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1. Photoluminescence study of MgGa2O4 spinel oxide films grown by molecular beam epitaxy

   https://doi.org/10.1063/5.0218242  

   Yang, Tianchen; Shou, Chengyun; Tran, Jason; Almujtabi, Abdullah; Mahmud, Quazi Sanjid; Zhu, Edward; Li, Yuan; Wei, Peng; Liu, Jianlin (August 2024, Applied Physics Letters)

   As a promising ultrawide bandgap oxide semiconductor material in the spinel family, magnesium gallate (MgGa2O4) exhibits great potential applications in power electronics, transparent electronics, and deep ultraviolet optoelectronics. However, few studies reveal its photoluminescence (PL) properties. In this work, MgGa2O4 films were grown by using oxygen plasma assisted molecular beam epitaxy. The bandgap of MgGa2O4 spinel films is determined to be around 5.4–5.5 eV, and all samples have transmittance over 90% in the visible spectral range. X-ray diffraction patterns confirmed that the spinel films were grown highly along ⟨111⟩ oriented. Power and temperature dependent PL studies were investigated. Optical transitions involving self-trapped hole, oxygen vacancy deep donor, and magnesium atom on gallium site deep acceptor levels were revealed. 

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2. Photoluminescence from Defects in Be‐Doped GaN

   https://doi.org/10.1002/pssb.202400656  

   Reshchikov, Michael_Alexander; McEwen, Benjamin; Shahedipour‐Sandvik, Shadi (February 2025, physica status solidi (b))

   While GaN is a crucial semiconductor material for bright light‐emitting devices, fabrication of p‐type GaN remains challenging since the Mg acceptor commonly used for p‐type doping is not shallow enough. Doping of GaN with Be is a promising path, yet no reliable p‐type GaN has been achieved by Be doping so far. One of the reasons is a poor understanding of point defects in Be‐doped GaN that can be studied by photoluminescence (PL). The yellow (YL_Be) band at 2.15 eV is the dominant PL band in Be‐doped GaN. In this work, a blue PL band named the BL_Beband is discovered. It has a maximum at 2.6 eV and a lifetime of 0.8 μs at temperatures below 100 K. The BL_Beband is observed in GaN samples with relatively high concentrations of Be (>10¹⁸ cm⁻³). Both the YL_Beand BL_Bebands likely originate from the isolated Be_Gadefect, namely from electron transitions via the −/0 and 0/+ thermodynamic transition levels of the Be_Ga. The 0/+ transition level is located at 0.1–0.2 eV above the valence band. Other broad PL bands in Be‐doped GaN were also observed and preliminarily attributed to Be‐containing complexes. 

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3. Low‐Temperature Synthesis of Stable CaZn ₂ P ₂ Zintl Phosphide Thin Films as Candidate Top Absorbers

   https://doi.org/10.1002/aenm.202402640  

   Quadir, Shaham; Yuan, Zhenkun; Esparza, Guillermo_L; Dugu, Sita; Mangum, John_S; Pike, Andrew; Hasan, Muhammad_Rubaiat; Kassa, Gideon; Wang, Xiaoxin; Coban, Yagmur; et al (September 2024, Advanced Energy Materials)

   Abstract The development of tandem photovoltaics and photoelectrochemical solar cells requires new absorber materials with bandgaps in the range of ≈1.5–2.3 eV, for use in the top cell paired with a narrower‐gap bottom cell. An outstanding challenge is finding materials with suitable optoelectronic and defect properties, good operational stability, and synthesis conditions that preserve underlying device layers. This study demonstrates the Zintl phosphide compound CaZn₂P₂as a compelling candidate semiconductor for these applications. Phase‐pure, ≈500 nm‐thick CaZn₂P₂thin films are prepared using a scalable reactive sputter deposition process at growth temperatures as low as 100 °C, which is desirable for device integration. Ultraviolet‐visible  spectroscopy shows that CaZn₂P₂films exhibit an optical absorptivity of ≈10⁴ cm⁻¹at ≈1.95 eV direct bandgap. Room‐temperature photoluminescence (PL) measurements show near‐band‐edge optical emission, and time‐resolved microwave conductivity (TRMC) measurements indicate a photoexcited carrier lifetime of ≈30 ns. CaZn₂P₂is highly stable in both ambient conditions and moisture, as evidenced by PL and TRMC measurements. Experimental data are supported by first‐principles calculations, which indicate the absence of low‐formation‐energy, deep intrinsic defects. Overall, this study shall motivate future work integrating this potential top cell absorber material into tandem solar cells. 

