Interlevel transitions and two-photon processes in Ge/Si quantum dot photocurrent

Abstract

Photocurrent spectra due to interlevel transitions of holes in Ge/Si quantum dots show several peaks in the range of 60–300 meV, which superlinearly increase with bias, indicating release of carriers by tunneling. The relative peak intensity drastically changes with applied voltage, its polarity, and the measurement system. Lower energy peaks, at 69 and 86 meV, are observed only with a Fourier transform IR (FTIR) spectrometer. The 69 and 86 meV transitions excite holes into intermediate levels from which they are re-excited to shallow levels in a two-photon process. This is observed with FTIR only where the sample is simultaneously exposed to a wide range of energies. Direct band-to-band excitation at 1.25 eV increases the midinfrared signals by orders of magnitude by pumping the intermediate levels. Placing dopants in the barrier greatly increases photocurrent intensity and reduces noise. One-dimensional and three-dimensional numerical analyses confirm our findings.

Broadband Ge/SiGe quantum dot photodetector on pseudosubstrate

“…The built-in voltage was found to be U 0 =0.68 and 0.94 V for U b <0 and U b >0, respectively. These values are typical for p-type Ge/Si QDIPs [9]. …”

“…Ge/SiGe QDIP is of wide detection window with the cutoff wavelength of about 12 μ m instead of 5 to 6 μ m for Ge/Si QDIPs of similar device structure [11]. Since the sample in FTIR experiments is simultaneously exposed to a wide range of photon energies, the spectra may display additional transitions due to two-photon processes [9]. The near-infrared photons with energies larger than the SiGe bandgap create electrons and holes mostly in the SiGe barrier.…”

“…The inherent feature of tunneling mechanism of carrier escape is the exponential dependence of PC intensity I on the applied voltage. Finkman and co-workers [9] proposed a simple equation which follows from the WKB approximation: …”

“…There are only few works announcing the long-wave operation of detectors based on Ge/Si quantum dots [9,12-14]. Since the long-wavelength photoresponse in this system originates from the bound-to-bound intraband transitions, superior performance of such devices is unlikely, and one is obliged to seek another approach.…”

Bidirectional photocurrent of holes in layers of Ge/Si quantum dots

“…Here and below, the positive photocur rent values correspond to the situation where holes move from the upper electric contact to the substrate. The photoresponse observed in the mid IR range is associated with the transitions of holes from the states coupled in quantum dots to the continuous spectrum [2,4,6,9]. The photocurrent in the sample with N B = 2 × 10 11 cm -2 at zero bias is negative (Fig.…”