Researchers at Kyoto University have announced a breakthrough with broad implications for semiconductor-based devices. The findings may lead to the development of ultra-high-speed transistors and high-efficiency photovoltaic cells.
Working with standard semiconductor material (gallium arsenide, GaAs), the team observed that exposing the sample to a terahertz (1,000 gigahertz) range electric field pulse caused an avalanche of electron-hole pairs (excitons) to burst forth. This single-cycle pulse, lasting merely a picosecond (10^-12 s), resulted in a 1,000-fold increase in exciton density compared with the initial state of the sample.
“The terahertz pulse exposes the sample to an intense 1 MV/cm^2 electric field,” explains Hideki Hirori, team leader and Assistant Professor at Kyoto University’s Institute for Integrated Cell-Material Sciences (iCeMS). “The resulting exciton avalanche can be confirmed by a bright, near-infrared luminescence, demonstrating a three-order of magnitude increase in the number of carriers.”
“Since terahertz waves are sensitive to water, our goal is to create a microscope that will allow us to look inside living cells in real time,” says Prof. Tanaka. “These just-released results using semiconductors are an entirely different field of science, but they demonstrate the rich potential that lies in the study of terahertz waves.”
(a) Generated THz pulses are focused onto the GaAs QWs sample, and the luminescence is detected by a CCD camera after it has passed through a spectrometer. (b) The geometry of the sample interfaces with air (nA=1), QWs (nQW=3.5) and a quartz substrate (nS=2.1). We assume here that the QWs with thickness (L=6 μm) on the quartz substrate has a homogeneous refractive index (nQW=3.5) represented by the average of the refractive indices of the wells (nw=3.6) and barriers (nb=3.4). ɛi(t, x) is incident THz electric field from the air. (c) Electron-initiated impact ionization transitions in the schematic GaAs band structure for momentum in the Δ direction. The lattice constant a of GaAs is 5.6 Å, and ±2π/a corresponds to ±1.1×1010 m−1. The diagram shows electrons and hole positions before and after the transition at the threshold.
The study of carrier multiplication has become an essential part of many-body physics and materials science as this multiplication directly affects nonlinear transport phenomena, and has a key role in designing efficient solar cells and electroluminescent emitters and highly sensitive photon detectors. Here we show that a 1-MVcm−1 electric field of a terahertz pulse, unlike a DC bias, can generate a substantial number of electron–hole pairs, forming excitons that emit near-infrared luminescence. The bright luminescence associated with carrier multiplication suggests that carriers coherently driven by a strong electric field can efficiently gain enough kinetic energy to induce a series of impact ionizations that can increase the number of carriers by about three orders of magnitude on the picosecond time scale.
(a) The sketch visualizes the distortion in the Coulomb potential of donors, causing the potential to widen and free electrons to be released, and subsequent evolution of unbound e-h gas generated by a series of impact ionizations into a pure population of excitons emitting luminescence. (b) Electric field dependences of carrier density obtained in the experiment (red open circle) and calculations using equation (2) for three different impact ionization rates γ Ical; one is infinity (grey solid line), and the others are derived from equation (3) with C=C0=870 ps−1 eV−2 (green one-dot-dashed line) and C=5C0 (blue two-dot-dashed line). We assumed N0=1013 cm−3. For the experimental (red open circles) and calculated data (grey solid line), the carrier density N is plotted together with the corresponding 〈nI〉. Panels (c) and (d) show the electron wavenumber k(t) calculated by using equation (2) for ɛ=1.05 and 0.47 MVcm−1, respectively. Dashed lines indicate the wavenumbers in the range of ±2.77×109 m−1, where the electron energy corresponds to the threshold energy Eth of 1.7 eV determined from the dispersion of the GaAs band structure. (e) Normalized electric field of the temporal profile of the incident THz pulse (grey dotted line) and of THz pulse with multiple reflections inside the sample (orange solid line)
In conclusion, we demonstrated extraordinarily high carrier multiplication in the typical semiconductor GaAs when it is subjected to a THz pulse of a strong electric field; the number of carriers increased by about three orders of magnitude. The carrier multiplication increase that occurs with an increase in the THz electric field was in agreement with phenomenological theory based on an impact ionization model including electron motion in k-space with a pristine band structure. In the future, a full quantum kinetic theory treatment of carrier dynamics in a band structure modified by an intense THz electric field should be performed to reveal the microscopic origin of the carrier multiplication. Moreover, an experiment with a time-delayed probe pulse would allow for the carrier multiplication dynamics to be directly probed. Our findings of efficient ultra-fast carrier multiplication bode well for future applications in ultra-high-speed devices such as high-quantum-efficiency THz-biased avalanche photodiodes that have femtosecond resolution and are sensitive to a single photon, and they may also be exploited in the development of efficient electroluminescent and photovoltaic nanoscale devices.