Diamond p–n junction NEA cathode

From recent studies based on total photoelectron yield spectroscopy, p-type and intrinsic diamond surfaces show good electron emission characteristics when they are terminated by hydrogen.^{Reference Takeuchi, Ogura, Ri, Kato, Okushi and Yamasaki9} However, these diamond films do not have conducting electrons to supply for electron emission from their surfaces. In the case of n-type diamond, which also shows NEA when its surface is hydrogen terminated, electrons are thermally excited to the conduction band from which they are expected to easily escape into vacuum. Still, a low electron emission yield was observed.^{Reference Takeuchi, Ogura, Ri, Kato, Okushi and Yamasaki9} In subsequent studies, this was attributed to charge transfer at the surface, which leads to upward band bending, thereby suppressing electron emission.^{Reference Yamada, Hasegawa, Kudo, Masuzawa, Okano and Nebel10} A convincing result of successful diamond p–n junction fabrication was reported in 2001, for the first time forming boron-doped and phosphorus-doped CVD diamond layers epitaxially on {111} single crystalline diamond substrates. The junctions showed clear diode characteristics and ultraviolet light emission at 235 nm, which proves minority carrier injection for both sides of p- and n-type diamond layers through the junction interface (i.e., electron injection from the n-type to the p-type layer [electrons are the minority carrier for p-type semiconductors] and hole injection from the p-type to the n-type layer [holes are the minority carrier for n-type semiconductors] through the p-n junction interface are taking place concurrently).^{Reference Koizumi, Watanabe, Hasegawa and Kanda11} This was a new opportunity to fabricate efficient electron emitters using electron injection into p-type diamond.

Hydrogen-terminated diamond surfaces naturally show NEA in vacuum with very high thermal and chemical stability due to the strong covalent bonding of C–H. The surface is stable even after exposure to air or heating to over 400°C. Based on this, exploratory research was performed with the aim of obtaining p–n junction cold cathodes fabricated from diamond. Initially, the electron efficiencies of these devices were lower than 0.01% due to poor junction fabrication. Most problems related to junction fabrication are due to the difficulties in obtaining n-type diamond.^{Reference Geis, Efremow, Woodhouse, McAleese, Marchywka, Socker and Hochedez12,Reference Brandes, Beetz, Feger and Wright13}

After establishing n-type diamond growth, followed by the successful formation of p–n junctions, a study of efficient p–n junction cold cathodes was first reported in Japan in 2006.^{Reference Koizumi, Ono and Sakai14} Diamond p–n junctions were formed on {111} single crystalline diamond surfaces because high-quality n-type diamond was only feasible in this orientation. A phosphorus doped n-type diamond layer was deposited as a bottom layer for the p–n junction on the {111}-oriented diamond substrate. Growth was performed by microwave plasma CVD. Phosphine (PH₃) was used as a dopant gas together with methane and hydrogen as source gases. The film thickness was 5 µm, with a phosphorus doping concentration of 1 × 10¹⁹ cm⁻³. On the surface of the n-type diamond, using diborane as the doping source gas, a boron-doped p-type diamond layer was formed with a thickness and doping concentration of 500 nm and 1 × 10¹⁸ cm⁻³, respectively. After the p–n stacking, mesa structures were fabricated by reactive ion etching in order to obtain p–n junction diodes.^{Reference Koizumi, Ono and Sakai14}

Electron emission measurements were performed in an ultrahigh vacuum (UHV) probe station. The sample was placed on the stage of the UHV probe station, which was heated to 200°C to obtain higher conductivity of the n-type layer, which has a large donor activation energy of 0.57 eV. A collector electrode, biased at 100 V, was placed above one of the p–n junction mesas with about 0.1 mm separation. The p–n junction showed rectifying diode characteristics, and electron emission from the p-type layer of the p–n junction was observed in the forward bias regime by which the diode turns on. The electron emission efficiency (electron emission current/diode current) of the first prepared device was 0.64%.^{Reference Koizumi, Ono and Sakai14}

