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Hong Kong team integrates on-chip light source with III-nitride electronics

Approach holds promise for synchronous RF/optical comms and more...

Integrating III-nitride-based light-emitting and electronic control devices would help make more compact optoelectronics systems such as on-chip lighting control, synchronous RF/optical communications, and opto-couplers for power conversion. But attempts to grow LED and HEMT structures on the same substrate have been hampered by the incompatibility of their optimised growth temperatures and by the complexity of integrated devices with different active layers.

Now a team from Hong Kong University of Science and Technology has reported producing GaN band-edge ultraviolet emission at 3.4eV at room 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.

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/cm-2 and mobility 2080cm2/V-1s-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 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). 

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.

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 metal, Pt/Au and found similar results concluding this is  a general property of metal-AlGaN/GaN Schottky diodes. 

From these results, the researchers think it would be possible to realise synchronous radio-frequency/optical communications using an AlGaN/GaN HEM-LET, and an all-on-chip opto-coupler for III-nitride power electronics.

They also suggest that the metal-AlGaN/GaN light-emitting Schottky diode provides an alternative for micro-display with unique advantages. The back electrode is served by a high-mobility 2DEG channel, and the pixel is defined by the top Schottky contact, eliminating th mesa etching process and current spreading design and allowing for a higher resolution and smaller pixel size.

'P-doping-free III-nitride high electron mobility light-emitting diodes and transistors' by Baikui Li et al, appears i Appl. Phys. Lett. 105, 032105 (2014); http://dx.doi.org/10.1063/1.4890238

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