Abstract

Resonant tunneling diode (RTD) devices have been investigated for photodetection at the mid-infrared region, at wavelengths around 10pm, by photon-assisted resonant tunneling through the double barrier (DB) structure of the device [1], Also, several authors have reported mid-infrared detection in resonant tunneling superlattices, where photons are resonant with the quantized levels of the multiple quantum well structure, i.e. it is based on intersubband absorption within the wells [1]. Here we report for the first time that an InGaAlAs RTD optical waveguide device operates as a broadband photodetector. We show that our device, that was originally designed as a electroabsorption modulator [2], is capable of detecting light in the visible (594nm), infrared (1064nm) and mid-infrared (10.8µm) ranges. To characterise the device as a broadband photodetector we used a He-Ne laser (λ = 594nm), a Nd:YAG laser (λ = 1064nm) and a CO₂ laser (λ = 10.8µm) as light sources. The first and the later operating CW, with powers of 5mW and 200mW, respectively . The chopped light from each of the above laser was shone on top of the device, rather than coupled to the device waveguide. Due to the waveguide configuration and top metallization only a fraction of the light reaches the RTD and we could not obtain the absolute responsivity of the device. The device was electrically connected to a computer controlled variable DC voltage supplier for biasing porpuses. The signal from the electrical connections was taken to a lock-in amplifier. Fig. 1 shows the experimentally obtained DC current versos DC voltage (I × V curve) and photo-generated signal versos DC voltage of the device when it is iluminated by the CO₂ laser. Continuos and dashed lines indicates upward and downward variation of the bias voltage, respectively. Despite the difference in photon energy, a similar figure is obtained when a He-Ne laser is used. Also important to point out is that the signal is independent on light polarisation, which indicates that previously reported photon-assisted tunneling and intersubband absorption are not taking place. We believe different absorption processes are taking place, as follows. The high energy photons at λ = 594nm (2.1eV) induce interband absorption, taking electrons high up in the conduction band. These hot electrons than loose their excess energy by optical phonon scattering. The thermalized photo-generated electrons can than tunnel through the DB structure, and also recombine. The tunneling process will lead to signal amplification in the RTD when it is biased in the negative dynamic resistance region. For low energy photons, however, the process is different. As the first level in the quantum well is only at about 130meV from the bottom of the conduction band, and the second level is at about 500meV, and considering the band tilt due to bias voltage, the photons at λ = 10.8µm (115meV) are not in resonance for photon-assisted tunneling or intersubband absorption processes to take place. Therefore these low energy photons are absorbed by intraband absorption outside the quantum well, followed by carrier thermalization and tunneling as described before. Therefore, either the intraband or interband absorption processes change the electron concentration near by the DB and produce a photo-generated modulated signal that is measured by the lock-in in a similar way. We also measured the response of the device to short (lOOps) optical pulses from the Nd:YAG laser operating Q-switched mode-locked with 100mW average power, which is shown in fig 2. It shows a rise-time (left-hand side of the pulse in fig. 2) of the order of 500ps and a fall-time (right-hand side) of 2ns. This corroborates with the proposed processes described above, since the rise-time is due to the thermalization process by phonon scattering, and the fall-time is due to carrier recombination.

© 2001 EPS