【Member Papers】High performance solar blind photodetector based on β-Ga₂O₃ achieved via face-to-face thermal annealing with oxygen plasma treatment

      A team led by Professor Bingsheng Li from Northeast Normal University published an article entitled "High performance solar blind photodetector based on β-Ga₂O₃ achieved via face-to-face thermal annealing with oxygen plasma treatment" in Applied Materials Today.

Project Support

      This work was supported by the National Natural Science Foundation of China under Grant 62274027 and 62404039, the Open Research Fund of Songshan Lake Materials Laboratory (2023SLABFK03) and 111 Center (B25030), the Postdoctoral Fellowship Program of CPSF (GZC20230416), the Fundamental Research Funds for the Central Universities (2412024QD010).

Background

      β-Ga₂O₃ has emerged as an ideal material for solar-blind ultraviolet detector applications due to its ultra-wide band gap and ultra-high chemical and physical stability. The photoconductive type, which features low cost and high gain, is one of the basic structures of β-Ga₂O₃-based photodetector devices. However, β-Ga₂O₃ materials inherently contain various defects such as oxygen vacancies (V_O) and gallium vacancies (V_Ga). These defects act as non-radiative recombination centers, affecting the transport of carriers and hindering the improvement of key device performance. Currently, thermal annealing is an effective method to improve crystal quality and reduce defect density. However, traditional annealing methods may cause damage to the surface of Ga₂O₃ thin films, and especially under high-temperature conditions, they may lead to the degradation of surface morphology and crystal quality. Therefore, it is crucial to develop an advanced post-treatment technology that can not only effectively eliminate internal defects but also protect the surface quality of the thin films.

Main Content

      This study proposed a novel plasma face-to-face annealing process to improve the material quality and device performance. Atomic oxygen in oxygen plasma can be adsorbed on the surface and diffuse into the film more efficiently. The face-to-face sample placement can protect the surface of Ga₂O₃ from damage and prevent the loss/re-evaporation of oxygen from the surface. For comparison, three different annealing processes were used in this study: (1) Annealed in air. In this process, the samples were placed face-up (uncapped) in the furnace. During the annealing, air was supplied into the furnace. (2) Annealed under oxygen plasma (OPA). In this process, oxygen plasma, which was created by RF power and consists of oxygen molecules (O₂), oxygen ions (O₂^+/-), and atomic oxygen (O), was supplied during annealing. The samples were placed face up (uncapped) in the furnace. (3) Oxygen plasma face-to-face annealing (OPA FTF). In this process, samples were placed face-to-face in the furnace, and oxygen plasma was supplied during annealing. The FWHM and root-mean-square (RMS) roughness of the β-Ga₂O₃ thin film treated by OPA FTF are 0.093° and 7.5 nm, respectively. The surface V_O concentration decreased to 2.88%, and the V_O concentration in the bulk decreased to 2.53%. The dark current of a metal-semiconductor-metal (MSM) photodetector is as low as 30 fA at 10 V bias. The device exhibits excellent performance with a light-to-dark current ratio of 6.06 × 10⁷ and a detection rate of 5.68 × 10¹⁵ Jones. This work provides a feasible process for the preparation of high-quality β-Ga₂O₃ thin films.

Highlight

      ● The oxygen plasma face-to-face annealing (OPA FTF) technique is proposed for the first time. The sample is placed face-to-face to block high-energy ion bombardment and inhibit the re-evaporation of surface oxygen at high temperature. Avoid the surface roughness caused by traditional annealing.

      ● The dual regulation of surface and bulk defects was successfully achieved. The surface and internal oxygen vacancies were reduced to 2.88 % and 2.53 %, respectively. The recombination of carriers is inhibited.

      ● The dark current of the MSM detector is as low as 30 fA at 10 V bias, the light-dark current ratio is as high as 6.06 × 10^7, and the detection rate is 5.68 × 10¹⁵ The performance of β-Ga₂O₃-based solar blind detector is greatly improved.

Conclusion

      In summary, high-quality β-Ga₂O₃ films and high-performance β-Ga₂O₃-based PDs can be obtained by post-growth annealing under optimized conditions. After annealing in air at an optimal temperature of 1000 °C, the crystallization quality was significantly improved, as indicated by the narrow FWHM of the XRD peak of 0.109°. Annealing β-Ga₂O₃ thin films with OPA FTF further improves the crystallization quality and surface morphology, with a FWHM of XRD peak of 0.093° and a RMS roughness of 7.5 nm. The face-to-face placement of samples protects the sample surfaces being damaged by high energy oxygen ions and prevents the loss/re-evaporation of oxygen from the β-Ga₂O₃ surfaces. The oxygen atoms supplied by the plasma source diffuse into the film to fill the oxygen vacancies, which effectively reduced the V_O concentration in Ga₂O₃ from 6.26% to 2.53% when comparing to an unannealed sample. The PD fabricated from the sample annealed with OPA FTF showed improved device performance, with a relatively large PDCR of 6.06 × 10⁷ and a high detectivity of 5.68 × 10¹⁵Jones. The device also exhibits good time response and long time stability. This is of great significance in applications that require efficient and accurate detection of UV light.