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The reflectivity of MoAgAu ohmic contacts on p-type GaN for flip-chip light-emitting diode

The reflectivity of MoAgAu ohmic contacts on p-type GaN for flip-chip light-emitting diode

The reflectivity of MoAgAu ohmic contacts on p-type GaN for flip-chip light-emitting diode

The re?ectivity of Mo/Ag/Au ohmic contacts on p-type GaN for

?ip-chip light-emitting diode (FCLED)applications

Ming-Jer Jeng,Ching-Chuan Shiue,Liann-Be Chang *

Department of Electronic Engineering,Chang-Gung University,259WenHwa 1st Road,Kweishan-Taoyuan 333,Taiwan

Received 5June 2007;received in revised form 13November 2007;accepted 7January 2008

Available online 17January 2008

Abstract

The Mo/Ag/Au contact for ?ip-chip light-emitting diode (FCLED)applications is examined on its contact resistance and light re?ectance.A high re?ectance of 90%is achieved in un-annealed contact,but a strong inter-diffusion of ohmic metals and GaN during the annealing process is found to result in poor re?ectance (55%at the wavelength of 465nm).The secondary ion mass spectrometry (SIMS)depth pro?les indicate that a wide inter-diffusion region existed in the annealed contacts;thus the low re?ectivity of the Mo/Ag/Au-annealed contacts can be attributed to the strong inter-diffusion of Au and Ag.

#2008Elsevier B.V .All rights reserved.

PACS :66.30.Jt

Keywords:Re?ectivity;Ohmic contact;GaN;FCLED;Light re?ectance;SIMS;Inter-diffusion

1.Introduction

The ?ip-chip light-emitting diode (FCLED)structures have been demonstrated having great potential applications in solid-state lighting [1–4].The FCLED means that the LED chip is ?ip-chip mounted on metal heat sinks.Light emits through a transparent substrates rather than an absorbing electrode contact layer.In the conventional top-emission LEDs,a large fraction of photons generated from the active region are blocked by p-electrodes or its bonding pads and reduce the light extraction ef?ciency.To enhance the light extraction ef?ciency by removing those electrode patterns from the top-emitting surface to the bottom one,a re?ective bottom contact was put together to re?ect downstream emitting light to top-emitting surface.It is known that many advantages exist in FCLED structures [1–4].In addition to the upward emitting light,the downward propagating light is re?ected up by a high re?ectance contact,increasing the light extraction.The heat generated in the LED ?ows directly through the interconnect metal of the submount,improving thermal conduction.A low

resistance and high re?ectance p-GaN contacts reduce spreading resistance.Thus,the FCLED structure can achieve a high power and a high brightness performance.In order to achieve such a high performance FCLED,the formation of high quality contact,which exhibits low resistance and high re?ectance,is a key issue.In literatures [5–7],many contact metals to p-GaN have been extensively studied and shown to be a good low resistance ohmic contact structure.A FCLED generally operates at high current state which rises a higher temperature in contact electrodes.If the electrode temperature does not spread out,the electrode will be possibly burned out.Hence,the ohmic contact metal with high melting point will be helpful for the high current operation.It is known that Mo has high melting point and good thermal conduction [8].It may be suitable to form ohmic contact for FCLED applications.Thus,Mo is used as ohmic metal in this work.In addition,Ag due to its high re?ectivity (>90%)is very good material for metallic re?ectors.However,Ag deposited directly on p-GaN shows a poor ohmic contact due to its low work function [9,10]and which results in a high contact resistance.A combination of Mo and Ag will be a good choice to form low resistance and high re?ective contact.It will be of great interest to investigate the ohmic contact and re?ectivity performance of the Mo/Ag/Au metallization.

http://www.wendangku.net/doc/d8ca3c3b8e9951e79a892703.html/locate/apsusc

Available online at http://www.wendangku.net/doc/d8ca3c3b8e9951e79a892703.html

Applied Surface Science 254(2008)4479–4482

*Corresponding author.Tel.:+88632118800x5793;fax:+88632118507.E-mail address:liann@http://www.wendangku.net/doc/d8ca3c3b8e9951e79a892703.html.tw (L.-B.Chang).