By improving the crystalline quality of the semiconducting diamond layers, the emission efficiency was increased up to 1.4%, as reported in 2009.^{Reference Koizumi and Kono15}Figure 1 shows the current-voltage characteristics of the latter diamond p–n junction cathode together with the electron emission efficiency plots. It is clear that the electron emission strongly increases with forward diode current increase. In the reverse bias regime, clear leakage in the diode current is observed, although no increase in the electron emission current appears. In subsequent work, electron emission current densities of ∼0.23 A/cm² have been obtained with excellent emission current stability.^{Reference Takeuchi, Koizumi, Makino, Kato, Ogura, Ohashi, Okushi and Yamasaki16} These results confirmed that the electron emission is driven by electron injection at the p–n junction interface. The emission area and energy of emitted electrons have been carefully analyzed using electron emission spectro-microscopy.^{Reference Kono and Koizumi17} The electron emission image clearly showed the emission coming from the p-type diamond surfaces on top of the mesa structures. The energy of the emitted electrons was about 4 eV over the Fermi level of p-type diamond (∼0.37 eV from the valence band maximum) with an energy spread of about 1 eV. It was considered that electrons, injected from the n-type layer, drift in the conduction band of p-type diamond and occupy empty surface states of the hydrogen-terminated surface. A part of those electrons escape to the vacuum when their energy is higher than the vacuum level. The emission current, formed by these electrons, can be measured with a collector electrode.

Figure 1.

Electron emission and diode characteristics of a diamond p–n junction cathode.^{Reference Koizumi and Kono15}

The application of p–n junction cold cathodes as high voltage switching devices has been successfully demonstrated.^{Reference Takeuchi, Koizumi, Makino, Kato, Ogura, Ohashi, Okushi and Yamasaki16}Figure 2 shows the concept drawing of a high voltage vacuum switch using a diamond p-i-n junction cold cathode. In the near future, it is expected to obtain efficient ultrahigh voltage switches, which can serve as DC high voltage transmission systems for electricity, preferably used in offshore wind power generation.

Figure 2.

A schematic drawing of the proposed vacuum switch and the electrical characterization circuit is shown. E represents the high voltage of the power supply, I_A the anode current (or the current of the high voltage line), Z the load, V_A the anode voltage, V_L the voltage drop at the load, V_G the control voltage of the electron emitter, and I_G the control current of the electron emitter. I_A = I_G + I_K, and in the on state, V_A = V_on, which is the on-voltage of this switch. In the off state, V_A = E.^{Reference Takeuchi, Koizumi, Makino, Kato, Ogura, Ohashi, Okushi and Yamasaki16}

Secondary electron emission from NEA diamond

While efficient electron emission at NEA diamond surfaces has been confirmed experimentally,^{Reference Himpsel, Knapp, van Vechten and Eastman1,Reference Bandis and Pate18–Reference Diederich, Aebi, Kuttel and Schlapbach20} electrons must be present in the conduction band in order to exploit this NEA emission capability. One practical approach is to inject electrons into the conduction band using energetic electron or photon beams.^{Reference Jenkins and Trodden21} In such a process, the impacting beam loses energy through electron scattering, whereby valence electrons are excited into the conduction band. These “secondary” electrons can then diffuse through the conduction band to the NEA surface, where they can be emitted into a vacuum (Figure 3a).

Figure 3.

(a) Schematic energy-band representation of the secondary-electron-emission process in diamond produced by an impacting electron beam with energy E₀. (b) Energy distribution curves measured from the hydrogenated surface of 8-μm-thick single crystal diamond flake in reflection and transmission configurations.^{Reference Yater, Shaw, Jensen, Feygelson, Myers, Pate and Butler22} (c) Secondary emission gain measured from NEA surfaces (cesiated, Cs; hydrogenated, H) of single-crystal natural diamond (C(100), C(111)) and polycrystalline (CVD) diamond films.^{Reference Yater and Shih25}

A key feature of such an emission from NEA diamond is the narrow energy spread of the emitted electrons (FWHM < 0.5 eV).^{Reference Himpsel, Knapp, van Vechten and Eastman1,Reference Yater, Shaw, Jensen, Feygelson, Myers, Pate and Butler22} This results from thermalization during the transport process as the secondary electrons (with an incident energy E₀) lose energy through phonon scattering. Consequently, the electron distribution that reaches the surface is dominated by very-low-energy electrons. At surfaces with positive electron affinity, this distribution is blocked by the energy barrier, and only the relatively few non-thermalized electrons can be emitted. However, at the NEA surface, the entire secondary electron distribution can be emitted, resulting in electron beams with low energy spread (Figure 3b).

