Investigation of transferred-electron oscillations in diamond

Abstract

The recent discovery of Negative Differential Mobility (NDM) in intrinsic single-crystalline diamond enables the development of devices for high frequency applications. The TransferredElectron Oscillator (TEO) is one example of such devices that uses the benefit of NDM to generate continuous oscillations. This paper presents theoretical investigations of a diamond TEO in the temperature range of 110 to 140 K where NDM has been observed. Our simulations map out the parameter space in which transferred-electron oscillations are expected to occur for a specific device geometry. The results are promising and indicate that it is possible to fabricate diamond based TEO devices. Published by AIP Publishing. [http://dx.doi.org/10.1063/1.4952766]The carrier drift velocity in semiconductors is normally proportional to the electric field at low fields and saturates at high fields; nonetheless, Gunn observed that the velocity could behave in a different way. He discovered that at high fields, the electron drift velocity in Indium Phosphide and Gallium Arsenide decreased with an increase in the electric field.1,2 This was later called Negative Differential Mobility (NDM) or Negative Differential Resistance (NDR). 3 The phenomenon generally appears in direct bandgap III-V and II-VI semiconductor materials with non-equivalent valleys, such as GaAs, 1 InP, 1 InAs, 4 ZnSe, 5 and CdTe. [5][6][7][8] When increasing the applied electric field, electrons with a lower effective mass at the central conduction band valley are heated up and transferred into satellite valleys where they have a higher effective mass, thus decreasing the drift velocity and causing NDM to occur. This effect is not expected to occur in indirect bandgap materials but has previously been reported for silicon at a temperature below 45 K. 9 Furthermore, the phenomenon has been observed in intrinsic Single-Crystalline Chemical Vapor Deposition (SC-CVD) diamond.10 With an applied electric field of 300 to 600 V/cm in the [100] direction, the NDM effect was observed in a temperature range of 110 to 140 K. Since diamond is an indirect bandgap material, this effect occurs in a somewhat different way. 10Diamond has six equivalent conduction band valleys oriented along the {100} directions with a uniquely low scattering rate between these valleys.11,12 Short wavelength lattice vibrations are required for intervalley scattering but these are, in diamond, absent at low temperatures due to the rigid lattice. Electrons in these valleys have a different longitudinal (m l ) and transversal effective mass (m t ), which cause a strong anisotropy in the transport properties of the polarized electrons. If an electric field is applied in the [100] direction, two valleys are aligned in parallel (on the (100) axis) and four valleys orthogonal to the field (on the (010) and (001) (010) and (001) axes respond with the transversal effective mass (m t ¼ 0.22 m 0 ). 13 At low electric fields, the electron drift velocity is proportional to the field. By increasing the electric field, the ...

Observation of transferred-electron oscillations in diamond

“…In all three cases the oscillator strength is at its maximum at 110 to 130 K, as would be expected from simulations [17]. This can be explained by the negligible rate of intervalley phonon scattering at low temperatures which effectively inhibits valley repopulation.…”

“…The current waveforms at the temperatures 90 and 110 K with bias voltages of 0 and 20 V are shown for device A in Figure 3. With a voltage applied, current oscillations are observed having a much larger amplitude at 110 K compared to 90 K. A comparison of the current waveforms in the frequency domain at different voltages and at temperatures of 90, 110 and 290 K are presented for In all three cases the oscillator strength is at its maximum at 110 to 130 K, as would be expected from simulations [17]. This can be explained by the negligible rate of intervalley phonon scattering at low temperatures which effectively inhibits valley repopulation.…”

“…Clearly the model presented in Ref. [17] does not explain the high-temperature behavior. In this model, the population ratios are defined by first order rate equations, and we believe that this approach may be too simplistic to capture the detailed dynamics of the valley population.…”

“…The possibility of using the NDM effect in diamond devices has been investigated with a theoretical model to establish if it is feasible to demonstrate a diamond based transferred-electron oscillator (TEO). [17] In this model the TED operates with an electric field applied in such a way that the internal space charge distribution and the internal electrical field distribution become unstable and cause oscillations in a resonant circuit. This paper presents experimental results of an actual diamond-based TEO, where oscillations have been observed for the temperature range 90 to 300 K. This is the first time oscillations have been observed in a bulk TEO based on an elemental semiconductor or an indirect-bandgap semiconductor.…”