Photodetection in the solar-blind deep ultraviolet (DUV) wavelength region is of central importance in various civil and industrial applications. Recently, the research of solid-state semiconductor DUV photodetectors has attracted much attention, stimulated by the development needs of its small size, energy-saving, and smart detection modules. For solar-blind DUV detection, however, sensitivity below the magnitude of fW is generally required in many key applications, including flame sensing, corona discharge observations, UV astronomy, and biological and chemical agent detection. Although this demand is in line with the current development trend of single-photon detection, it poses a very challenging task for the signal response capability of solid-state DUV detectors.

As a representative form of carrier existence and transport in a low-dimensional structure, two-dimensional electron gas (2DEG) has many critical applications in high-gain and high-speed heterojunction field-effect transistors (FETs)/phototransistors (FEPTs) due to its inherent characteristics of high electron density and high mobility. The FEPTs with a 2DEG based on GaAs, InP, and GaN material systems have demonstrated highly sensitive detection performance in the terahertz, infrared, and UV bands. A prominent problem affecting the performance of such structured phototransistors is the low signal-to-noise ratio and inferior sensitivity caused by the 2DEG existing in both dark and illumination conditions.

Research efforts are usually from the perspective of the device structure, namely introducing a Schottky barrier or a PN junction gate to deplete the 2DEG in the channel below, rather than looking for solutions from the perspective of material properties. Such a solution, although these works have brought significant progress to AlGaN solar-blind UV FEPTs, suffers from the increased complexity of the device fabrication and the electrical connection and it is difficult to ensure very low dark current.

To solve the above problems, Prof. Jiang Hao and his team from Sun Yat-sen University reported a novel photodetector based on a quasi-pseudomorphic structure to realize self-depletion and photorecovery of full-channel two-dimensional electron gas, and the two-terminal field-effect phototransistor with built-in photogate can directly detect the sub-fW solar-blind signal. The relevant research results were published in Photonics Research, Vol. 11, Issue 7, (Jiabing Lu, Zesheng Lv, Hao Jiang. lGaN solar-blind phototransistor capable of directly detecting sub-fW signals: self-depletion and photorecovery of full-channel 2DEG enabled by a quasi-pseudomorphic structure [J]. Photonics Research, 2023, 11(7): 1217).

Fig. 1(a) shows a schematic cross section of the device structure. The structure is composed of quasi-pseudomorphic epilayers. This was accomplished by using a relatively thin AlGaN channel layer together with a thin graded Al_xGa_1-xN layer to suppress strain relaxation. In the polarization consisting of spontaneous polarization (P_sp) and piezoelectric polarization (P_pz), the strain-dependent P_pz has a large adjustable degree. The quasi-pseudomorphic epilayer structure can regulate P_pz to eliminate the 2DEG at the barrier/channel interface, namely, self depletion effect of two-dimensional electron gas channel. In the case of such a heterostructure, the barrier layer is compressively strained, which makes the P_sp partially offset by P_pz in the opposite direction, thus reducing electron surface density (σ_e) or even making σ_e=0. Fig. 1(b) shows a schematic diagram of the resulting planar phototransistor array. In addition, the sub-fW signal detection capability of the device can be seen from Fig. 1(c), a maximum detectivity of 4.5 × 10²¹ Jones was calculated at an optical power of 0.6 fW, demonstrating the extremely weak signal sensing capability of such photodetectors in the solar-blind band. Research group also established the device photocurrent calculation model, and the calculated values (solid lines) of the model are in good agreement with the measurements (points), confirming that the main mechanism of the high sensitivity of our phototransistor is the built-in polarization photogating effect.

Fig.1 (a) Cross-sectional diagram of a phototransistor with ohmic-contact electrodes (left) and directions of the spontaneous and piezoelectric polarization in our compressively strained Al_0.55Ga_0.45N/Al_0.47Ga_0.53N heterostructure (right). (b) Schematic diagram of the planar phototransistors array. (c) Light power dependence of detectivity at 10 V bias.

This work provides a simple, effective, and easily integrated architecture for carrier control and supersensitive photodetection based on polarization semiconductors. Our research team will further carry out research on the structure of built-in polarization photogating field-effect phototransistors based on other polarization materials, further achieving breakthroughs in the ultrasensitive solid-state photodetection in other wavelengths, including visible light.