High-transparency Ni/Au ohmic contact to p-type GaN

High-transparency Ni/Au ohmic contact to p -type GaN

J. K. Sheu and Y. K. Su

Department of Electrical Engineering, National Cheng Kung University, Tainan, Taiwan, Republic of China G. C. Chi

and P. L. Koh

Department of Physics, National Central University, Chung-Li, Taiwan, Republic of China M. J. Jou, C. M. Chang, C. C. Liu, and W. C. Hung

Epistar Corporation, Hsinchu, Taiwan, Republic of China

~Received 9 July 1998; accepted for publication 22 February 1999!

In this study, a very thin Ni/Au bilayer metal film was prepared by electron beam evaporation and thermal alloying to form ohmic contact on p-type GaN film. After thermal alloying, the current–

voltage (I – V) characteristic of Ni/Au contact on p-type GaN film exhibited ohmic behavior. The Ni/Au contacts showed a specific contact resistance of 1.7 310

V cm

at an alloying temperature of 450 °C. In addition, the light transmittance of the Ni/Au ~2 nm/6 nm! bilayer on p-type GaN was measured to be around 85% at 470 nm. These results suggest that a suitable metallization technology for the fabrication of light emitting devices can be achieved. © 1999 American Institute of Physics. @S0003-6951~99!00816-5#

GaN based semiconductors have recently been applied to fabricate optoelectronic devices such as light emitting di- odes ~LEDs! and laser diodes ~LDs! in the blue and violet light region.

They also have the potential for fabricating electronic devices operating at high temperatures up to 300 °C due to their superior physical properties, such as wider band gap, high breakdown field, and high thermal conductivity.

In order to improve the performance of LEDs with reduced contact resistance, reliable ohmic con- tacts on both n- and p-type GaN films are required. An im- proved ohmic contact with Ti/Al deposited on n-type GaN layer with a carrier concentration of ;1310

cm

has been demonstrated by Lin et al.

The specific contact resis- tance was as low as 8 310

cm

after the sample was annealed at 900 °C for 30 s by rapid thermal annealing

~RTA!. Several studies about ohmic contact on n-type GaN films deposited with other metals have been reported.

In contrast to the many studies of ohmic contact on n-type GaN, very few detailed investigations of ohmic contact on p-type GaN were reported.

In a GaN based LED, Ni/Au are com- monly used as ohmic contact on p-type GaN top layer.

However, the low doping level of the p-type GaN layer may result in a non-ohmic contact, thereby degrading the perfor- mance of devices. In addition, the current coming from the top electrode cannot be spread effectively through the entire chip due to the high resistivity of the p-type GaN. One way to avoid this so-called current crowding problem, thereby obtaining high efficiency and uniform light emission from the device, is to design the pattern of the top electrode to be interdigitated finger shape. However, this type of electrode reduces the emitted light output. Thus, a low resistive p elec- trode with high transparency is an important issue for fabri- cation of GaN-based LED, although the well known trans- parent conducting films such as indium tin oxide ~ITO! and cadmium tin oxide ~CTO! have been used as the electrode of

light emitting diodes.

However, in our study, ITO trans- parent conducting film on n-GaN ~with carrier concentration of 3 310

cm

!

and p-GaN ~with carrier concentration of 3 310

cm

! showed Schottky contact characteristics after thermal annealing. If the carrier concentration of GaN in- creases to the order of larger than 1 310

cm

, the forma- tion of ITO ohmic contact on GaN may be possible. How- ever, the hole concentration of p-GaN is hardly higher than 1 310

cm

. In this study, we used a thin Ni/Au ohmic contact on p-type GaN film. We have demonstrated that both low contact resistance and high light transmittance can be achieved at the same time.

The Mg-doped GaN was grown on a c-face sapphire substrate by metalorganic vapor phase epitaxy ~MOVPE!.

Trimethylgallium ~TMGa! and ammonia (NH

) were used as the sources for Ga and N, respectively; hydrogen was used as a carrier gas. Before growing the thicker high-temperature GaN epitaxial layer, a low-temperature nucleation layer nominally 30 nm thick was deposited at 525 °C. The high- temperature Mg-doped GaN epitaxial layer was grown at 1050 °C at a growth rate of 3 m ^{m/h. CP}

Mg was employed as the p-type dopant. The as-grown Mg-doped GaN film was semi-insulating with a resistivity greater than 10