Additionally, large current amplification can be achieved when energetic electron beams impact a NEA diamond film. Specifically, incident electrons with energy E₀ generate a cascade of secondary electrons, with the internal gain being approximately equal to E₀/(2.5 × E_g) (where E_g is the bandgap energy).^{Reference Klein23,Reference Jensen, Yater, Shaw, Myers, Pate, Butler and Feygelson24} Consequently, a single impacting electron can produce ∼73 secondary electrons for each keV incident energy, thereby providing a current multiplication mechanism in the diamond. Importantly, if a sufficiently large percentage of secondary electrons reach the surface, emission currents can be obtained that are substantially higher than the incident current. In fact, emission gains as high as ∼115 and ∼85 were measured at E₀ = 2.9 keV from hydrogenated p-type single-crystal and polycrystalline diamond films, respectively^{Reference Yater and Shih25,Reference Shih, Yater, Pehrsson, Butler, Hor and Abrams26} (Figure 3c).

In spite of such results, high emission gain cannot be achieved at all hydrogenated diamond surfaces. Rather, the emission gain depends on the dynamics of the generation, transport, and emission processes.^{Reference Jenkins and Trodden21} Specifically, the internal gain increases with E₀, but the penetration depth of the primary beam increases as well. Thus, to optimize the transport efficiency of secondary electrons, high crystal quality is important to limit scattering and traps at defects and grain boundaries, while sufficient conductivity and downward band bending are needed to promote electron transport to the surface.

To optimize the emission probability, a uniform NEA must be present across the surface that is stable and robust in a practical operating environment. In general, the hydrogenated diamond surface meets these criteria, as it is stable in vacuum up to ∼800°C, and the NEA property can be restored after exposure to air by moderate annealing (∼400°C) to remove surface adsorbates.^{Reference Thoms, Pehrsson and Butler27} In fact, the hydrogenated diamond surface is exceptionally robust and stable when compared to surface coatings (e.g., CsO, BaO) used in state-of-the-art thermionic and photocathode materials. However, in situ annealing of the diamond may not be possible in practical device applications, and the hydrogenated surface may also degrade following sustained high-energy particle impact (e.g., electrons, photons, ions).^{Reference Shih, Yater, Pehrsson, Butler, Hor and Abrams26,Reference Mearini, Krainsky, Wang, Dayton, Ramesham and Rose28,Reference Hopman, Verhoeven, Bachmann, Wilson and Kroon29} Strategies are needed to overcome these challenges, and research efforts are exploring possible approaches. For example, a robust, air-stable NEA surface has recently been demonstrated on lithium-covered oxygen-terminated diamond that exhibits the same high emission efficiency as the hydrogenated surface.^{Reference O’Donnell, Edmonds, Ristein, Tadich, Thomsen, Wu, Pakes and Ley30}

Additionally, a transmission approach has been demonstrated that uses back-side electron beam impact to avoid damage at the NEA emitting surface.^{Reference Yater, Shaw, Jensen, Feygelson, Myers, Pate and Butler22,Reference Chang, Wu, Ben-Zvi, Burrill, Kewisch, Rao, Smedley, Wang, Muller, Busby and Dimitrov31} Due to the longer transport through the film, a fully thermalized electron distribution reaches the surface with very low energy spread (∼0.3 eV) (Figure 3b).^{Reference Yater, Shaw, Jensen, Feygelson, Myers, Pate and Butler22} However, efficient transport becomes especially critical in order to achieve high emission gain. Therefore, it is necessary to use very thin films (e.g., microns thick) of high-purity single-crystal diamond, along with an internal field to accelerate the electrons to the surface. While challenging, the successful development of a high-gain transmission-mode secondary emitter would permit the current amplification of existing electron sources and thereby provide previously unattainable emission performance.

Exciting opportunities exist to exploit the current amplification capabilities of NEA diamond, as well as the low energy spread of the emitted electrons, in a wide range of device applications, including cathodes for vacuum electronics, electron beam lithography, electron multiplier technology, and high-resolution imaging and spectroscopy.

High conductivity UNCD films

UNCD films possess a very interesting granular structure that can be easily altered. In fact, most of the approaches for improving the conductivity of UNCD films via “doping” end up with modification of the granular structure of the films. One typical example is the enhancement of the electron field emission (EFE) properties of UNCD films via the ion implantation process. It has been demonstrated that P-, O-, N-, and Cu-ion implantation^{Reference Hu, Ye, Liu, Shen, Chen and Hu32–Reference Panda, Sundaravel, Panigrahi, Magudapathy, Krishna, Nair, Chen and Lin36} as well as Au-ion irradiation^{Reference Chen, Chen, Wang, Lee, Lin, Guo and Chang37–Reference Sankaran, Chen, Sundaravel, Lee, Tai and Lin39} improved the EFE properties of UNCD films. The conductivity of the UNCD films increased with dose for Au-ion implantation, achieving a value as high as σ = 185 S/cm when 1 × 10¹⁷ ions cm² of Au-ions were implanted into the UCND films; the enhanced EFE properties were preserved even after annealing.^{Reference Chen, Teng, Tang, Sundaravel, Amirthapandian and Lin38,Reference Sankaran, Chen, Sundaravel, Lee, Tai and Lin39} The EFE process of such films can be turned on at 4.88 V/μm with an EFE current density of J_e = 6.52 mA/cm² at an applied field of 8.0 V/μm. TEM examination revealed that the Au ions formed clusters in the UNCD films that induced the formation of a nanographite phase surrounding the clusters.

