GaN-based Indium-tin-oxide light emitting diodes
with nanostructured silicon upper contacts
C.H. Kuo, S.J. Chang and H. Kuan
Abstract: GaN-based indium-tin-oxide (ITO) light emitting diodes (LEDs) with p-GaN, nþ-short period superlattice (SPS) and nanostructured silicon contact layers were fabricated. It was found that surface of the ITO LED with nanostructured silicon layer was very rough. It was also found that 20 mA forward voltages measured from ITO LEDs with p-GaN, nþ-SPS and nanostructured silicon contact layers were 6.01, 3.25 and 3.26 V, respectively. Compared with ITO LED with nþ-SPS, it was found that output power of ITO LED with nanostructured silicon contact was 17% larger. Furthermore, it was found that ITO LED with nanostructured silicon contact was more reliable.
1 Introduction
Wide bandgap III-nitride semiconductors are important devices that can be used in various applications, such as high power and high temperature electronics [1 – 3], ultra-violet photo-detectors[4 – 6], light emitting diodes (LEDs) and laser diodes (LDs)[7 – 10]. Conventional nitride-based LEDs use an Mg-doped GaN layer as the top p-contact layer and use Ni/Au as semi-transparent p-metal [11]. However, it has been shown that transmittance of Ni/Au is only around 60 – 75%. In order to enhance LED output intensity, one can use highly transparent indium-tin-oxide (ITO) to replace Ni/Au. Recently, we reported the fabrica-tion of nitride-based ITO LEDs with an nþ-short period superlattice (SPS) tunnel contact layer [12, 13]. It has been shown that we can simultaneously achieve a reason-ably small specific contact resistance and a high upper contact transmittance by using such a combination. However, we need to control the growth of the nþ-SPS carefully.
On the other hand, it is known that external quantum effi-ciency of LEDs depends on the refractive index and mor-phology of the top surface layer. The light extraction efficiency of GaN-based LED is limited mainly by the large difference in refractive index between GaN film and the surrounding air. The critical angle for photons to escape from GaN film is determined by Snell’s law. Although we cannot change the refractive index of GaN, we can enhance light output by roughening the GaN sample surface [14]. For a LED with roughened surface, the angular randomisation of photons can be achieved by surface scattering from the roughened top surface of the LED. In other words, we should be able to enhance LED
output intensity by changing surface morphology of the LEDs. In this study, we use a low resistive nanostructured silicon contact layer to replace the nþ-InGaN/GaN SPS tun-nelling contact layer. It was found that we could simul-taneously achieve rough surface and good contact resistance using this method. The physical and electrical properties of the fabricated nitride-based ITO LEDs with nanostructured silicon contact layers will also be discussed.
2 Experiment
The InGaN/GaN multi-quantum well (MQW) LEDs used in this study were all grown by metalorganic chemical vapour deposition (MOCVD). Details of the growth pro-cedures are already been reported elsewhere[11 – 14]. The structure of the device proposed in this study consists of a 30-nm-thick GaN nucleation layer, a 4-mm-thick Si-doped n-GaN cladding layer, an InGaN/GaN MQW active region, a 50-nm-thick Mg-doped p-Al0.15Ga0.85N cladding layer, a 0.25-mm-thick Mg-doped GaN layer, and a nanos-tructured silicon layer. The nanosnanos-tructured silicon layer was grown at 7508C using diluted SiH4 and H2 as the source materials. During the growth of this layer, we kept the growth time at 2 min whereas the flow rates of SiH4 and H2 were kept at 40 sccm and 60 slm, respectively. Atomic force microscopy was then used to characterise surface morphologies of the samples. Surfaces of the samples were then partially etched until the n-type GaN layers were exposed. ITO layers were subsequently evapor-ated onto the sample surfaces to serve as the upper contacts. On the other hand, Ti/Al/Ti/Au contacts were deposited onto the exposed n-type GaN layers to serve as the n-type electrodes. The epitaxial wafers were then lapped down to about 90 nm. We then used scribe and break to complete the fabrication of 300 mm 300 mm blue InGaN/GaN LED chips. For comparison, ITO LED chips with nþ-SPS tunnel contact layers and with only p-GaN cap layers were also fabricated. These LED chips were then packaged into LED lamps. Room temperature electroluminescence (EL) characteristics of these fabricated LED lamps were then evaluated by injecting different amount of DC current into these LED lamps. The output power was then measured using the molded LEDs with an integrated sphere detector from top of the devices. The reliabilities #The Institution of Engineering and Technology 2007
doi:10.1049/iet-opt:20060027
Paper first received 13th March and in revised form 9th July 2006
C.H. Kuo is with the Institute of Optical Science, National Central University, Chung-Li 320, Taiwan
S.J. Chang is with the Institute of Microelectronics & Department of Electrical Engineering, National Cheng Kung University, Tainan 701, Taiwan H. Kuan is with the Department of Electrical Engineering, Far East College, Hsin-Shih 744, Taiwan
E-mail: [email protected]
IET Optoelectron., 2007, 1, (3), pp. 110 – 112 110
of these LEDs were then evaluated by injecting a 50 mA DC current into these devices at 808C.
