Secondary electron amplification using single-crystal CVD diamond film

Abstract

The current amplification characteristics of an unbiased 8.3-μm-thick single-crystal CVD diamond film are examined using secondary-electron-emission measurements. In particular, the intensity and energy distribution of transmitted and reflected secondary electrons are measured and used to examine the transport and emission properties that govern the current amplification process. Overall, the measurements confirm the excellent transport and emission properties of single-crystal CVD diamond, as compared to polycrystalline CVD diamond films studied previously. Specifically, the transmitted and reflected energy distributions measured from the single-crystal diamond are nearly identical, with a sharp, narrow (FWHM = 0.35 eV) emission peak dominating the spectra. However, the transmitted distributions are more fully thermalized as a result of the longer transport distances. In fact, transmitted electrons are detected even after traveling more than 8 μm through the film, which demonstrates the potential for excellent transport efficiency. Maximum transmission gains of 3–4 are obtained, which is encouraging under such field-free conditions. However, the results of the study indicate that the transmission process is being limited by diffusive transport in the unbiased diamond film.

Introduction

New classes of high-power, high-frequency vacuum electronic devices are being designed and developed for various commercial and defense applications [1]. However, the cathode requirements for such devices are quite severe since high current densities (~ 100 A/cm²) must be produced from small sub-mm size beams. In addition, future mm-wave and THz device applications may impose further constraints on the cathode such as the need for spatially-distributed or density-modulated beams, cold operation, or compatibility with microfabricated structures. Although research is ongoing to develop improved thermionic cathodes [2], [3] and high-performance field emission sources [4], [5], the capabilities of existing cathode technology cannot meet the demands of these emerging vacuum electronic devices. To address this deficiency, we are developing a current amplifier stage based on chemical-vapor-deposited (CVD) diamond technology that would be used in conjunction with an existing cathode to produce higher-brightness electron beams. Specifically, the diamond amplifier would serve to multiply the beam current produced by a primary cathode while also converting the current into a cold electron distribution having low energy spread.

This current amplifier approach was motivated by the secondary-electron-emission (SEE) characteristics of diamond. In the SEE process, current amplification is produced when a primary beam impacts a semiconductor, such as diamond, and generates secondary electrons through the excitation of valence electrons into the conduction band. In this process, each incident electron produces many secondary electrons, with the internal gain being proportional to the incident beam energy E_o. For diamond, the internal gain can be estimated as ~ E_o / 2.5 E_g [6], [7], where the energy gap E_g = 5.47 eV.

Once generated, the secondary electrons diffuse through the material and thermalize to the bottom of the conduction band. As such, they can reach the surface in a narrow, low-energy distribution. Usually, the emission of these electrons is blocked by the energy barrier present at positive-electron-affinity surfaces (where the electron affinity is the energy difference between the vacuum level E_vac and the conduction band minimum E_c). However, the hydrogenated diamond surface has a negative electron affinity (NEA) [8], and therefore the secondary electrons should be easily emitted into vacuum.

In fact, very high emission gains (i.e., ratio of emitted current to incident current) have been achieved in studies using incident electron [9], [10], [11] and ion [12], [13] beams. Such high gains reflect not only efficient emission at the NEA surface but also efficient electron transport to the surface. For example, in our previous SEE studies [11], emission gains of 80–130 were measured at our highest beam energy of 3 keV from single-crystal and polycrystalline type 2b diamond. An analysis of the energy-dependent gain curves suggested long electron escape distances of ≥ 5 μm in the (100) diamond sample and ≥ 1.3 μm in the polycrystalline diamond sample [14]. It should be noted, however, that these gain measurements were obtained in a reflection configuration in which electron impact and emission occurred at the same surface. As such, the actual secondary-electron transport distances were relatively short (≤ 0.1 μm) due to the limited penetration depth of primary electrons with E_o ≤ 3 keV.

In order to confirm the long escape depths predicted in diamond, the secondary electrons must be generated at greater distances from the emitting surface. This can be accomplished by increasing E_o, which results in deeper primary electron penetration and hence deeper secondary electron generation. However, in a reflection configuration, the risk of electron-induced surface modification increases at higher E_o. Instead, a transmission configuration can be used whereby high-energy electrons are injected into the back side of the diamond such that the hydrogenated front surface is not impacted. Using this approach, we directed an electron beam at the back surface of thin (0.15–2.5 μm) polycrystalline diamond films and then studied the transmission of secondary electrons through the films [15], [16], [17]. In our studies, electron transmission was detected when electrons were generated within ~ 1.3 μm of the emitting surface, in good agreement with the predicted escape depth in our polycrystalline diamond. However, the transmission gains were substantially lower (< 5) than the reflection gains. From an analysis of the data, we concluded that the low transmission gain was most likely due to increased electron scattering at grain boundaries as well as low diffusive-transport efficiency over the longer transport distances [17].

Therefore, it is proposed that the use of single-crystal diamond and an internal electric field are necessary to achieve the high transport efficiency needed for high-gain current amplification (e.g., gain ~ 100). In this study, we evaluate the transmission characteristics of single-crystal CVD diamond films grown at NRL, and we examine the mechanisms that govern the current amplification process.