The photoelectric conversion process in semiconductor materials is the basis of photodetectors and solar devices, and has been a hot research topic in the field of semiconductor materials and physics. According to the traditional semiconductor physics theory, in low-dimensional materials, photo-generated carriers relax to the ground state after they are formed. Due to the quantum confinement, photo-generated carriers hardly escape the confinement barrier to form an effective photocurrent. Therefore, the application of low-dimensional semiconductor materials in the field of photovoltaic and detectors has been difficult to succeed.
Recently, doctoral students Wang Wenqi, Wu Haiyan, Yang Haojun, and Wang Lu, Ma Ziguang and Jiang Yang, associate PhD candidates from Institute of Physics, Chinese Academy of Sciences, under the guidance of Prof. Chen Hong, worked with Prof. Liu Wuming's research group and used resonance excitation Emission spectroscopy (ie, the use of laser energy in the band gap between low-dimensional materials and their barriers only selectively excites electrons and holes in low-dimensional materials without forming photo-generated carriers in the barrier) InGaN quantum wells, InGaAs quantum wells, InAs quantum dots and other material systems are observed in the PN junction of the carrier efficient escape phenomenon.
As shown in Fig. 1, in the NIN structure without PN junction (the undoped I region consists of 10 cycles of InAs / GaAs quantum dot structure), the limiting potential can not escape even in the presence of higher applied bias carriers Base, only through the way of radiation composite light. In another test sample, only one N-type doping region in the NIN structure was changed to a P-type doping region to form a PIN structure containing a PN junction. In the same resonant excitation photoluminescence spectroscopy experiment, it was observed experimentally that more than 85% of the carriers no longer participate in the luminescence under zero bias short circuit. In the meantime, significant photocurrent generation was observed in the circuit. By adjusting a circuit current in series with a variable resistor, it is found that there is a linear inverse relationship between the current in the circuit and the integral intensity of the light emission of the quantum dot, which directly confirms the photocurrent formed by the photo-generated carriers in the quantum dot. In addition, by calculating the photoelectric conversion efficiency of the sample, it is estimated that the absorption coefficient of the material in the sample is exponentially increased.
(A) Resonance excitation spectrum of (a) nin structure sample under open circuit and 0.7V reverse bias condition; (b) Resonance excitation spectrum of pin structure sample under open circuit and 0V reverse bias condition; (c) Quantum dot luminous intensity and circuit current curve
The above phenomenon can not be explained by classical hot electron emission, tunneling phenomenon and middle band theory. The project team proposed a new physical model, as shown in Figure 2: Low-dimensional materials without pn junctions relax to the ground state after absorbing light and can not escape low-dimensional materials under the action of an electric field. Low-dimensional materials with pn junctions Absorb light directly after the escape of low-dimensional material without relaxation to the ground state.
Figure 2. (a) Traditional Low-Dimensional Semiconductor Photon Absorption and Carrier Transport Process (b) Low-Dimensional Semiconductor Photon Absorption and Carrier Transport Process Observed in Experiments
The efficient extraction of photo-generated carriers in low-dimensional semiconductor materials and the increase of the absorption coefficient of the materials lead to the preparation of photoelectric conversion devices based on transition between low-dimensional semiconductor materials. To verify the practical effect of this phenomenon, the project team also prepared InGaAs / GaAs multiple quantum well infrared detector prototype device. With no surface anti-reflective film, the device achieves an external quantum efficiency of 34% with an absorption layer thickness of only 100 nm. Using this numerical calculation, the absorption coefficient of the active layer in the device reaches 3.7 × 104 cm-1, which is obviously higher than the measured value in the traditional transmission spectroscopy experiment. (Journal of Infrared and Millimeter Waves, in press)
The above findings not only provide a theoretical basis for the application of low-dimensional semiconductor materials in the field of photovoltaic, but also provide a new technical route for the development of high-temperature infrared detectors.
GaAs-based InAs quantum dots and InGaAs quantum well systems are reported in a recent publication by Chin. Phys. B Vol. 25, No. 9, and China Physics Letters. LETT. Vol. 33, No. 10, 2016, 106801), an experimental phenomenon in GaN-based InGaN quantum wells will be published in the recently published Chin. Phys. B, in press. Based on this work, the project team has applied for a Chinese invention patent and submitted each of the invention patents in Japan and the United States.
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