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4. Tuning the optoelectronic properties of enargite (Cu3AsS4) solar cells by Ag alloying: A DFT-informed synthesis

   https://doi.org/10.1063/5.0170314  

   Pradhan, Apurva A; Yao, Canglang; McClary, Scott A; Weideman, Kyle G; Blach, Daria D; Khandelwal, Shriya; Andler, Joseph; Rokke, David J; Huang, Libai; Handwerker, Carol; et al (November 2023, Applied Physics Letters)

   The enargite phase of Cu3AsS4 (ENG) is an emerging photovoltaic material with a ∼1.4 eV bandgap and is composed of earth abundant elements with favorable defect properties arising from the differing ionic radii of the constituent elements. Unfortunately, ENG-based photovoltaic devices have experimentally been shown to have low power conversion efficiencies, possibly due to defects in the material. In this joint computational and experimental study, we explore the defect properties of ENG and employ synthesis approaches, such as silver alloying, to reduce the density of harmful defects. We show that shallow copper vacancies (VCu) are expected to be the primary defects in ENG and contribute to its p-type character. However, as shown through photoluminescence (PL) measurements of synthesized ENG, a large mid-bandgap PL peak is present at ∼0.87 eV from a band edge, potentially caused by a copper- or sulfur-related defect. To improve the properties of ENG films and mitigate the mid-bandgap PL, we employed an amine-thiol molecular precursor-based synthesis approach and utilized silver alloying of ENG films. While silver alloying did not affect the mid-bandgap PL peak, it increased grain size and lowered film porosity, improving device performance. In conclusion, we found that incorporating silver such that [Ag]/([Ag] + [Cu]) is 0.05 in the film using an amine-thiol based molecular precursor route with As2S3 as the arsenic source resulted in improved photovoltaic device performance with a champion device of efficiency 0.60%, the highest reported efficiency for an Cu3AsS4 (ENG)-based device to date. 

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5. Photoluminescence from defects in GaN

   https://doi.org/10.1117/12.2645878  

   Reshchikov, Michael A. (March 2023, Photoluminescence from defects in GaN)

   Morkoç, Hadis; Fujioka, Hiroshi; Schwarz, Ulrich T. (Ed.)

   We present the most recent results of photoluminescence (PL) studies, classification of defects in GaN and their properties. In particular, the yellow luminescence band (labeled YL1) with a maximum at 2.17 eV in undoped GaN grown by most common techniques is unambiguously attributed to the isolated CN acceptor. From the zero-phonon line (ZPL) at 2.59 eV, the /0 level of this acceptor is found at 0.916 eV above the valence band. The PL also reveals the 0/+ level of the CN at 0.33 eV above the valence band, which is responsible for the blue band (BLC), with the ZPL at 3.17 eV. Another yellow band (YL2) with a maximum at 2.3 eV, observed only in GaN grown by the ammonothermal method, is attributed to the VGa3H complex. The nitrogen vacancy (VN) causes the green luminescence (GL2) band. The VN also forms complexes with acceptors such as Mg, Be, and Ca. These complexes are responsible for the red luminescence bands (the RL2 family) in high-resistivity GaN. The results from PL studies are compared with theoretical predictions. Uncertainties in the parameters of defects are discussed. 

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