss spectra to…
The density of states and the band diagrams were computed for diamond, cubic boron nitrde (cBN), and hexagonal boron nitride (hBN) using a Korringa-Kohn-Rostoker (KKR) scheme to investigate the shift of the Fermi level by impurity-atom doping below 10 at.%. The dopant atoms were B and N for diamond, Be, Si, and C for cBN, and Be and C for hBN. It was found that the Fermi level was located at the valence band maximum or the conduction band minimum in the following seven cases: (i) the B concentration was 0.3 at.% in B-doped diamond, (ii) the N concentration was 0.4 at.% in N-doped diamond, (iii…
The transferred-electron oscillator (TEO) is a device used in microwave applications that utilizes the negative differential mobility (NDM) effect to generate continuous oscillations. Recently, NDM was observed in intrinsic single-crystalline chemical vapor deposition (SC-CVD) diamond. The occurrence was explained by the electron repopulation between its different conduction band valleys. This paper presents the results of constructing a diamond TEO based on the NDM effect. A series of experiments has been performed for varying voltages, temperatures and resonator parameters on three SC-CVD di…
We report time-dependent photocurrent and transport measurements of sub-bandgap photoexcited carriers in nitrogen-rich (type Ib), single-crystal diamond. Transient carrier dynamics are characteristic of trapping conduction with long charge storage lifetimes of ~3 hours. By measuring the photoexcited Hall effect we confirm that the charge carriers are electrons and by varying the excitation energy we observe a strong turn-on in the photoconduction at ~1.9 eV. These findings shed new light on sub-bandgap states in nitrogen doped single-crystal diamond.
Recently, a number of new two-dimensional (2D) materials based on puckered phosphorene and arsenene have been predicted with moderate band gaps, good absorption properties and carrier mobilities superior to transition metal dichalcogenides. For heterojunction applications, it is important to know the relative band alignment of these new 2D materials. We report the band alignment of puckered CaP$_3$, CaAs$_3$ and BaAs$_3$ monolayers at the quasiparticle level of theory (G$_0$W$_0$), calculating band offsets for isolated monolayers according to the electron affinity rule. Our calculations sugges…
The spin-split indirect bandgap in hybrid-halide perovskites provides a momentum-space realisation of a photon-ratchet intermediate band. Excited electrons thermalise to recombination-protected Rashba pockets offset in momentum space, building up the charge density to have sufficient flux to the higher lying conduction band. This effect could be used to form an intrinsic intermediate band solar cell with efficiencies beyond the Shockley-Queisser limit if a selective low-electron affinity contact can be made to the higher conduction state. This concept is supported by analysis of the many-body …
Polarons are quasi-particles made from electrons interacting with vibrations in crystal lattices. They derive their name from the strong electron-vibration polar interaction in ionic systems, that induces associated spectroscopic and optical signatures of such quasi-particles in these materials. In this paper, we focus on diamond, a non-polar crystal with inversion symmetry which nevertheless shows characteristic signatures of polarons, better denoted "nonpolarons" in this case. The polaronic effects are produced by short-range crystal fields with only a small influence of long-range quadrupol…
In the nitrogen-incorporated ultrananocrystalline diamond ((N)UNCD) films, representing an $n$-type highly conductive two-phase material comprised of $sp^3$ diamond grains and $sp^2$-rich graphitic grain boundaries, the current is carried by a high concentration of mobile electrons within the large-volume grain boundary networks. Fabricated in a simple thin-film planar form, (N)UNCD was found to be an efficient field emitter capable of emitting a significant amount of charge starting at the applied electric field as low as a few V/$μ$m which makes it a promising material for designing electron…
The protocols for the control and readout of Nitrogen Vacancy (NV) centres electron spins in diamond offer an advanced platform for quantum computation, metrology and sensing. These protocols are based on the optical readout of photons emitted from NV centres, which process is limited by the yield of photons collection. Here we report on a novel principle for the detection of NV centres magnetic resonance in diamond by directly monitoring spin-preserving electron transitions through measurement of NV centre related photocurrent. The demonstrated direct detection technique offers a sensitive wa…
Silicon is the most popular material used in electronic devices. However, its poor optical properties owing to its indirect band gap nature limit its usage in optoelectronic devices. Here we present the discovery of super-stable pure-silicon superlattice structures that can serve as promising materials for solar cell applications and can lead to the realization of pure Si-based optoelectronic devices. The structures are almost identical to that of bulk Si except that defective layers are intercalated in the diamond lattice. The superlattices exhibit dipole-allowed direct band gaps as well as i…
We present a simplified computational scheme in order to calculate the effects of electron correlations on the energy bands of diamond and silicon. By adopting a quasiparticle picture we compute first the relaxation and polarization effects around an electron set into a conduction band Wannier orbital. This is done by allowing the valence orbitals to relax within a self-consistent field (SCF) calculation. The diagonal matrix element of the Hamiltonian leads to a shift of the center of gravity of the conduction band while the off-diagonal matrix elements result in a small reduction of the condu…
Although the direct or indirect nature of the bandgap transition is an essential parameter of semiconductors for optoelectronic applications, the understanding why some of the conventional semiconductors have direct or indirect bandgaps remains ambiguous. In this Letter, we revealed that the existence of the occupied cation d bands is a prime element in determining the directness of the bandgap of semiconductors through the s-d and p-d couplings, which push the conduction band energy levels at the X- and L-valley up, but leaves the Γ-valley conduction state unchanged. This unified theory unamb…
Oxygen vacancies at the SnO2(110) and (101) surface and subsurface sites have been studied in the framework of density functional theory by using both all-electron Gaussian and pseudopotential plane-wave methods. The all-electron calculations have been performed using the B3LYP exchange-correlation functional with accurate estimations of energy gaps and density of states. We show that bulk oxygen vacancies are responsible for the appearance of a fully occupied flat energy level lying at about 1 eV above the top valence band, and an empty level resonant with the conduction band. Surface oxygen …
A technique to measure the band gap of dielectric materials with high refractive index by means of energy electron loss spectroscopy (EELS) is presented. The technique relies on the use of a circular (Bessel) aperture and suppresses Cherenkov losses and surface-guided light modes by enforcing a momentum transfer selection. The technique also strongly suppresses the elastic zero loss peak, making the acquisition, interpretation and signal to noise ratio of low loss spectra considerably better, especially for excitations in the first few eV of the EELS spectrum. Simulations of the low loss inela…
We report on the generation of bulk photocurrents in materials driven by non-resonant bi-chromatic fields that are circularly polarized and co-rotating. The nonlinear photocurrents have a fully controllable directionality and amplitude without requiring carrier-envelope-phase stabilization or few-cycle pulses, and are generated with photon energies much smaller than the band gap (reducing heating in the photo-conversion process). We demonstrate with ab-initio calculations that the photocurrent generation mechanism is universal and arises in gaped materials (Si, diamond, MgO, hBN), in semi-meta…