Gruen et al.^{Reference Arenal, Bruno, Miller, Bleuel, Lal and Gruen40,Reference Lin, Sankaran, Chen, Lee, Chen, Lin and Tai41} demonstrated that using CH₄/N₂ plasma instead of CH₄/Ar plasma yields needle-like diamond grains. It was also observed that UNCD films grown at 700°C (designated as N-UNCD) showed very good conductivity (σ = 200 S/cm) and superb EFE properties of E₀ = 6.13 V/μm and an EFE current density of J_e = 3.36 mA/cm² at 8.8 V/μm.^{Reference Sankaran, Kurian, Chen, Dong, Lee, Tai and Lin42} TEM examination revealed that they contain needle-like diamond grains encased in a few layers of graphite. On the other hand, diamond films with a hybrid granular structure were developed using a unique two-step microwave plasma-enhanced (MPE)-CVD process in which a layer of UNCD film was first grown in CH₄/Ar plasma to serve as a nucleation layer for subsequent deposition of a nano-diamond film using CH₄/(50%Ar-50%H₂) plasma.^{Reference Cheng, Chiang, Horng, Chen, Wang and Lin43,Reference Wang, Chen, Cheng and Lin44} The second MPE-CVD process resulted in the formation of large diamond aggregates evenly distributed in the matrix of ultra-small grains (i.e., a hybrid granular-structured diamond [HiD]). The HiD films showed even better EFE properties than the UNCD films (i.e., lower turn-on field of (E₀)_HiD = 6.6 V/μm and a higher EFE current density of (J_e)_HiD = 1.0 mA/cm² at 30 V/μm applied field).^{Reference Cheng, Chiang, Horng, Chen, Wang and Lin43} N-ion implantation/annealing of the HiD films further improved the EFE properties of these films.^{Reference Panda, Chen, Sundaravel, Panigrahi and Lin45}

These observations demonstrated that UNCD films are versatile in terms of modification of their granular structure. All of the ion-implantation, N₂-plasma MPE-CVD, and two-step MPE-CVD processes thus can modify the granular structure of the UNCD films, rendering them more conductive. The key to enhancing the conductivity of the films is the induction of a nanographitic phase encasing the diamond grains.

Diamond nanostructures as electron field emitters

To further enhance the EFE behavior, two approaches have been adopted to fabricate electron field emitters with a high aspect ratio: (1) the use of nanoscale tips as templates for growing high conductivity UNCD films and (2) directly fabricating diamond nanowires (DNWs) via a reactive ion etching (RIE) process. Si-nanowires (SiNWs) are good candidates for this application. The UNCD films can be grown on SiNWs with full coverage, exhibiting good EFE properties (i.e., with a low turn-on field of E₀ = 4.4 V/μm and a large EFE capacity of J₂ = 13.9 mA/cm² at an applied field of E_a = 12.0 V/μm).^{Reference Tzeng, Lee, Lee, Lin and Chiu46,Reference Tzeng, Liu, Lee, Lin, Lin, Lee and Chiu47} The alternative is to fabricate Si-pyramids (Si_pyramid, 4 μm in size and 3.5 μm in height with a pyramid separation of 10 μm) using an RIE process^{Reference Chang, Lou, Chen, Chen, Lee, Tai and Lin48} that are less sharp but are widely separated. Superior EFE behavior was observed. The evident feature of the HiD/Si-pyramid emitters is that the template fabrication process is robust since standard photolithographic and RIE processes are the only techniques required for making these nanostructures.

