Van der Waals Integrated Silicon/Graphene/AlGaN Based Vertical Heterostructured Hot Electron Light Emitting Diodes

Silicon-based light emitting diodes (LED) are indispensable elements for the rapidly growing field of silicon compatible photonic integration platforms. In the present study, graphene has been utilized as an interfacial layer to realize a unique illumination mechanism for the silicon-based LEDs. We designed a Si/thick dielectric layer/graphene/AlGaN heterostructured LED via the van der Waals integration method. In forward bias, the Si/thick dielectric (HfO2-50 nm or SiO2-90 nm) heterostructure accumulates numerous hot electrons at the interface. At sufficient operational voltages, the hot electrons from the interface of the Si/dielectric can cross the thick dielectric barrier via the electron-impact ionization mechanism, which results in the emission of more electrons that can be injected into graphene. The injected hot electrons in graphene can ignite the multiplication exciton effect, and the created electrons can transfer into p-type AlGaN and recombine with holes resulting a broadband yellow-color electroluminescence (EL) with a center peak at 580 nm. In comparison, the n-Si/thick dielectric/p-AlGaN LED without graphene result in a negligible blue color EL at 430 nm in forward bias. This work demonstrates the key role of graphene as a hot electron active layer that enables the intense EL from silicon-based compound semiconductor LEDs. Such a simple LED structure may find applications in silicon compatible electronics and optoelectronics.


Fabrication of AlGaN layer on Sapphire
An AlGaN layer with an Mg concentration of 5 x10 19 cm -3 and a thickness of 800 nm was grown on the sapphire substrate by metal-organic chemical vapor deposition (MOCVD). The hole concentration is about 5×10 17 cm -3 for the AlGaN layer (through Hall effect measurement of the hole concentration). After ultrasonic cleaning in acetone, ethanol, and DI water respectively for 5 min, the samples were dried by N2. Subsequently, Ni (20 nm)/Au (50 nm) contacts were achieved by the electron-beam evaporation method onto one end of the sample and followed by intermediate annealing at 600°C in N2 conditions for 30 min to reduce the contact resistance. [1]

CVD growth of graphene
Continuous films of polycrystalline Graphene (monolayer and bilayer) were primarily grown on a few micrometer thick Cu foils (Alfa Aesar). The polished Cu or moisture-free Cu samples were cut into 5 cm length and 2cm width strips and positioned in a hot wall furnace consisting of a 22-mm ID fused silica tube heated with a split tube furnace. A typical growth process of graphene flow is (1) load the fused silica tube with the Cu foil, evacuate, backfill with hydrogen, heat to 1000 o C, and maintain H2(g) pressure of 69 Pa under a 40 sccm flow; (2) stabilize the Cu film at the desired temperatures, up to 1000℃, and introduce 5 sccm of CH4(g) for the desired period of 45 minutes at a total pressure of 92 Pa; (3) after exposure to CH4, the furnace was cooled to room temperature. The experimental parameters (temperature profile, gas composition/flow rates, and system pressure) are shown in Fig. S1. The cooling rate was varied from 300℃/min to about 400℃/min which resulted in films with no discernable differences.
Graphene films were removed from the Cu foils by etching in an aqueous solution of iron nitrate or the ferric chloride. The etching time was found to be a function of the etchant concentration, the area, and thickness of the Cu foils. The PMMA method was used to transfer the graphene from the Cu foils. The surface of the graphene-on-Cu is coated with poly-methyl methacrylate (PMMA) and after the Cu is dissolved, and the PMMA/graphene is lifted from the solution. [2] The graphene films are easily transferred to other desired substrates such as the TEM grid, SiO2/Si, sapphire, SiC, etc. with significantly fewer holes or cracks (< 5% of the film area).

Fabrication of Graphene/AlGaN heterostructure.
Double layered graphene was prepared by layer-by-layer transfer method. Graphene/Cu was used as a substrate to lift the graphene/polymethylmethacrylate (PMMA) membrane floating on the DI water. After that the sample allowed for naturally drying, then it was kept for annealing at about 105°C for 30 min. This sample was further treated for standard PMMA assisted graphene transfer method to obtain graphene/graphene/PMMA (bilayer graphene/PMMA) membrane. Then, the PMMA supported double-layer graphene was transferred onto the p-AlGaN and washed with acetone, and kept for drying. [3,4] The thickness of the double layer was expected to be 0.81 nm. [5]

Characterization, electrical and optical measurement.
Raman spectra were measured by a Renishaw micro Raman spectrometer with an excitation laser of 532 nm. The current-voltage (I-V) characteristics were measured by a Keithley 2400 multimeter. Electroluminescence (EL) spectra were recorded by a fiber optic spectrometer (Ocean view, QE pro). The Raman spectrum of graphene is shown in Fig. S2. The thickness of the n-Si was about 400 µm. The XRD spectrum of Si/HfO2 and Raman spectrum of Si/SiO2 are shown in Fig. S3. The Fig. S4a and S4b shows the I-V curve of the graphene free Si/Al2O3(10 nm)/AlGaN LED, and the EL of the LED at forward bias applied voltages respectively. The schematic illustration of the device structure of Si/SiO2/DLG/AlGaN LED is shown in Fig. S5. The energy band diagram and illumination mechanism of Si/SiO2/DLG/AlGaN LED are shown in Fig. S6. Figure S2. The Raman spectra of graphene. In the Raman characterization, graphene as excited with a laser of 532 nm, the Stokes phonon energy shift triggered by laser excitation generates two main peaks in the Raman spectrum. Graphene can be recognized by the location and character of its G (1580 cm -1 ) and 2D (2690 cm -1 ) peaks.