Visible and infrared rare-earth-activated electroluminescence from indium tin oxide Schottky diodes to GaN:Er on Si
Applied Physics Letters
Visible and infrared rare-earth-activated electroluminescence ͑EL͒ has been obtained from Schottky barrier diodes consisting of indium tin oxide ͑ITO͒ contacts on an Er-doped GaN layer grown on Si. The GaN was grown by molecular beam epitaxy on Si substrates using solid sources for Ga, Mg, and Er and a plasma source for N 2 . RF-sputtered ITO was used for both diode electrodes. The EL spectrum shows two peaks at 537 and 558 nm along with several peaks clustered around 1550 nm. These emission
... . These emission lines correspond to atomic Er transitions to the 4 I 15/2 ground level and have narrow linewidths. The optical power varies linearly with reverse bias current. The external quantum and power efficiencies of GaN:Er visible light-emitting diodes have been measured, with values of 0.026% and 0.001%, respectively. Significantly higher performance is expected from improvements in the growth process, device design, and packaging. Green light emission from semiconductor light-emitting diodes ͑LEDs͒ is more difficult to achieve than red or infrared ͑IR͒ light because of the lack of semiconductors with direct band gap in the range of 2.2-2.4 eV. GaP, which has an indirect band gap of 2.27 eV, has been utilized 1 for green LEDs, but after years of development still has rather inefficient external quantum efficiencies in the range of 0.1%-1%. Nakamura and co-workers 2,3 have demonstrated efficient green emission ͑2%-6% external quantum efficiency͒ with InGaN alloys and quantum well structures, but at the price of a highly complicated device structure and, hence, highly complicated processing. At the same time, progress is being made using II-VI materials ͑ZnTeSe LEDs with 5.3% external quantum efficiency 4 ͒, but device lifetime remains an issue. Green emitting devices grown on Si substrates would be ideal because of the obvious advantage of Si integrated circuit compatibility. IR electroluminescence ͑EL͒ has been demonstrated using Si doped with Er by implantation or during growth. 5-10 The IR photoluminescence ͑PL͒ 11-15 and cathodoluminescence 16,17 of Er-doped III-V compound semiconductors have been investigated by several groups. Recently, a review 18 of this subject by Zavada and Zhang indicates that wide-band-gap III-N semiconductors may be the ideal hosts for Er-doped devices. IR EL from Erimplanted GaN ͑Ref. 19͒ and from in situ Er-doped GaN ͑Ref. 20͒ have also been demonstrated, but no visible rareearth-based light emission was observed. We have recently reported strong visible ͑green͒ rareearth-activated PL from in situ Er-doped GaN grown by molecular beam epitaxy ͑MBE͒ on sapphire ͑Ref. 21͒ and on Si ͑Ref. 22͒ substrates. The same strong green rare-earthactivated emission was also observed 23 in EL from simple Al Schottky diodes on GaN:Er. In this letter, we report on the performance characteristics of Er-doped GaN LEDs emitting in both the visible and IR regions. The GaN:Er LEDs consist of Schottky diodes which use a transparent indium tin oxide ͑ITO͒ layer for both positive and negative electrodes. The relative sizes of the electrode dictate the diode behavior of the device. Under reverse bias, the smaller electrode is negative and the larger electrode is positive. The Er-doped GaN was grown using a Riber MBE-32 system on 2 in. p-Si ͑111͒ substrates. Solid sources were used to supply the Ga, Mg, and Er fluxes, while a STVA radio-frequency ͑RF͒ plasma source supplied atomic nitrogen. Prior to MBE growth, the Si substrate was cleaned in acetone, methanol, and DI water, followed by HF to remove all native oxide. After insertion into the MBE chamber, a thin, low-temperature GaN buffer layer was first grown followed by three GaN layers: ϳ0.6 m of undoped GaN, ϳ0.6 m of Er-doped GaN, and ϳ0.6 m of Mg-doped GaN. The GaN layers were grown at a substrate temperature of 750°C. The ITO contacts were formed using RF sputtering in conjunction with a lift-off process. The ITO target had a com-a͒ Permanent address: Air Force Research Laboratory, Wright-Patterson AFB, OH 45433. b͒ Electronic mail: email@example.com FIG. 1. Photograph of visible rare-earth-activated emission from ITO/ GaN:Er Schottky barrier LED ͑0.125 cm 2 area͒ operating with 19 mA current.