Figure 3.8. X-ray photoelectron spectra-Cu2p spectrum of 1 ~ 30 layers h-BN films on Cu foil used to identify the element present.
layers with various thicknesses from 1 to 15 layers. Mono-, bi-, and 4-layer samples show measurable low-bias conductance, which we ascribe to direct tunneling. Thicker samples are insulating at low bias and show sharp increases at a breakdown voltage that increases with thickness.
The inset in the Figure 4b shows the conductance as a function of sample thickness, which decays exponentially, as expected for direct tunneling. The current densities at the two metal electrodes and through the h-BN layers of different thicknesses were investigated as a function of voltage, as plotted in Figure 3.9. (c). As shown in Figure 3.9. (c), the measured currents of the thin h-BN films agreed well with the Poole-Frenkel (PF) emission model, indicating that a trap-assisted PF emission mechanism dominated the transport mechanism for the leakage current in our h-BN films. Figure 3.9. (c) shows the PF plot using the following equation:
( ) exp 0
T
r
PF c
q qVd
I V AqN Vd
kT
where A, q, Nc, µ, ΦT, V, d, and h are the effective area, electron charge, density of state in the conduction band, electronic mobility in the oxide, trap energy level in the h-BN, voltage, h-BN thickness, and Planck constant, respectively.
Figure 3.9. (a) A schematic diagram of the Au / h-BN / Au (MIM) capacitors fabricated on a Si substrate. (b) Characteristic I−V curves for Au / h-BN / Au devices with different thicknesses of BN insulating layer: red curve, monolayer of BN; orange, bilayer; green, four layer; navy, 10 layers; and purple, 15 layers. The inset of (a) is typical J-V characteristics of a MIM capacitor, described by the field-assisted tunneling model. The h-BN thickness range was less than 5 nm. The inset shows a PF emission plot (J/V versus 1/V1/2). (d) The breakdown characteristics as a function of the h-BN film thickness.
Finally, for thicker (> 1 nm) h-BN films, we performed irreversible dielectric breakdown measurements to determine the hard-breakdown voltage (Figure 3.10) and corresponding field strength of the ultrathin h-BN. Figure 3.9 (d) plots the breakdown field strength as a function of the h-BN thickness. In h-BN films with thicknesses less than 5 nm, the breakdown voltage increased linearly with h-BN thickness, indicating very high quality films at the few-layer limit. Breakdown field strength approaching ~ 4.3 MV/cm were observed for 4.5 nm thick BN films.
Figure 3.10. (a) The characteristic I−V curves for Au / h-BN / Au devices with 3 nm h-BN layer. (b) The characteristic I−V curves for Au / h-BN / Au devices with 4.5 nm h-BN layer.
3.7 Single photon emitters (SPE) of h-BN layers synthesized by APCVD method130
Solid-state single photon emitters (SPEs) are fundamental light sources for scalable quantum technologies including quantum computing, quantum precision measurement, and quantum secure communication. Over the years, SPEs have been established in a variety of solid-state systems such as zero dimensional (0D) GaAs and InGaAs quantum dots (QDs), one dimensional (1D) carbon nanotubes (CNTs) and InP nanowires, as well as three dimensional (3D) wide-bandgap diamonds and GaN. Each emitter type is characterized by the spectral diffusion, i.e., temporal variations in emission energy around a nominal value, which poses a challenge to the use of solid-state quantum emitters as sources of indistinguishable single photons. However, there are many factors associated with SPEs that limit their applications. For instance, most of SPEs in QDs only work at cryogenic temperatures; the brightness and purity for SPEs from color centers in CNTs are low and the quantum efficiency of the color centers in silicon vacancy (Si-V) in diamond is low (3.5%). To date, empirical evidences have suggested that two dimensional (2D) hexagonal boron nitride (h-BN) is the most promising SPEs material with bright, linearly polarized, high quantum efficiency (50 ~ 100%), and optically stable SPEs operational at room temperature (RT) with high photon purity. Furthermore, defects in 2D h-BN also exhibit high sensitivity to the surrounding environment, which lead to tunable properties and allow predetermined positioning of SPEs by strain engineering. Efforts to engineer SPEs in h-BN have been demonstrated on a range of electron irradiation, laser processing, ion implantation, wet etching, annealing, and plasma processing methods on chemical vapor deposition (CVD) growth h-BN. Among the aforementioned methods, the CVD process is the most reliable method to grow large area h-BN films with thickness control that can host a high density of SPEs. However, the zero-phonon lines (ZPLs) of the h-BN quantum emitters have demonstrated a wide range of energies (1.6 eV ~ 2.4 eV), which implies the existence of multiple defect species by using CVD method, and it has been a significant challenge to reduce the ZPL energy distribution. A recent study demonstrates a new capability to deterministically place the ZPL between either 550–
600 nm or 600–650 nm by using the gettering mode of the atmospheric pressure chemical vapor deposition (APCVD) method that manipulates the boron diffusion through the copper to determine the defect species formation. The ability to control the defect formation during h-BN growth provides a cost effective way to reduce the ZPL energy distribution and is also an important way to understand
studies indicated that the borazane on a catalytic surface underwent a two-stage weight loss process.
The 2D cross-linking reaction of the B-H and N-H groups is initiated from 125 °C to 200 °C, and then dehydrogenation from unaligned chain branches continues from 600 °C to 1000 °C. The nanoparticles are most likely a complex mixture of the poly-aminoborane in the reaction, and they partially dehydrogenate derivatives. The dehydrogenation of BH2NH2 become easier by increasing Ni content in the Ni-Cu alloy, which implies that the introduction of the Ni can enhance the decomposition of the poly-aminobrane and help the reactions of desorption or the formation of the Ni-B and Ni-N phases. The enhanced ability of the Ni-Cu alloy in decomposing polyaminoborane residues improves the cleanliness of the alloy surface during the h-BN growth. Thus, by controlling different chemical reaction pathways, we have demonstrated further control of the ZPL energy distribution.
Figure 3.11. Quantum emission from APCVD h-BN from standard substrate copper. (a) Raman spectra of the h-BN. (b) Three representative PL spectra of the h-BN color center. (c) Autocorrelation measurements on the h-BN color center, confirming the quantum nature of the single photon emitters.
(d) ZPL histograms showing the spectral distribution of the h-BN SPEs.
Figure 3.12. Quantum emission from APCVD h-BN from Cu-Ni alloy. (a) Raman spectra of the h- BN. (b) Three representative PL spectra of the h-BN color center. (c) Autocorrelation measurements on the h-BN color center, confirming the quantum nature of the single photon emitters. (d) ZPL histograms showing the spectral distribution of the h-BN SPEs.
From Figure 3.11 and 3.12, we found that the APCVD grown h-BN on two different substrates (Cu and Cu-Ni alloy) hosted different luminescent defect centers, with the representative room- temperature PL spectra from each displayed in Figure 3.11 (b) and 3.12 (b). To confirm the single photon nature of the emission, we recorded second-order autocorrelation measurements using a Hanbury Brown–Twiss interferometer for all three growth types, with a representative
demonstrated that the Cu-Ni substrate allowed for deterministic selection of SPEs with a particular emission energy, providing a template for rational incorporation of h-BN SPEs with desired properties. This result also suggests that SPE defects with ZPL energies in the region of 550 – 600 nm (Figure 3.11 (d)) are of a different structural nature from those in the region of 600 – 650 nm (Figure 3.12 (d)).