An improved non-alloyed ohmic contact Cr/Ni/Au to n-type GaN with surface treatment

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2008 J. Phys. D: Appl. Phys. 41 175107

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IOP PUBLISHING JOURNAL OF PHYSICS D: APPLIED PHYSICS

J. Phys. D: Appl. Phys. 41 (2008) 175107 (4pp) doi:10.1088/0022-3727/41/17/175107

An improved non-alloyed ohmic contactCr/Ni/Au to n-type GaN with surfacetreatmentHyun Kyong Cho1, Sun-Kyung Kim1 and Jeong Soo Lee21 LED New Technology Lab, LG Innotek, Seoul 137-724, Korea2 LG Electronics Advanced Research Institute, Seoul 137-724, Korea

E-mail: hkchoa@lginnotek.com

Received 22 May 2008, in final form 24 June 2008Published 21 August 2008Online at stacks.iop.org/JPhysD/41/175107

AbstractThe Cr/Ni/Au non-alloyed ohmic contact resistance on n-type GaN is obtained by chemicalsurface treatment of n-type GaN films following the laser lift-off of the sapphire substrate. Theeffects of n-GaN surface treatments on the metal/GaN interface were studied using x-rayphotoelectron spectroscopy. Nitrogen vacancies at the n-type GaN surface are thereforeproduced and act as donors for electrons, improving the non-alloyed ohmic contact resistanceinduced by the reduction in native oxygen by the surface treatment of chemical solutions. Inaddition, the n-GaN surface treatment reduces the forward voltage (Vf) of the vertical LEDs.(Some figures in this article are in colour only in the electronic version)

High power light-emitting diodes (LEDs) are in high demandfor solid-state lighting applications and are expected to replaceconventional lighting applications such as incandescent andfluorescent lamps [1, 2].

These LEDs require high current injection and theygenerate high temperatures during the operation of the LED.High current injection reduces the internal quantum efficiencyof the active layers in the LED by raising the junctiontemperature, and this causes the brightness and operatingvoltage of the LED to deteriorate. However, the high currentinjection can be avoided by fabricating the LED with a verticalstructure, where a laser lift-off (LLO) process is used to transferthe GaN film from the sapphire substrate and integrate GaNwith thermally and electrically conductive substrate materialssuch as Si, GaAs and Cu to distribute the paths of currentflows and to act as a heat sink [35]. Also, surface patterningfor producing photonic crystal (PhC) structures and surfacetexturing have been generated using the dry etching methodto improve the extraction efficiency of vertical GaN-basedLEDs [68].

Unfortunately, these techniques destroy the surface ofthe GaN film and thus increase the resistivity of the contactregion. The surfaces of compound semiconductors have veryactive chemisorption and production of the native oxide onthe surface. A residual native oxide would still exist or

regenerate on the surface prior to metal deposition. In order toreduce the surface product and improve the contact resistivity,considerable work has been performed on vertical injectionLEDs [9, 10].

Generally, the thermal annealing process of metalactivation is used for improving the contact resistivity inconventional lateral GaN-based LEDs, but this inevitablyinduces thermal damage such as decomposition and a spikyinterface within the vertical LED structure because the hightemperature process that is used for vertical GaN-based LEDsis incompatible with other substrates (Si or metal substrates).Therefore, the pretreatment of the surface before metaldeposition is the key to reducing the contact resistance on GaN.

In our work, we created a non-alloyed ohmic contactwith surface treatment using chemical wet-etching on an air-exposed n-GaN surface after LLO and a subsequent undopedGaN etch.