V cm. In order to obtain p-type GaN film, thermal annealing was per- formed at 750 °C to activate the dopant. The ambient gas was N

and the annealing time was 30 min. The hole concentra- tion of the p-type GaN film was determined by Hall mea- surements at room temperature. For the Hall effect measure- ment, a sample of 5 35 mm

size was cut from the wafer and metal ~Ni/Au! dots were evaporated in the four corners to obtain electrical contacts in the Van der Pauw geometry. The Hall effect measurement showed that the film was p type with a hole concentration of 3 310

cm

and a mobility of 12 cm

/V s. Prior to metal deposition, the p-type GaN sample was cleaned by HCl:H

O ~1:1! solution, and followed by deionized ~DI! water rinsing. The metals were then deposited onto the p-GaN by thermal evaporator at a chamber pressure of ;1310

Torr. In this study, Ni/Au contact was formed

a!Electronic mail: [email protected]

APPLIED PHYSICS LETTERS VOLUME 74, NUMBER 16 19 APRIL 1999

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on p-GaN with 2 nm of Ni and 6 nm of Au, respectively.

Specific contact resistance was evaluated by using a circular transmission line model ~TLM!. The circular TLM pattern was formed by standard photolithographic techniques. After the formation of the TLM pattern, thermal alloying with tem- perature ranging from 400 to 500 °C and temperature interval of 50 °C in N

ambient were performed on the Ni/Au metal contacts. The current–voltage (I – V) characteristics were measured at room temperature using an HP4145B semicon- ductor parameter analyzer. The light-transmission character- istics of Ni/Au contact were measured by a Hitachi U3000 photospectrometer.

Figure 1 shows the I – V characteristics of Ni/Au dot contacts on p-type GaN films. The diameter and the inter- spacing of dot contacts were 130 and 170 m m, respectively.

The samples were alloyed at temperatures ranging from 400 to 500 °C. Only the as-deposited sample exhibited a nonlin- ear I – V characteristic. The 450 °C alloyed sample exhibited the best I – V characteristic among all samples. The improve- ment of I – V characteristics may be attributed to the interfa- cial reactions taking place between Ni, Au, and GaN and their alloys extending into GaN film. However, as the alloy- ing temperature increases to 500 °C, the I – V characteristics became worse and unstable. This could be attributed to poor adhesion of the metallic contact on GaN film due to the fully depletion of Ni, thereby resulting in the interfacial roughen- ing. On the other hand, as the thermal alloying was per- formed to the Ni/Au ~2 nm/6 nm! contact on p-GaN, Au readily diffuses through Ni into the metal/GaN interface which reacts with GaN to form the Au–Ga intermetallic phase and then mixing with the Ga–Ni intermetallic phase at the vicinity of the interface. Considering these possible in- terfacial reactions, it may be the cause of achieving the im- proved I – V characteristics of Ni/Au contact on p-type GaN.

However, if the thickness of the Ni of the Ni/Au bilayer is increased to 25 nm, the I – V characteristics could be im- proved by increasing the alloying temperature up to 700 °C.

In our study, the outdiffusion of Ni was clearly observed by Auger electron spectroscopy ~AES! depth pro- files as the thermal alloying was performed to the Ni/Au contact on p-GaN. Ni and Au also diffused into the GaN film and reacted with the GaN film at the vicinity of the

interfacial to form interfacial compounds, such as Ga

Ni

, Ga

Ni

, GaAu, and GaAu

.

To identify the possible reac- tions, the measurements of x-ray diffraction ~XRD! in a graz- ing incident angle configuration had been performed. Figure 2 shows the XRD spectra of alloyed Ni/Au contacts on p-type GaN. After 450 °C alloying, in addition to the peaks of ~0002! GaN, ~0004! GaN, ~111! Ni, and ~111! Au, an extra peak at around 2 u 528.4 were also detected, as shown Fig. 2. Note that this small peak at around 2 u 528.4 may be identified to be ~110! GaAu

or ~201! Ga

Au

. It indicates that the Au–Ga intermetallic phase was formed at the vicin- ity of the interface after thermal alloying. Other intermetallic phases should also exist at the vicinity of metal/

semiconductor interface but these reaction phases might be too thin to be detected in our present XRD system. Some further studies, such as the analysis of high resolution trans- mission electron microscopy and high resolution and high power x-ray diffraction in grazing incident angle configura- tion, will be conducted to understand the interfacial reaction.

These interfacial chemical products can substantially influ- ence the electrical properties of the contacts. As illustrated in Fig. 1, linear I – V curves were obtained after thermal alloy- ing. This may be attributed to the formation of Ga–Ni and/or Ga–Au compounds at the metal/semiconductor interface and then mixing, thereby reducing the barrier height. Although, Foresi and Moustakas

observed that metal contacts on GaN should have barrier heights which depend directly on the difference of the work function between the metal and GaN.

