Preparation and ultraviolet detection performance of Cu doped β-Ga2O3 thin films

At a Glance

Metadata	Details
Publication Date	2023-01-01
Journal	Acta Physica Sinica
Authors	Wei Liu, Qiu-Ju Feng, Ziqi Yi, Yu Chen, Shuo Wang
Citations	2
Analysis	Full AI Review Included

Executive Summary

This research successfully synthesized p-type Cu-doped beta-Ga₂O₃ (β-Ga₂O₃) thin films using Chemical Vapor Deposition (CVD) and demonstrated their high performance as solar-blind ultraviolet (UV) photodetectors.

P-Type Conductivity Achieved: Hall measurements confirmed successful p-type doping, with the highest hole concentration reaching 1.69 x 10¹⁶ cm^-3 in the film with the highest Cu content (Sample C).
CVD Synthesis: A cost-effective CVD method was utilized on sapphire substrates, yielding films with high crystallinity, confirmed by XRD analysis showing Cu²⁺ substitution into the Ga₂O₃ lattice.
High Sensitivity: The fabricated Metal-Semiconductor-Metal (MSM) photodetector (Sample C) achieved a maximum photo-to-dark current ratio (I_light/I_dark) of 3.81 x 10² under 254 nm UV illumination at 10 V bias.
Exceptional Efficiency: The device demonstrated a high responsivity (R) of 1.72 A/W and a corresponding External Quantum Efficiency (EQE) of 841% at a low light intensity of 64 µW/cm².
Fast Response: The detector exhibited rapid switching characteristics, with a rise time (τ_r) of 0.11 s and a decay time (τ_d) of 0.13 s.
Bandgap Engineering: Increased Cu doping concentration resulted in a bandgap redshift, narrowing the optical bandgap from 4.87 eV (low Cu) to 4.80 eV (high Cu).

Technical Specifications

Parameter	Value	Unit	Context
Material System	Cu-doped β-Ga₂O₃	N/A	Grown on Sapphire (Al₂O₃)
Doping Type	p-type	N/A	Confirmed by Hall measurement
Max Hole Concentration	1.69 x 10¹⁶	cm^-3	Sample C (Highest Cu content)
Max Photo-to-Dark Current Ratio	3.81 x 10²	N/A	Sample C, 10 V bias, 254 nm
Peak Responsivity (R)	1.72	A/W	Sample C, 64 µW/cm² light intensity
Peak External Quantum Efficiency (EQE)	841	%	Sample C, 64 µW/cm² light intensity
Rise Time (τ_r)	0.11	s	Sample C response time
Decay Time (τ_d)	0.13	s	Sample C recovery time
Optical Bandgap (E_g) Range	4.87 to 4.80	eV	Decreases with increasing Cu content
Operating Wavelength	254	nm	Solar-blind UV
Operating Bias	10	V	Standard test voltage
Detector Structure	MSM	N/A	Au interdigitated electrodes
Electrode Finger Width/Spacing	20 / 20	µm	Au electrodes
Max Detectivity (D*)	5.65 x 10¹²	Jones	Calculated at 64 µW/cm²

Key Methodologies

The Cu-doped β-Ga₂O₃ thin films were grown on sapphire substrates using a low-cost Chemical Vapor Deposition (CVD) method, followed by MSM device fabrication.

Substrate Preparation: Sapphire substrates were cleaned sequentially using acetone, ethanol, and deionized water via ultrasonic agitation (10 min per step).
Precursor Mixing: The reaction source consisted of mixed powders: Ga₂O₃ (99.99%), CuO (99.99%), and Carbon (C, 99.99%). Carbon served as the reducing agent.
CVD Growth Parameters:
- Growth Temperature: 1000 °C.
- Reaction Time: 30 min.
- Carrier Gas (Ar) Flow Rate: 200 mL·min^-1.
- Reaction Gas (O₂) Flow Rate: 50 mL·min^-1.
Doping Control: Three samples (A, B, C) were prepared by varying the Ga₂O₃/CuO mass ratio (25:1, 25:2, and 25:3, respectively) to control the Cu doping concentration.
Structural and Compositional Analysis:
- Scanning Electron Microscopy (SEM) assessed surface morphology (films became rougher with increased Cu content).
- X-ray Diffraction (XRD) confirmed the monoclinic β-Ga₂O₃ structure and showed a shift in the (201) peak to lower angles, verifying Cu²⁺ incorporation.
- Energy Dispersive Spectroscopy (EDS) confirmed Cu molar percentages (1.3% to 2.4%).
Electrical and Optical Characterization: Hall effect measurements confirmed p-type conductivity. UV-Vis spectroscopy determined the optical absorption and bandgap.
Device Fabrication: Au interdigitated electrodes (1000 µm length, 20 µm width, 20 µm spacing) were deposited onto the Cu-doped β-Ga₂O₃ films using electron beam evaporation to form the MSM photodetector structure.