The GaN-based epilayer structure used in this workwas grown on a sapphire substrate by metal organicchemical vapour deposition (MOCVD). The LED epilayersconsist of an undoped GaN layer (2 m), an Si-dopedn-GaN layer (3 m), a five-period InGaN/GaN multiquantum well and a 100 nm thick Mg-doped p-GaNlayer. Deposition and annealing of a 280 nm thicktransparent indium tin oxide (ITO) film was performed

0022-3727/08/175107+04$30.00 1 2008 IOP Publishing Ltd Printed in the UK

J. Phys. D: Appl. Phys. 41 (2008) 175107 H K Cho et al

to serve as a p-contact to the p-GaN. Subsequently,an Ni(10 )/Ag(2000 )/Ni(1000 )/Cu(70 m) layer wasdeposited by an e-beam evaporator on the p-GaN surface. Themulti-metal layer acted not only as a reflective mirror layer butalso as a conductive seed layer on which a 70 m thick Cu waselectroplated. Using an ArF excimer laser with a wavelengthof 193 nm, an LLO process was performed to separate thesapphire substrate from the GaN-based LED structure. TheGaN attached with Cu was rinsed in an HCl : DI (1 : 1) solutionin order to remove the uppermost low quality Ga nucleationlayer. The air-exposed undoped GaN surface was dry etchedusing inductively-coupled-plasma reactive ion etching (ICP-RIE) with a Cl2/BCl3 etchant by 2 m in order to expose then-GaN layer.

The specific contact resistivities were measured using thelinear transmission line method (TLM). For the measurementof contact resistivity on n-GaN of the vertical LED, an activerectangular mesa region was defined using ICP-RIE. TheTLM test structure was patterned with a photoresist. Contactdimensions were 10050 m2, and the TLM spacing betweenthe contact pads was 5, 10, 15, 20 and 25 m.

Prior to metal evaporation of Cr/Ni/Au (200 /250 /5000 ) using the electron beam evaporator, the surfaces ofthe samples were treated with various chemical etchants suchas buffered oxide etch (BOE). The currentvoltage (IV )characteristics of the contacts were measured using a four-point-probe arrangement at room temperature. Also, x-rayphotoelectron spectroscopy (XPS) (PHI5400 system) withAl K was used to analyse the surface state in n-GaN withdifferent surface treatments.

Figure 1 shows the change in surface morphology inn-GaN as measured by atomic force microscopy (AFM) ofthe samples after LLO and LLO/ICP. For the samples thatwere processed with LLO operation, the rms roughness wasincreased to 46 , while that of as-grown samples was 2 . Nodistinct change in roughness was found after the dry etchingof GaN for exposing n-GaN in the LLO/ICP sample (39 ).

Figure 2(a) shows the IV curves of Cr/Ni/Au non-alloyed ohmic contacts to n-GaN with and without BOEtreatment. The IV curves were measured between the TLMpads with a gap spacing of 5 m. The ohmic contact exhibitslinear IV behaviour after BOE surface treatment, while theIV curve for the as-is sample (without surface treatment)was non-linear. The specific contact resistivity as a functionof the BOE cleaning duration is shown in figure 2(b). Thespecific contact resistivity was 1.2 104 cm2 after BOEcleaning for 10 min. As the cleaning time was increased to10 min, the contact resistivity was decreased. However, thecontact resistivity of the Cr/Ni/Au ohmic metal on the n-GaNthat was grown on the sapphire substrate was lower than thatof the result by two orders of magnitude.

Generally, the ohmic contact resistivity for n-GaN alongwith the Ga-face surface grown on the sapphire substrate waslower than 3 106 cm2 using Cr/Ni/Au. In our case, theremaining n-GaN epitaxial films of the vertical structure LEDsafter removing the sapphire substrate displayed an N-face(0 0 0 1) surface. The (0 0 0 1) surface (Ga-face) is composedof three nitrogen dangling bonds that point upwards to the

RMS:46

RMS:39

(a)

(b)

Figure 1. The surface morphology in n-GaN as measured by AFMof the samples after LLO (a) and LLO/ICP (b).

c-plane surface, while the (0 0 0 1) surface (N-face) hasa single nitrogen dangling bond that points upwards. Thedifference in the structures of the surfaces affects the devicecharacteristics, especially the ohmic contact properties. TheGa-face films have larger surface band bending than the N-facefilm by about 3.5 eV and 1.4 eV in the theoretical calculationand experimental results, respectively [1113]. It was foundthat the Ga-face GaN has a larger surface band bending than theN-face GaN, resulting in a higher Schottky barrier height [12].

