Thermally enhanced photoinduced electron emission from nitrogen-doped diamond films on silicon substrates

This work presents a spectroscopic study of the thermally enhanced photoinduced electron emission from nitrogen-doped diamond films prepared on $p$-type silicon substrates. It has been shown that photon-enhanced thermionic emission (PETE) can substantially enhance thermionic emission intensity from a $p$-type semiconductor. An $n$-type diamond/$p$-type silicon structure was illuminated with $400–450$ nm light, and the spectra of the emitted electrons showed a work function less than 2 eV and nearly an order of magnitude increase in emission intensity as the temperature was increased from ambient to $\sim 400 °\mathrm{C}$. Thermionic emission was negligible in this temperature range. The results are modeled in terms of contributions from PETE and direct photoelectron emission, and the large increase is consistent with a PETE component. The results indicate possible application in combined solar/thermal energy conversion devices.

Figure 1

Band schematics of the proposed diamond-Si structure showing electrons being excited in the absorbing substrate and contributing to the photon-enhanced emission through the low work function surface.

Figure 2

Simulation results of the two models on an ideal electron emitter, which has a band gap of 1.12 eV, electron affinity of 0.5 eV, and is under illumination of 400 nm light with a photon flux of ${10}^{15}{\mathrm{cm}}^{-2}$ per second. The thermionic contribution is obtained by calculating the emission current with no photon illumination.

Figure 3

Photoinduced emission spectra from nitrogen-doped diamond films on a $p$-type doped Si substrate, obtained by subtracting the thermionic emission contribution from the combined emission spectra. The corresponding photon energies are labeled with solid lines relative to $E_F$. The “pure” thermionic emission spectra are shown in (d), and were collected at each temperature without illumination.

Figure 4

Temperature dependence of integrated photoinduced emission spectral intensity and thermionic emission intensity, obtained from a nitrogen-doped diamond film on a $p$-type doped Si substrate, showing results with different excitation energies. Results obtained from an N-doped diamond film on a Mo substrate [20] are also included for comparison. Combined curves of the two models are shown for the three wavelengths respectively.