NONRESONANT TUNNELING IN SHORT-PERIOD SUPERLATTICES WITH OPTICAL CAVITIES

M.S. Kagan1, I.V. Altukhov1, S.K. Paprotskiy1, A.N. Baranov2, R. Teissier2, A. A. Usikova3, N.D. Ilinskaya3, A.D. Buravlev3, V.M. Ustinov3

1V.A. Kotelnikov Institute of Radio Engineering and Electronics, Russian Ac. Sci., Moscow, Russia; kagan@cplire.ru2IES, Universit Montpellier 2, CNRS, Montpellier, France3 A.F. Ioffe Physico-Technical Institute, Russian Ac. Sci., St. Petersburg, Russia OUTLINE1. Motivation2. Superlattices 3. Measurements4. Current-voltage characteristics 5. Nonresonant tunneling 6. Effect of resonant cavity

Emission power vs frequencyEmission power vs frequencyEmission power vs frequency

SuperlatticesL. Esaki and R. Tsu, IBM J. Res. Devel. 14, 61 (1970). Note, that all semiconductor THz lasers operate at cryogenic temperatures and can not work at room temperature (kT=25 meV) because the energies of quanta in THz range are around 10 meV and to obtain the population inversion is practically impossible. So, we need to reject the laser scheme and to look for another way to get THz generation at room T. The reasonable idea seems to look for a semiconductor system with a fast negative differential conductivity (NDC), which can produce THz oscillations in a suitable resonant cavity.

Superlattices InAs/AlSb superlattices:Cap layer: n+-InAs (n = 1x1019 cm-3) - 1 m, 60 periods of 4.5nm InAs/3.5nm AlSb InAs(Si) QWs: n = (0.5 - 2) 1017 -3 Substrate: n+-InAs, n = 1x1019 cm-3 GaAs/AlAs superlattices:100 periods of 4nmGaAs/2nmAlAs Energy spectrum and wave functions.40 kV/cm. T = 300 KResonant cavity with the current leadCavities: ring-shaped gold contacts formed THz optical cavities for free space l ~ 110 to 160mm. Whispering gallery mode

Current-voltage characteristics of InAs/AlSb SLs without cavity. T = 77 K. Rectangular pulse, matched load resistance.

At low voltages, the current saturation or negative differential conductivity (NDC) were observed caused by Esaki-Tsu mechanism of miniband transport at overlapping broadened confined states in periodic QWs.

Time dependences of voltage and currentInAs/AlSb superlattices. =300 , matched load resistance. Current oscillations are due to NDC of the sample exciting parasitic resonant circuits.

InAs/AlSb SLs with optical cavities Time dependences of current and voltage. I-V characteristics in two polarities. = 300 . Triangular pulse, small load resistance. The current saturation on initial part of I-V curve is connected with static domain formation resulted from NDC.

Current-voltage characteristics of InAs/AlSb SL. T = 300 K. Triangular pulses, small load resistance (1 Ohm). Inset: scheme of nonresonant tunnelingNonresonant tunneling and the influence of optical cavity (Purcell effect)Purcell factor lc/n is the wavelength within the material, Q is the quality factor, V is the mode volume of the cavity.

(E.M. Purcell, Phys. Rev. 69, 681, 1946.)

The Purcells paper, where he first entered this factor is one of the shortest (one paragraph) and at the same time one of the most cited in the modern physics. E.g., in 2011 there were 1681 citings. Using for estimation Q=100 we get in our case Fp ~ 1000.

Measurements.GaAs/AlAsTime dependences of current and voltage. I-V curves. = 300 . Triangular pulse, small load resistance.

Measurements.GaAs/AlAsTime dependences of current and voltage. I-V curves. = 300 .

Current-voltage characteristics of GaAs/AlAs SL. T = 300 K. Triangular pulses, small load resistance (1 Ohm).GaAs/AlAs superlattices with optical cavities100 periods of 4nmGaAs (QW)/2nmAlAs on heavily doped GaAs substrate.

ConclusionVertical transport in short-period InAs/AlSb and GaAs/AlAs superlattices (SLs) was studied. The periodic maxima observed in the current-voltage characteristics of these SLs in the nonresonant tunneling regime were attributed to the influence of optical cavity on optical transitions within quantum wells (Purcell effect). Certainly, the additional confirmation is desirable for this explanation, both theoretical and experimental. In particular, it is necessary to calculate the probabilities of phonon-assisted and radiative transitions between the confined states in the neighboring QWs. It would be useful, as well, to study the current-voltage characteristics of resonant-cavity samples with different frequencies.

It has been suggested to use the SLs as an amplification medium for coherent THz electromagnetic field due to Bloch oscillations. This principle is well known, while its practical realization has been problematic for a long time because of NDC arisen in SLs, which is necessary for Bloch wave amplification. The existence of NDC causes a parasitic formation of the Gunn domains inside the superlattice, which destroys the mechanism of Bloch gain.