0169-4332/$–see front matter #2008Elsevier B.V .All rights reserved.doi:10.1016/j.apsusc.2008.01.039

2.Experiments

The GaN?lms were grown on c-plane sapphire substrates using the metal organic chemical vapor deposition(MOCVD) method.An unintentional doped GaN buffer layer was initially grown on the sapphire substrate,followed by n-type Si doped GaN layers and p-type Mg doped GaN layers.Thermal annealing was performed at7008C for30min under a N2 ambient to activate the Mg dopants.In Mo/Ag/Au contact formation process,Mo(1nm)/Ag(150nm)/Au(50nm)metal was deposited in sequence on the p-type GaN by e-gun evaporation.Some contacts were not annealed and some contacts were annealed at7008C for5min in the ambient of N2.It is known that ohmic contact properties are strongly dependant on the annealing temperature[5–7].For Mo/Ag/Au ohmic contacts on p-type GaN,various annealing temperatures in the range of500–8008C were also performed to?nd the best annealing temperature.The annealing temperature of7008C exhibits the lowest contact resistance.Thus,all the investiga-tions in this work focus on the annealing temperature of7008C. To study the correspondent contact characteristics and re?ectivity,samples with various Mo thicknesses(1,3and 5nm)were examined on p-type GaN contacts.Modi?ed transmission line methods(MTLM)[11]were adopted by the pattern formation of photolithographic technique to measure the respective speci?c contact resistance.Its advantage is to process simplicity and eliminate the possibility of obtaining misleading results from CTLM.The dot radius was75m m and spacing between dot circular centers was300m m.A spectro-photometer was used to measure the re?ectivity at the wavelength of465nm.The depth pro?les of ohmic metals and p-GaN were analyzed by secondary ion mass spectrometry (SIMS)measurements.

3.Results and discussions

Fig.1shows the current–voltage(I–V)characteristics of the Mo/Ag/Au contacts with and without anneal treatment.The I–V curves for the un-annealed contacts were highly nonlinear, but they were linear for the annealed contacts.This shows that the contacts were not ohmic prior to the anneal treatment.The contact resistance of the Mo/Ag/Au annealed samples with a value of1.69?10à3V cm2has one order less than that of un-annealed contacts(8.01?10à2V cm2).It is known that ohmic contact properties are strongly dependant on the annealing temperature[5–7].For Mo/Ag/Au ohmic contacts on p-type GaN,various annealing temperatures in the range of 500–8008C were performed to?nd the best annealing temperature.The variation of contact resistance as a function of anneals temperature is shown in Fig.2.Clearly,the contact resistance decreases with the increase of annealing tempera-ture up to7008C.Further increase of the annealing temperature results in a signi?cant increase of contact resistance.A contact resistance of1.69?10à3V cm2can be obtained at an anneal temperature of7008C.In literatures [12,13],different metallization stacks will result in different anneal behaviors.In Kumadas’results,a reduced Mo?lm with an anneal temperature of8508C can obtain low resistivity.In Mohammeds’results,metallization stacks of Ti/Mo/Au exhibited the lowest resistivity at an anneal temperature of 9008C.In this work,the multilayer contacts of Mo/Ag/Au shows a minimum resistivity at7008C.According to Kumadas’report[12],the high resistivity at low annealing temperature is attributed to the poor crystalline of Mo.The high resistivity at high annealing temperature is due to the Mo crack which is generated by the high temperature.Fig.3shows the I–V characteristics of the Mo/Ag/Au contacts treated by various chemical solutions(HCl,HCl+HNO3, HCl+(NH4)2S and HCl+HNO3+(NH4)2S)before contact deposition.The contact resistance of the samples with HCl+HNO3solution treatment exhibits the lowest value of 6.71?10à4V cm2.It is possibly due to the HCl+HNO3 solution can effectively remove the native oxide in GaN surface.No oxide layer exists between ohmic metal and GaN. Thus,good ohmic contact can be formed.However,in other solution treatment,the contacts exhibit a non-ohmic

The reflectivity of MoAgAu ohmic contacts on p-type GaN for flip-chip light-emitting diode

behavior. Fig.1.The current–voltage(I–V)characteristics of Mo/Ag/Au contacts with

and without an annealing temperature of7008C for5min in N2

The reflectivity of MoAgAu ohmic contacts on p-type GaN for flip-chip light-emitting diode

ambient.

Fig.2.The variation of contact resistance as a function of annealing tempera-

ture.

M.-J.Jeng et al./Applied Surface Science254(2008)4479–4482

4480

The re?ectivity of Mo/Ag/Au contact with and without an annealing temperature of 7008C for 5min in N 2ambient is shown in Fig.4.A high re?ectance of 90%is achieved in the un-annealed contact,but poor re?ectance of 55%is found in annealed contact.It is known that the re?ectivity of the contact without annealing treatment is usually better than that with annealing treatment [14].But the contacts without annealing treatment will result in poor ohmic behavior.Three different thickness of Mo,i.e.,1,3and 5nm,are also examined.Fig.5shows the re?ectivity of Mo/Ag/Au contact with three different thickness of Mo (1,3and 5nm).It was found that the thicker the Mo thickness,the lower the re?ectance.The thick Mo layer may result in large absorption and reduce re?ective intensity.But the thickest Mo with thickness of 5nm exhibits the lowest contact resistance as shown in the inset of Fig.5.