3 Results and discussion
Figs. 1aandbshow atomic force microscope images of ITO LEDs with nþ-SPS tunnel contact layer and with nanostruc-tured silicon contact layer, respectively. It can be seen that surface of ITO LED with nþ-SPS tunnel contact layer is smooth with a root-mean-square (RMS) roughness less than 3.3 nm. In contrast, surface of ITO LED with nanos-tructured silicon contact layer is much rougher with a RMS roughness larger than 15.7 nm. Fig. 2 shows room temperature EL spectra of the two LEDs with the same 20 mA DC injection current. It was found that EL peaks of these two LEDs both occurred at 465 nm. It was also found that integrated EL intensity of ITO LED with nanos-tructured silicon contact layer was 20% larger than that of ITO LED with nþ-SPS tunnel contact layer. Such an enhancement could be attributed to the much rougher surface of ITO LED with nanostructured silicon contact layer so that more photons can be emitted from sample surface.
Fig. 3 shows intensity – current – voltage (L – I – V) characteristic of ITO LEDs with nþ-SPS tunnel contact layer and with nanostructured silicon contact layer. For comparison, I – V characteristic of ITO LED with p-GaN contact layer was also plotted. It can be seen from Fig. 3 that 20 mA forward voltages measured from ITO LEDs
with p-GaN and with nþ-SPS contact layers were 6.01 and 3.25 V, respectively. Similar results have also been reported previously[12, 13]. In other words, the operation voltage measured from the ITO LED with p-GaN cap layer was much higher than that measured from the ITO LED with nþ-SPS contact layer. Such a large operation voltage can be attributed to the Schottky contact formed when ITO was deposited directly onto p-GaN. On the other hand, the much smaller operation voltage observed from ITO LED with nþ-SPS can be attributed to the facts that ITO forms good ohmic contact on n-GaN, carriers can tunnel through the nþ-p junction and carriers can spread out easily in the in-plane directions [12, 13]. As also shown inFig. 3, it was found that the 20 mA forward voltage measured from the ITO LED with nanostructured silicon contact layer was 3.26 V, which was almost identical to that observed from ITO LED with nþ-SPS contact layer. We believe some of the Si atoms might diffuse into p-GaN so as to form a highly doped thin nþ-GaN layer. Thus, we can significantly reduce the LED operation voltage without growing the complicated nþ-SPS structure. As also shown in Fig. 3, it was found that 20 mA output powers were 7.17 and 8.40 mW for the ITO LEDs with nþ-SPS contact layer and with nanostructured silicon contact layer, respectively. In other words, output power of ITO LED with nanostructured silicon contact layer was 17% larger than that of ITO LED with nþ-SPS contact layer. Such an enhancement could again be attributed to the rough surface when nanostructured silicon contact layer was used. It should be noted that the nanostructured silicon contact layer was very thin that it is not possible to determine its exact thickness by secondary ion mass spec-troscopy (SIMS).
Fig. 1 AFM images of ITO LEDs
a nþ-SPS tunnel contact layer and
b nanostructured silicon contact layer
400 500 600
ITO on Nanostructured Si layer
ITO on n+-SPS EL int e n sit y (a.u.) Wavelength (nm) Room temperature @20mA
Fig. 2 Room temperature EL spectra of the two LEDs with the same 20 mA DC injection current
0 20 40 60 80 100 0 2 4 6 8
ITO on nanostructured Si layer) ITO on n+-SPS ITO on p-GaN
Current (mA) V o ltage (V ) 0 5 10 15 20 25 Output Power (mW)
Fig. 3 L – I – V characteristics of ITO LEDs with nþ-SPS tunnel contact layer and with nanostructured silicon contact layer.
For comparison, I – V characteristic of ITO LED with p-GaN contact layer was also plotted
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Fig. 4 shows life tests of relative luminous intensity measured from ITO LEDs with nþ-SPS tunnel contact layer and with nanostructured silicon contact layer, normal-ised to their respective initial readings. During life test, both LEDs were driven by a 50 mA current injection at 808C. After 72 h stress, it was found that luminous intensity only decreased by 14% for ITO LED with nanostructured silicon contact layer. In contrast, luminous intensity decreased by 17% for ITO LED with nþ-SPS tunnel contact layer, during the same period. The slightly better reliability of ITO LED with nanostructured silicon contact layer is probably due to the extremely rough surface of the nanostructured silicon contact layer with a larger contact area. Thus, carriers could be injected into the LED easier with better current spreading. The larger output intensity also suggests smaller photon re-absorption and heat generation in the LED. As a result, we can achieve better device lifetime from ITO LED with nanos-tructured silicon contact layer.
4 Conclusion
In summary, GaN-based ITO LEDs with p-GaN, nþ-SPS and nanostructured silicon contact layers were fabricated. It was found that surface of the ITO LED with nanostruc-tured silicon layer was very rough. It was also found that 20 mA forward voltages measured from the ITO LEDs with p-GaN, nþ-SPS and nanostructured silicon contact layers were 6.01, 3.25 and 3.26 V, respectively. Compared with ITO LED with nþ-SPS, it was found that output power of ITO LED with nanostructured silicon contact was 17% larger. Furthermore, it was found that ITO LED with nanostructured silicon contact was more reliable.
5 Acknowledgements
The authors would like to acknowledge the financial support from the National Science Council for their research grant of NSC 90-2112-M-008-046 and NSC-95-2221-E-008-001.
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ITO on Nanostructured Si layer ITO on n+-SPS
Relative EL Int
e
nsity (%)
Time (hr)
Fig. 4 Life tests of relative luminous intensity measured from ITO LEDs with nþ-SPS tunnel contact layer and with nanostruc-tured silicon contact layer, normalised to their respective initial readings
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