However, as generally known, chemical reactions between the metal and the semiconductor can substantially influence the electrical properties of metal-semiconductor contact. In a practical metal/semiconductor system with Fermi level pin- ning, the Schottky barrier height is empirically expressed as f

5S f

1C, where S is a slope parameter and C is a con- stant. In other words, the formation of the interfacial com- pounds near the metal/GaN interface during thermal alloying can possibly lead to a decrease of the S parameter, thereby influencing the barrier height.

In this study, the circular TLM was used to measure the specific contact resistance. Circular pattern design avoids the need for isolation of the contact structures by ion implanta- tion or mesa etching.

Table I shows the specific contact resistance and light transmittance of Ni/Au contacts on

FIG. 1. The I – V curves of Ni/Au contacts on p-type GaN films alloyed at various temperatures.

FIG. 2. The XRD spectrum of 450 °C alloyed Ni/Au contacts on p-type GaN layers.

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Appl. Phys. Lett., Vol. 74, No. 16, 19 April 1999 Sheuet al.

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p-type GaN films as a function of alloying temperature. The light transmittance for samples alloying at 450 and 500 °C are 83% and 88%, respectively. Note that the typical thick- ness of Ni and Au are 2 and 6 nm, respectively. For the 450 °C alloyed sample, the lowest value of specific contact resistance was found to be 1.7 310

V cm

. As mentioned previously, the contact resistance of Ni/Au bilayer on p-type GaN films became larger as the alloying temperature is above 500 °C. It is well known that the specific contact re- sistance will be reduced if the thickness of the Ni/Au bilayer and the carrier concentration of p-GaN are increased. How- ever, the increase of the Ni/Au bilayer will reduce the light transmission. Thus, there is a tradeoff between the value of contact resistance and the light transmittance. Figure 3 shows the measured light-transmission spectra of as-deposited, 400, 450, and 500 °C alloyed Ni/Au contacts on p-type GaN film.

In our experiments, the p-type GaN films were grown on double polished sapphire substrates. These high-transparency p-GaN wafers were used as the references to calibrate the light-transmission measurements. As shown in Fig. 3, the light transmission increases as alloying temperature is in- creased. This is presumably due to the gradual decrease in Ni thickness with increasing alloying temperature. The thick- ness reduction is believed to be a consequence of Ni diffu- sion and its reaction with GaN at the metal/semiconductor interface, as mentioned above. Also, the Au was found to

participate in the interfacial reaction with GaN near the metal–semiconductor interface.

As shown in Table I, the value of transmittance for Ni/Au contacts are 60%, 83%, and 88% ~at 470 nm! for samples of 400, 450, and 500 °C al- loyed, respectively. In general, to achieve good electrical property of Ni/Au contact on p-GaN, an optimum alloying temperature window, depending on the thickness of Ni/Au, needs to be chosen.

In conclusion, after thermal alloying, the I – V character- istic of very thin Ni/Au contacts on p-type GaN films was demonstrated to be ohmic. For alloying temperature of 450 °C, the Ni/Au contacts to p-type GaN showed a specific contact resistance of 1.7 310

V cm

. In addition, the light- transmission characteristics of the Ni/Au ~2 nm/6 nm! bi- layer on p-type GaN was measured. It clearly indicated that the transmittance of Ni/Au contacts increase with increasing alloying temperature. The optimum light-transmitting ohmic contacts alloyed at 450 °C typically exhibit a transmittance of 83%. GaN based light emitting diodes with low operation voltage and high efficiency can be successfully fabricated.

The authors would like to thank I.-Ping Huang and Y. C.

Ten for metal deposition and XRD measurement, respec- tively. They also acknowledge the financial support from the National Science Council for Research Grant Nos. NSC87- 2811-E-006-001 and NSC87-2115-E-008-001.

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FIG. 3. The transmission spectra of as-deposited, 400, 450, and 500 °C alloyed Ni/Au contacts on p-type GaN films.

TABLE I. Specific contact resistance and transmittance~at 470 nm! of the Ni/Au contacts on p-type GaN films as function of alloying temperature.

Transmittance Specific contact resistance

400 °C, 5 min 60% 2.35310²²V cm²

450 °C, 5 min 83% 1.70310²²V cm²

500 °C, 5 min 88% 2.43310²²V cm²

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