Parvaneh Ravadgar

Effects of crystallinity and point defects on optoelectronic applications of β-Ga2O3 epilayers

This study evaluates the effect of crystallinity and point defects on time-dependent photoresponsivity and the cathodoluminescence (CL) properties of β-Ga₂O₃ epilayers. A synchrotron high-resolution X-ray technique was used to understand the crystalline structure of samples. Rutherford backscattering spectroscopy was used to determine the net chemical composition of the samples to examine the type and ratio of their possible point defects. The results show that in functional time-dependent photoresponsivity of photodetectors based on β-Ga₂O₃ epilayers, point defects contribution overcomes the contribution of crystallinity. However, the crystalline structure affects the intensities and emission regions of CL spectra more than point defects.

P-side up AlGaInP-based light emitting diodes with dot-patterned GaAs contact layers.

High-brightness p-side up AlGaInP-based red light emitting diodes (LEDs) with dot-patterned GaAs contact layer and surface rough structure are presented in this article. Initial LED structure of p-GaP/AlGaInP/GaAs is epitaxially grown using metal organic chemical vapor deposition technique. Using novel twice transferring process, the p-GaP layer is remained at the top side as both the current spreading and-window layer. Dot patterned GaAs contact dots are formed between main structure and rear mirror to improve light reflection and current spreading. Moreover, the surface of p-GaP window is further textured by nano-sphere lithography technique for improving the light extraction. Significant improvement in output power is found for AlGaInP LEDs with GaAs contact dots and roughened p-GaP window as compared with those of LEDs with traditional n-side up and p-side up structures without roughened surfaces.

See-Through

This paper demonstrates the high-temperature operation of fully transparent solar-blind deep ultraviolet (DUV) metal-semiconductor-metal (MSM) photodetectors (PDs) employing β-Ga₂O₃ thin films with transmittance up to 80% from 400 to 900 nm without image blurring. Even at a bias up to 200 V, the β-Ga₂O₃ MSM PDs show dark current as low as ~1 nA. The dark current of β-Ga₂O₃ MSM PDs under significantly different oxygen concentration in the ambiences are similar, indicating that the high inertness to surface effect. Moreover, the responsivity and the working temperature of β-Ga₂O₃ MSM PDs at 10 V bias are 0.32 mA/W and as high as 700 K, respectively. Full recovery after 700-K operation demonstrates reliability and robustness of β-Ga₂O₃ PDs. The superior see-through features, electrical tolerance, inertness to surface effect, thermal stability, and solar-blind DUV photoresponse of β-Ga₂O₃ MSM PDs support the use in next-generation DUV PDs applications under harsh environments.

\hbox{Ga}_{2}\hbox{O}_{3}

The β-Ga2O3 films were grown on (0001) sapphire at 500 °C by metal organic chemical vapor deposition. In the analysis of crystal structure, we found that the (-201) oriented single crystal β-Ga2O3 epilayer can be obtained under low chamber pressure of 15 torr. Moreover, a metal-semiconductor-metal solar-blind deep ultraviolet photodetector was fabricated with the β-Ga2O3 epilayer. As the bias voltage is 5 V, the photodetector exhibits a relatively low dark current about 0.2 pA, which induced by the highly resistive nature of the β-Ga2O3 thin films. From the responsivity result, it can be observed that photodetector shows a maximum responsivity at 260 nm, revealing the β-Ga2O3 photodetector was really solar-blind. The responsivity of the photodetector was as high as 20.1 A/W with an applied bias of 5 V and an incident light wavelength of 260 nm. The improved performance is attributed to the high quality of β-Ga2O3 epilayer.

Solar-Blind Photodetectors for Use in Harsh Environments

Single crystal β-Ga2O3 epitaxial layers have been prepared on c-axis (0001) sapphire substrates using metalorganic chemical vapor deposition technique at relatively low temperature. Post-annealing of β-Ga2O3 single crystals up to 800 °C does not affect the crystallinity, explored by x-ray diffraction, showing that β-Ga2O3 epitaxial layers are highly (-201) oriented. Metal-semiconductor-metal devices are fabricated on single crystals to study their photoresponsivity. A significant improvement in performance of post annealed-based devices is observed, attributed to point defect reduction. Annealing of as-grown samples results to a significant decrease in both oxygen and gallium vacancies, which are sources of current leakage.

Growth and characterization of Ga 2 O 3 on sapphire substrates for UVsensor applications

Highly (-201) oriented β-Ga2O3 films prepared by metal-organic chemical vapor deposition on (0001) sapphire substrates, undergone different post annealing temperatures to study their resistivity under harsh environment. Both of Rutherford backscattering spectrometry and cross-sectional transmission electron microscopy (TEM) results are exposing a harmony between oxygen vacancies and gallium interstitials. TEM characterization of samples determines a relationship between interstitials and formation of screw dislocations. Cathodoluminecsnece investigated under different applied voltages is found to be applicable to study chemistry of the bulk and surface of β-Ga2O3.