In this work, the optical properties of thin-film
α
-Ga
2
O
3
are examined and contrasted with those of
β
-Ga
2
O
3
thin films grown on similar substrates. Thin films of
α
-Ga
2
O
3
were synthesized ...employing mist chemical vapor deposition (mist CVD) on double-side polished
c
-plane sapphire. Optical properties were studied through absorbance, photoluminescence (PL), and photoluminescence excitation (PLE). Absorbance measurements showed spectra consistent with that previously reported in the literature, with
E
g
~ 5.19 eV for
α-
Ga
2
O
3
. Despite an absorption edge laying at higher energies, the PL spectra in
α-
Ga
2
O
3
films yield similar features to those of metal-organic chemical vapor deposition (MOCVD) grown
β
-Ga
2
O
3
thin-films. In both cases, when excited above the band gap, the PL spectra consist of UV, UV′, blue and green broad bands. As the excitation photon energy was varied, the relative intensity of these PL bands changed. This is attributed to a non-uniform defect concentration distribution across the sample depth. With smaller concentration of defects associated with blue and green emission in the bulk compared to the surface in the
α
-phase films.
Growth of high quality III–V compound nanowires using simple chemical vapor deposition method with safe, inexpensive precursors is extremely important for a wide range of applications from solar ...cells, detectors, to light emitting devices. However, the most-often used deposition approach using compound powers suffers from incongruent sublimation of group III and V elements, leading to premature exhaust of group V elements and eventually to defective, non-stoichiometric nanowires with poor electronic and optical properties. In this paper, we report new results of our efforts in resolving these challenges using as an example the growth of GaP nanowires, an important widegap semiconductor. Our results reveal fascinating roles played by the elemental P source in addition to GaP powder such as restoration of chemical balance, inhabit of the incongruent sublimation of GaP, and the growth of highly stoichiometric GaP NWs with high optical quality and maximized band edge emission. The effects of growth parameters such as growth time, orientations of silicon substrate, growth temperature, and precursor combinations are studied and correlated to stoichiometry of NWs, and to the existence and degree of deep defect states and band edge emission. Our study sheds new light onto the important interplays among all these important factors. Our strategy is not only important for the growth of GaP nanowires on silicon but also for other III–V compounds as well using CVD and inexpensive compound powder sources.
Growth of high‐quality InP film is demonstrated by directly co‐evaporating elemental In and P in a simple low‐cost chemical vapor deposition (CVD) reactor. By exploring the interplay between the ...metal‐assisted vapor–liquid–solid and vapor–solid mechanisms, the authors present the growth of highly stoichiometric InP thin films or nano‐network in a wide range of growth temperature from 560 to 720 °C. Photoluminescence (PL) measurements reveal high optical quality of the as‐grown InP film with PL bandwidth (≈49 meV) comparable to commercial InP wafer (≈41 meV). The versatility of this approach is demonstrated through successful growth of thin films on diverse substrates such as silicon, InP, and fused quartz. The effects of catalysts are also studied by comparing growths based on Au‐catalyst with In‐self‐catalyst (without Au). The temperature dependence study of Au‐catalyzed growth shows that higher growth temperature results in formation of larger grain sizes and continuous void‐free polycrystalline InP films. Preliminary solar cell devices are fabricated on a p‐type silicon with an indium‐tin‐oxide contact layer. It is believed that the new growth strategy provides a simpler, lower cost (both precursors and substrates) approach for producing high‐quality InP thin film for the fabrication of high efficiency, low‐cost solar cells.
Growth of InP film/nano‐network using metal‐catalyzed approach is demonstrated. By exploring the interplay between the vapor–liquid–solid and vapor–solid mechanisms, a highly stoichiometric InP with high optical quality in a wide range of growth temperature is obtained. The versatility of this approach is demonstrated through successful growth of InP on diverse substrates. Furthermore, preliminary solar cell devices are fabricated.
We report on n-type degenerate doping in β-(Al0.26Ga0.74)2O3 epitaxial thin films grown by metalorganic vapor-phase epitaxy and modulation doping in β-(Al0.26Ga0.74)2O3/β-Ga2O3 heterostructures. ...Alloy composition is confirmed using high-resolution X-ray diffraction measurements. Carrier concentration in the thin films is proportional to the silane molar flow. Room-temperature Hall measurements showed a high carrier concentration of 6 × 1018 cm-3 to 7.3 × 1019 cm−3 with a corresponding electron mobility between 53-27 cm2 V-1 s-1 in uniformly doped β-(Al0.26Ga0.74)2O3 layers. Modulation doping is used to realize a total electron sheet charge of 2.3 × 1012 cm−2 in a β-(Al0.26Ga0.74)2O3/β-Ga2O3 heterostructure using a uniformly doped β-(Al0.26Ga0.74)2O3 barrier layer and a thin spacer layer.
We report on the growth and characterization of metalorganic vapor-phase epitaxy-grown β-(AlxGa1−x)2O3/β-Ga2O3 modulation-doped heterostructures. Electron channel is realized in the heterostructure ...by utilizing a delta-doped β-(AlxGa1-x)2O3 barrier. The electron channel characteristics are studied using transfer length method, capacitance-voltage and Hall measurements. A Hall sheet charge density of 1.06 × 1013 cm−2 and a mobility of 111 cm2 V−1 s−1 is measured at room temperature. The fabricated transistor showed a peak current of 22 mA mm−1 and an on-off ratio of 8 × 106. A sheet resistance of 5.3 k /square is measured at room temperature, which includes contribution from a parallel channel in β-(AlxGa1-x)2O3.
In this work, we use photoluminescence spectroscopy (PL) to monitor changes in the UV, blue, and green emission bands from n-type (010) Ga2O3 films grown by metalorganic vapor phase epitaxy induced ...by annealing at different temperatures under O2 ambient. Annealing at successively higher temperatures decreases the overall PL yield and UV intensity at nearly the same rates, indicating the increase in the formation of at least one non-radiative defect type. Simultaneously, the PL yield ratios of blue/UV and green/UV increase, suggesting that defects associated with these emissions increase in concentration with O2 annealing. Utilizing the different absorption coefficients of 240 and 266 nm polarization-dependent excitation, we find activation energy for the generation of non-radiative defects of 1.34 eV in the bulk but 2.53 eV near the surface. We also deduce activation energies for the green emission-related defects of 1.20 eV near the surface and 2.21 eV at low temperatures and 0.74 eV at high temperatures through the films, whereas the blue-related defects have activation energy in the range 0.72-0.77 eV for all depths. Lastly, we observe hillock surface morphologies and Cr diffusion from the substrate into the film for temperatures above 1050 °C. These observations are consistent with the formation and diffusion of VGa and its complexes as a dominant process during O2 annealing, but further work will be necessary to determine which defects and complexes provide radiative and non-radiative recombination channels and the detailed kinetic processes occurring at surfaces and in bulk amongst defect populations.
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