Fig.6shows the SIMS depth pro?les of the Mo/Ag/Au contact annealed at 7008C for 5min in N 2ambient.It is observed that a strong inter-diffusion of ohmic metals and GaN occurs.It is known that Ag is an excellent material for metallic re?ectors due to its high re?ectivity in the visual light wavelength region.But Au in-diffusion and Mo out-diffusion which cause the strong inter-diffusion of ohmic metals and GaN in the Mo/Ag/Au samples result in the poor re?ectance.Thus,the low re?ectivity of Mo/Ag/Au contacts can be attributed to the intermixing of Mo,Ag and Au metals.In addition,the out-diffusion of Ga and N atoms in Mo/Ag/Au contacts is serious.It may be considered as another reason for re?ectivity degrada-tion.However,an inter-diffusion between contact metals and GaN will be helpful for lowering contact resistance.In our previous study [15],it was shown that Au in-diffusion plays an important role in the intermixing.If a Mo layer can effectively barricade Au in-diffusion to the metal–semiconductor inter-face,high re?ectivity would be exhibited.In order to clarify this mechanism,the Ni/Ag/Mo/Au con?guration with a diffusion barrier metal of Mo is fabricated.Fig.7(a)and (b)shows the SIMS depth pro?les and the re?ectivity of the Ni/Ag/Mo/Au contacts.In Fig.7(a),it is found that Mo can effectively block Au in-diffusion and result in the high re?ectivity.It is

The reflectivity of MoAgAu ohmic contacts on p-type GaN for flip-chip light-emitting diode

known

Fig.3.The I –V characteristics of the Mo/Ag/Au contacts treated by various chemical solutions (HCl,HCl +HNO 3,HCl +(NH 4)2S and HCl +HNO 3+(NH 4)2S)before contact

The reflectivity of MoAgAu ohmic contacts on p-type GaN for flip-chip light-emitting diode

deposition.

Fig.4.The re?ectivity of Mo/Ag/Au contact with and without an annealing temperature of 7008C for 5min in N 2

The reflectivity of MoAgAu ohmic contacts on p-type GaN for flip-chip light-emitting diode

ambients.Fig.5.The re?ectivity of Mo/Ag/Au contact with three different thickness of Mo (1,3and 5

The reflectivity of MoAgAu ohmic contacts on p-type GaN for flip-chip light-emitting diode

nm).

Fig.6.The SIMS depth pro?les of the Mo/Ag/Au contact annealed at 7008C for 5min in N 2ambients.

M.-J.Jeng et al./Applied Surface Science 254(2008)4479–44824481

that higher-melting-point metals generally exhibit lower-bulk diffusivities [8].Mo has a signi?cantly higher melting point and can stop Au inter-diffusion [15,16].Once Ag layer intermixed with Au the re?ectivity will worsen.Although the detailed mechanism is unclear now,it seems to be a reasonable explanation for low re?ectivity of our Mo/Ag/Au contact due to the intermixing of Au and Ag.Therefore,in Fig.7(b),the Ni/Ag/Mo/Au contact has a light re?ectance as high as 91%.It is noted that thermal change of surface geometry due to anneal treatment may be a reason for re?ectivity reduction.It is known that Mo is easily oxidized to form a MoO 3layer.The forming layer of MoO 3will result in a roughness at surface and then cause a poor re?ectivity.But,in a similar con?guration,a Mo layer in Ni/Ag/MO/Au ohmic contacts can posses re?ectivity as high as 91%.If the roughness layer of MoO 3is a reason for re?ectivity reduction,the Mo layer in Ni/Ag/MO/Au contacts will result in worse re?ectivity.Oppositely,Ni/Ag/Mo/Au

contacts obtain a high re?ectance.However,as shown in the inset of Fig.7(b),the contact resistance of the Ni/Ag/Mo/Au contacts is 5.77?10à3V cm 2.4.Conclusions

Contact resistance and light re?ectance of the Mo/Ag/Au contact are investigated.It is known that ohmic contact properties are strongly dependant on the annealing temperature.A low contact resistance of 1.69?10à3V cm 2on the Mo/Ag/Au contact can be obtained at an anneal temperature of 7008C but with a poor re?ectance of 55%.On the other hand,a high re?ectance of 90%is achieved in the un-annealed contact.SIMS pro?les reveal that the poor re?ectivity in the annealed Mo/Ag/Au contacts is due to the strong inter-diffusion of both contact metals and GaN interface.Acknowledgement

This work was supported by the National Science Council of Taiwan,ROC.,under Contract NSC NSC94-2215-E-182-006.References

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The reflectivity of MoAgAu ohmic contacts on p-type GaN for flip-chip light-emitting diode

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Fig.7.(a)The SIMS depth pro?les of the Ni/Ag/Mo/Au contact and (b)The re?ectivity of the Ni/Ag/Mo/Au contact.The inset shows current–voltage curves of this contact.

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