After the surface treatment of n-GaN using BOE, the peakof the GaN bond in the Ga 3d core level shifts towards higherbinding energies (not shown). The shift of the Ga 3d corelevel peak is closely associated with the shift of the surfaceFermi level, increasing the conductivity of electrons at then-GaN surface. Figure 3 shows XPS spectra of O 1s core levelswith LLO, LLO/ICP and subsequent BOE cleaning. All O lsspectra were deconvoluted into two components. OGa andOC bonds can be seen. The peak intensity corresponding toO ls on n-GaN increased after the LLO and subsequent u-GaNetching by ICP. This shows that a large number of Ga oxideswere formed. When the etched sample was treated with BOE,the peak of the Ga oxides was reduced. This suggests that theGa oxides that were unintentionally formed under the LLO andCl2 plasma could be removed by the BOE treatment.

Also, the relative Ga/N ratio from the n-GaN sampleof LLO/ICP is normalized to one. The Ga/N ratio of theBOE treated sample exceeds that of the untreated sample.

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J. Phys. D: Appl. Phys. 41 (2008) 175107 H K Cho et al

Figure 2. (a) The IV curves of Cr/Ni/Au non-alloyed ohmiccontacts to n-GaN with and without BOE treatment. (b) The specificcontact resistivity as a function of the BOE cleaning duration.

The increase in the Ga/N ratio after LLO, ICP etching andsubsequent BOE cleaning is indicative of the formation ofa Ga-rich layer near the surface due to the generation of Nvacancies.

Figure 4 shows the IV characteristics of the verticalGaN LED with LLO, u-GaN etching and subsequent surfacetreatment with BOE, compared with the vertical LED withoutthe surface treatment. The chip size is 500 500 m2. It isseen that the forward voltage (Vf) drop in the vertical LEDat 60 mA is 3.3 V, which is about 39% lower than that of theLED that had no surface treatment. Compared with the LEDwithout the surface treatment, the reduction in Vf of the LEDwith the surface treatment is mainly attributed to low contactresistivity because the surface treatment using BOE enablesrelatively much less surface plasma damage during ICP.

The Cr/Ni/Au non-alloyed ohmic contact resistance of then-GaN in the vertical GaN LED was reduced by the BOE

532 534 536 538 540 542 544 546

O-CO-Ga

LLO/ICP/BOE

LLO/ICP

O1s

Nor

mal

ized

Inte

nsity

(arb.

units

)

Binding Energy (eV)

LLO

Figure 3. XPS spectra of O 1s core levels with LLO, LLO/ICP andsubsequent BOE cleaning.

-2 -1 0 1 2 3 4 5

20

0

40

60

80

100VLED (w/o surface treatment)VLED (with surface treatment)

Curre

nt (m

A)

Voltage (V)Figure 4. The IV characteristics of the vertical GaN LED withLLO, u-GaN etching and subsequent surface treatment with BOE,compared with the vertical LED without the surface treatment.

surface treatment of n-GaN following LLO and undoped GaNetching. XPS shows the reduction in the native oxide on then-GaN surface following the BOE treatment. The specificcontact resistivity of 1.2 104 cm2 was achieved in non-alloyed Cr/Ni/Au contacts on n-GaN of the vertical GaN LEDwith BOE treatment. This suggests that the Ga oxides thatwere unintentionally formed under the LLO and Cl2 plasmacould be removed by BOE treatment. Also, the increase inthe Ga/N ratio after LLO, ICP etching and subsequent BOEcleaning is indicative of the formation of a Ga-rich layer nearthe surface due to the generation of N vacancies.

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