Commercial Applications

The development of p-type β-Ga₂O₃ and high-performance solar-blind UV detectors based on this material opens doors for applications requiring high sensitivity and immunity to background visible light.

Aerospace and Defense:
- Missile tracking and guidance systems (solar-blind detection prevents false alarms from sunlight).
- Space-to-space communication links.
Safety and Security:
- High-speed, reliable flame and fire detection systems in industrial plants, aircraft, and hazardous environments.
Environmental Monitoring:
- Detection and monitoring of atmospheric ozone (O₃) and other UV-absorbing pollutants.
Power Electronics (Future Potential):
- While this study focuses on detection, β-Ga₂O₃’s intrinsic properties (4.9 eV bandgap, 8 MV/cm breakdown field) are foundational for next-generation high-power switches and rectifiers.
Biomedical and Sterilization:
- UV dosage monitoring in sterilization equipment (e.g., water purification, medical tools).

View Original Abstract

Solar-blind UV photodetectors (SBPs) have attracted great attention because they are widely used in missile tracking, fire detection, biochemical analysis, astronomical observations, space-to-space communications, etc. At present, it is found that wide bandgap semiconductor materials such as AlxGa1-xN, Mg1Zn1-xO, diamond and β-Ga2O3 are ideal semiconductor materials for developing high-performance SBPs. The ultra-wide band gap semiconductor material, β-Ga2O3, has a large band gap width of 4.9 eV, strong breakdown electric field, absorption edge located in the solar blind ultraviolet band (200-280 nm), and it also has high transmittance in the near ultraviolet and the whole visible band. Therefore, β-Ga2O3 is a very suitable material for making solar blind UV photodetectors. However, the p-type β-Ga2O3 is difficult to dope, which limits the further development of β-Ga2O3 devices. In this work, the β-Ga2O3 thin films with different Cu doping content are grown on sapphire substrates by chemical vapor deposition method, and the morphology, crystal structure and optical properties of β-Ga2O3 films are measured. The test results show that the surfaces of β-Ga2O3 films with different Cu content are relatively smooth, and the (<inline-formula><tex-math id=“M2”>\begin{document}$ \bar 201 $\end{document}</tex-math><alternatives><graphic xmlns:xlink=“http://www.w3.org/1999/xlink” xlink:href=“19-20230971_M2.jpg”/><graphic xmlns:xlink=“http://www.w3.org/1999/xlink” xlink:href=“19-20230971_M2.png”/></alternatives></inline-formula>) diffraction peak positions shift toward the lower degree side with the increase of Cu content, which indicates that Cu2+ replaces Ga3+ and enters into the β-Ga2O3 lattice. The optical absorption spectrum measurement indicates that the energy gaps of samples are evidently narrowed with the increase of Cu doping concentration. Hall measurements indicate that the Cu doped β-Ga2O3 thin films have a p-type conductivity with a hole concentration of 7.36 × 1014, 4.83 × 1015 and 1.69 × 1016 cm-3, respectively. In addition, a photoconductive UV detector with metal-semiconductor-metal structure is prepared by evaporating Au on a Cu-doped β-Ga2O3 thin film, and its UV detection performance is studied. The results show that the photocurrent value of the device increases with Cu content increasing. The photo-to-dark current ratio (Il/Id) is about 3.8×102 of 2.4% Cu content device under 254 nm-wavelength light at 10 V. The rise time and decay time are 0.11 s and 0.13 s, respectively. Furthermore, the responsivity and external quantum efficiency can reach 1.72 A/W and 841% under 254 nm-wavelength light with a light intensity of 64 μW/cm2.

Tech Support

Original Source

DOI: https://doi.org/10.7498/aps.72.20230971