We demonstrate photovoltaic and photoconductive responses to near-infrared light in devices formed by depositing a film of gel permeation chromatography purified PbS quantum dots (QDs) on top of n-SiC epitaxial layers with natively grown, low-leakage 10–15 monolayer thick epitaxial graphene (EG) Schottky contacts. The QD film layer was removable by selective chemical etching, resetting the EG/SiC Schottky diode: the sub-bandgap response could be restored in subsequent PbS-QD depositions. The EG in these devices simultaneously forms Schottky contacts to SiC and ohmic contacts to PbS-QD, enabling electrical screening and isolation of these interfaces from each other. After PbS-QD deposition, the diodes exhibit photovoltaic and photoconductive responses at photon energies far below the SiC bandgap, extending to the NIR gap of the QD film. Scanning photocurrent microscopy illustrates that this is due to charge transfer from the QD film to the n-type 4H-SiC through a trap-limited, rectifying PbS-QD/SiC heterojunction with ideality n = 2 in parallel with the EG/SiC Schottky diode. The photoconductive gain at this QD/SiC interface could be useful for IR detection in wide-bandgap platforms. Response times as fast as 40 ms are suitable for imaging applications, although careful contact design is required to optimize work-function matching and spreading resistance.
We report an Ultrawide Bandgap Al0.4Ga0.6N channel metal-oxide-semiconductor heterostructure field effect transistor with drain currents exceeding 1.33 A mm−1 (pulse) and 1.17 A mm−1 (DC), around a 2-fold increase over past reports. This increase was achieved by incorporating a hybrid barrier layer consisting of an AlN spacer, n-doped Al0.6Ga0.4N barrier and a thin reverse graded AlxGa1–xN (x from 0.60 to 0.30) cap layer. To enhance current spreading, a “perforated” channel layout comprising of narrow channel sections separated by current blocking islands was used. A composite ALD deposited ZrO2/Al2O3 film was used as gate dielectric. A breakdown field above 2 MV cm−1 was measured.
All reagents for synthesis were used as received. Lead (II) oxide (PbO, 99.9 %), anhydrous acetonitrile (ACN, 99.8 %), anhydrous toluene (99.8 %), and 1,2-ethanedithhiol (EDT, 98 %) were purchased from Alfa Aesar. Bis(trimethylsilyl) sulfide ((TMS)2S, 95 %) and 1-octadecene (ODE, 90 %) were purchased from Acros Organics. Molecular sieves (4Å) were purchased from Mallinckrodt and activated by heating under vacuum prior to transferring into a nitrogen glovebox.For purification, methyl acetate (MeOAc, 99 %) was purchased from Millipore Sigma and dried under activated molecular sieves in a nitrogen glovebox following degassing under partial vacuum. Polystyrene Bio Beads (S-X1, 200-400 mesh) were purchased from Biorad. Bio-Beads were used to pack a GPC column following Shen et al. 1 with modifications as follows. Bio-Beads were swollen under ambient conditions with toluene. Next, toluene was evacuated from the swollen Bio-beads under partial vacuum and the medium was transferred to a nitrogen glovebox. The Bio-Beads were swollen a second time in the glovebox with anhydrous toluene and used to prepare the purification column following Shen et al. 1 QD synthesis, purification, and analysis:Oleate-capped, colloidal PbS quantum dots (QD) were synthesized following a reported method with slight modifications, as follows 2 . The as-synthesized QDs were initially purified by precipitation and redissolution (PR) using dried MeOAc and anhydrous toluene under air-free conditions. The sample was then dried under partial vacuum, transferred to a nitrogen glovebox, and re-dispersed in anhydrous toluene. QDs were then purified by gel-permeation chromatography (GPC) in the glovebox using reported methods 1 . An estimated concentration of 20-30 mg/mL for the GPC-purified QDs in anhydrous toluene was achieved by volume reduction under partial vacuum. The sample was filtered through a syringe filter (0.1 μm PTFE membrane) prior to the deposition of thin films.Routine absorbance spectra were obtained on a Cary 5000 UV-vis-NIR spectrometer in dual beam mode. For samples in solution, a quartz cuvette with a path length of 1 cm was used. For thin films, glass slides were used as substrates on a solid sample holder. Epitaxial substrates:All SiC substrates used for epitaxial growth were highly doped (~10 19 ) n + 4H-SiC 8°offcut, diced into 1 x 1 cm 2 samples from a 3-inch wafer purchased from Cree (www.cree.com). The substrates were degreased with organic solvents, deoxidized with hydrofluoric acid and rinsed with de-ionized water before being blown dry with Argon for growth. Formation of QD films by spin coating:QD thin films were deposited following a layer-by-layer (LBL) spin coating and ligand exchange procedure using EDT as the exchange ligand. In a typical thin film, formation of a layer of the LBL ligand-exchanged film was conducted as follows. First, ~15-25 μL of GPC-purified PbS QDs in anhydrous toluene (20-30 mg/mL) were deposited on a rotating substrate at 3000 rpm in a nitrogen glovebox. Next, 3 drops of an EDT/anhydrous ACN s...
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