In some applications, such as microplasma devices, the cathode materials are subjected to continuous ion bombardment. To fabricate electron field emitters, which combine good EFE properties with high resistance to Ar-ion bombardment in a microplasma, high conductivity N-UNCD films were directly reactive ion etched using Au nanodots as masking materials,^{Reference Sankaran, Kunuku, Lou, Kurian, Chen, Lee, Tai, Leou, Chen and Lin49} resulting in high conductivity diamond nanowires (N-DNWs) with σ = 275 S/cm (Figure 4a). The N-DNWs exhibit high EFE properties with a turn-on field of (E₀)_N-DNWs = 2.04 V/μm and a large EFE current density of (J_e)_N-DNWs = 4.84 mA/cm² at an applied field of E_a = 3.2 V/μm (Figure 4b). These N-DNWs emitters also show good stability against the plasma bombardment when used as a cathode material in a microplasma device (inset, Figure 4c).

Figure 4.

Scanning electron microscope micrograph of ultrananocrystalline diamond (UNCD) nanorods after (a) and before (b) the reactive ion etching process; (c) the electron field emission properties of reactive ion etched UNCD nanorods, with the TEM microstructure and a selected area electron diffraction image of the UNCD films shown as an inset. (d) The lifetime test for a microplasma device, in which the UNCD nanorod was used as a cathode (the inset shows the typical plasma illumination images of the device).^{Reference Sankaran, Kunuku, Lou, Kurian, Chen, Lee, Tai, Leou, Chen and Lin49}

The other approach of developing the electron emitter is to utilize the advantage of sp ²- and sp ³-bonded carbons that was demonstrated in the NCD-coated carbon nanotubes (CNTs).^{Reference Sankaran, Srinivasu, Leou, Tai and Lin50} Coating a thin layer of NCD film on CNTs markedly improved the stability of CNT emitters, although it slightly degraded the EFE properties. Such a composite material utilized both the advantage of the high conductivity/large aspect ratio of CNTs and the negative electron affinity/high hardness of the diamond film, resulting in emitters that possess very good EFE properties with high stability against plasma bombardment. The NCD-coated CNTs possess a low turn-on field (E₀) of 2.19 V/μm and an emission current density (J_e) of 2.33 mA/cm² at an applied field of 4.76 V/μm. There was no noticeable current degradation or fluctuation over a period of 250 min at an applied voltage of 900 V, whereas the bare CNT emitters lasted only 33 min at an applied voltage of 360 V.^{Reference Sankaran, Srinivasu, Leou, Tai and Lin50}

Flexible electronics possess great potential for applications and are of large current interest. The high EFE emitters described previously are all based on Si substrates and are very rigid. For the purpose of fabricating flexible EFE emitters, a transfer process was adopted in one study, as shown schematically in Figure 5a.^{Reference Sankaran, Tai and Lin51} The flexible EFE emitters are comprised of pyramidal N-UNCD tips on a polynorbornene-based polymer, shown in the photograph in Figure 5a. This EFE emitter exhibited good EFE properties; it can be turned on at (E₀)_flexible = 2.2 V/μm, achieving a high EFE current density of (J_e)_flexible = 5.10 mA/cm² at an applied field of E_a = 6.0 V/μm (Figure 5b). These EFE emitters show high brightness on a flexible phosphor screen, which was separated from the emitters by a flexible PTFE spacer (∼1 mm) (inset, Figure 5b). Moreover, the EFE current density can last more than 144 min without showing any sign of degradation. Such EFE behavior is superior to those of flexible electron field emitters previously reported (Table I).^{Reference Sankaran, Tai and Lin51–Reference Verma, Das, Lahiri and Choi55}

Figure 5.

(a) The process for fabrication of flexible diamond electron field emitters: (i) chemical etching of Si to form inverted cavities, (ii) growth of ultrananocrystalline diamond (UNCD) films on the Si containing the inverted cavities, (iii) coating of a thin Au layer, (iv) coating of a 10-mm-thick polynorbornene-based polymer (PNB) film, (v) etching of the Si materials, resulting in the UNCD pyramids on PNB, and (vi) a micrograph showing the flexible diamond electron field emitters. (b) The electron field emission properties of the flexible diamond electron field emitters, using 2, 4, 6, and 8 μm UNCD pyramids as emitting materials. The inset in (b) shows the typical field emission image for the devices using phosphor-coated polymer as the anode.^{Reference Sankaran, Tai and Lin51}

Table I.

Comparison of electron field emission properties of flexible C-UNCD pyramidal microtips with various flexible field-emitting materials.

^a E₀: the turn-on field defined as the applied field corresponding to the J_e-value indicated in the parenthesis.

^b J_e: the EFE current density corresponding to the applied field specified.

^c β: the field enhancement factor estimated from Fowler-Nordheim model (equation 2).

^d τ: the life-time stability measurement under the current density indicated in the parenthesis.