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temperature, at a small forward bias larger than -2V, from a simple metal-AlGaN/GaN Schottky diode. Their findings were published in Applied Physics Letters 105 (2014).


Schottky-drain electrode in an AlGaN/GaN HEMT


The researchers’ goal was to produce electroluminescence (EL) at room temperature from metal-AlGaN/GaN Schottky diodes on a conventional doping-free III-nitride heterostructure suitable for HEMTs. (EL was first discovered in a metal-SiC structure in 1907. EL emissions from Schottky diodes on Si, II-VI, and III-V semiconductor have also been reported by research groups over the last 30 years).


By employing a semi-transparent Schottky-drain electrode in an AlGaN/GaN HEMT, the team succeeded in building a UV high electron mobility light-emitting transistor (HEM-LET) in a relatively straightforward manner. Figure a) below presents the schematic device structure of the device demonstrated in this work.


removed by dilute alkaline solution. Then the semitransparent Schottky metal Ni/Au (5/6 nm) was deposited using an e-beam evaporator. A high electron mobility light-emitting transistor (HEM-LET) with ohmic source and semitransparent Schottky drain was made simultaneously.


Ni/Au (20/200 nm) was used as the gate metal and a SiN x/AlO (15/8 nm) stack insulator used as the gate dielectric for the HEM-LET, in order to single out the Schottky drain. Finally, the device was annealed at 400degC for 10 minutes in N2 ambience.


EL emission at room temperature


The team measured the current- voltage characteristics, and the EL and photoluminescence (PL) spectra. (A 266nm laser was used to excite the AlGaN/GaN heterostructure for the PL measurement).


The team used an AlGaN/GaN heterostructure consisting of a 21nm Al 0.25Ga 0.75 N barrier and 3.8µm GaN buffer, grown by MOCVD on a 4inch p-type S (111) substrate. The heterostructure contained a 2DEG channel of density 1013 mobility 2080cm2


/cm-2 /V-1 s-1 at room temperature.


They defined the ohmic contacts using photolithography and formed them with Ti/Al/Ni/ Au metallisation annealed at 850degC for 30s in N 2 ambience. Remote plasma pretreatment in an atomic-layer-deposition (ALD) machine was used to remove the residual native oxide and nitridise the surface. The passivation and surface protection layer was an AlN/SiN x (4/50nm) stack.


After the Schottky area was defined with photolithography, the SiN x was dry etched by a low-power plasma process and the AlN thin film


With semi-transparent Ni/Au (5/6 nm) Schottky metal, the team reported clearly seeing EL emission from the Ni/Au-Al 0.25Ga 0.75 N/GaN Schottky diode at room temperature when the forward bias is higher than 2.2V. The EL intensity becomes stronger at a higher bias.


and


They found that the EL spectra consisted of not only yellow and blue luminescence but also a narrow GaN band-edge UV component at 3.4eV, similar to the PL spectra of the AlGaN/GaN heterostructure as shown in graph c) above.


Both the EL and PL spectra are from the GaN layer; no emission from the thin AlGaN barrier layer was detected. The yellow/blue is due to radiative transition of electronics from conduction band of a shallow donor to a deep acceptor in the GaN layer. Its relative intensity, compared with GaN band- edge UV emissions decreased with increasing bias/ current or laser excitation intensity in both the LE and PL spectra


The team also experimented with another Schottky Issue VI 2014 www.compoundsemiconductor.net 143


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