W ohmic contact for highly doped n-type GaN films

W ohmic contact for highly doped n-type GaN ®lms

C.F. Lin

_{*, H.C. Cheng}

_{, G.C. Chi}

a_{Department of Electronics Engineering and Institute of Electronics & Semiconductor Research Center, National Chiao Tung}

University, 1001 Ta Hsueh Road, Hsinchu, 30049, Taiwan

b_{Department of Physics, National Central University, Chungli, Taiwan}

Received 1 October 1999; received in revised form 27 October 1999; accepted 2 November 1999

Abstract

Ohmic contacts with low resistance and low barrier height were fabricated on n-type GaN ®lms using W metallization. The n-type GaN ®lms were grown by low pressure metalorganic chemical vapor deposition (LP-MOCVD) with Si as the dopant. Ohmic characteristics were studied for GaN ®lms with Si carrier concentration varying from 1.4  1017 _{to 1.8  10}19_cmÿ3_{. The speci®c contact resistivity were reduced with increasing Si-doping}

concentration on as-deposited W/GaN ®lms, and the lowest value was 3.6  10ÿ4 _Ocm2 _{on n-type GaN with a}

concentration of 1.8  1019_cmÿ3_{. After rapid thermal annealing (RTA) treatment on W/GaN ®lms at 10008C for}

30 s, the speci®c contact resistivity were reduced to 1  10ÿ4 _Ocm2_{. The W metal on n-type GaN ®lms, show that}

good thermal stability varied the annealing temperatures. The barrier height of as-deposited W metal on GaN is calculated to be 0.058 eV. 7 2000 Elsevier Science Ltd. All rights reserved.

Gallium nitride (GaN) is a promising material for UV/blue light emitting diodes and high power-tem-perature devices. These electronic and optical elec-tronic devices include visible light emitting diodes (LED) [1], metal semiconductor ®eld e€ect transistors (MESFET) [2], high electron mobility transistors (HEMT) [3], UV photo conductive detectors [4] and UV photovoltatic detectors. Rapid and promising developments on III±V nitride materials for the appli-cation of optical devices as well as electrical devices have occurred. Laser diodes [5], and microwave ®eld e€ect transistors (FETs) have been demonstrated. A problem in achieving high performance GaN-based devices is the realization of good reliable metal con-tacts. Conventional metallization gives high contact

re-sistance, thereby limiting the performance of GaN-based devices [6]. Research on both ohmic and Schottky contacts for GaN are of current interest [7± 9]. However, Lin and co-workers achieved low resist-ance ohmic contacts to n-GaN using Ti/Al bilayer metallization by annealing at 9008C for 30 s. The low-est value for the speci®c contact resistivity is 8  10ÿ6

Ocm2_{. The performances of GaN devices have often}

been limited by contact resistances [10]. Early attempts for better ohmic metals include those of Foresi and Moustakas [11] and Lin et al. [12]. Foresi and Mousta-kas employed Al and Au, and achieved about 10ÿ3

Ocm2 _{after a 5758C anneal for 10 min. Recently, Fan}

et al. [13] realized a very low speci®c contact resistance of 8.9  10ÿ8_Ocm2_{utilizing reactive ion etching (RIE)}

treatment and multi-metal-layers (Ti/Al/Ni/Au) [14]. W was found to produce low resistance ohmic contact to n+ _{GaN (r}

c0 10ÿ4 Ocm2) with little interaction

between the semiconductor and the metal up to 8008C [15]. And the formation of the b±W2N and W±N

inter-Solid-State Electronics 44 (2000) 757±760

0038-1101/00/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved. PII: S0038-1101(99)00312-3

* Corresponding author. Tel.: +886-3-5712121-55673; fax: +886-3-5724241.

facial phases were deemed responsible for the electrical integrity.

In this letter, Ohmic contacts of W metal were sput-tered on n+ _{Si-doped GaN ®lms with various doping}

concentrations. Electrical characteristics were observed using the rectangular transmission line measurements method (TLM). The barrier height of W on n-type GaN was also calculated using the various alloy tem-peratures.

GaN ®lms were grown on c-face sapphire substrates. The LP-MOCVD system is a horizontal type reactor with water cooling, and the pressure of growth chamber is at 100 mbar. Trimethylgallium (TMGa) and ammonia (NH3) were used as the Ga and N

sources, and hydrogen (H2) was used as the carrier

gas. The SiH4was used as the source of n-type dopant.

Optical-grade polished sapphires with the (0001)-orien-tation (C face) were used as the substrates. Before ®lm growth, the substrate was heated at 11308C for 10 min in a pure H2 ambient. A thin GaN bu€er layer (250

AÊ) was deposited at 5308C ®rst, then the epitaxial GaN was grown at 11008C. The growth rate was about 2 mm/h. The surfaces of the ®lms were smooth and the thicknesses of these specimens used in this work were about 2 mm, measured by scanning electron microscopy (SEM). GaN ®lms were patterned and then etched by reactive ion etching (RIE) to generate the mesa area for the TLM measurements. The pattern of metalization was de®ned using a photoresist lift-o€ technique. The dimensions of linear con®guration of rectangular pads were 200 mm wide and 100 mm long.

The gaps between the contact pads varied, from 20 to 120 mm with 20 mm as the increment. Before the evap-oration, specimens were cleaned in HCl enchant for about 1 min. The 2000 AÊ W metal were sputtered with Ar+ _{plasma on n-type GaN ®lms, varying with the}

doping concentration. The base pressure of the sputter system was below 2  10ÿ5 _{torr with the di€usion}

pump. The Ar ¯ow rate was ®xed at 6 sccm and the operation pressure was 5 mtorr. In our growth pro-cedure, there is a thin undoped GaN layer between the GaN bu€er layer and Si doped GaN ®lm. The undoped GaN layer is a so called second bu€er layer. The growth condition is similar to the Si doped GaN ®lm, just without the doping source SiH4. These

samples were alloyed by RTA treatment varying the temperature from RT to 10008C for 30 s in an N2

ambient.

The various speci®c contact resistivies with the RTA treated temperatures were shown in Fig. 1. For the as-deposited W metal pads on n-type GaN, the rc values

were reduced from 1.5  10ÿ2 _Ocm2 _{to 3.6  10}ÿ4

Ocm2 _{on di€erent Si doping level GaN ®lms varied}

from 8.4  1017_{to 1.8  10}19_cmÿ3_{. But the W metal}

on GaN ®lms with 1.4  1017 _cmÿ3 _{carrier density}

shows the slightly Schottky property from the I±V curves. When the contact deposited on GaN ®lm with high doping concentration, the tunneling process will dominate, and the speci®c contact resistivity rcis given

as [16]:

Fig. 2. A plot of the calculated values of rcas a function of

1=pND:

Fig. 1. The speci®c contact resistivity as the function of RTA alloy temperatures on di€erent Si-doping concentration n-type GaN ®lms.

C.F. Lin et al. / Solid-State Electronics 44 (2000) 757±760 758

r_c1 exp  qfBn E00  ˆ exp " 2pEsm …h=2p†  _f Bn  ND p # , …1†

where rs is the semiconductor permittivity, m is the

e€ective mass and h the Planck constant. Eq. 1 shows that in the tunneling process, the speci®c contact resis-tivity strongly depends on the doping concentration. As the Si-doping concentration of the GaN ®lms is increased, the slope of the I±V curve is improved with the reducing contact resistance. The speci®c contact resistivity, rc, is obtained from the I±V data by

measurement resistance vs TLM pattern's gap spaces. Fig. 2 shows the plot of the measured values of rcas a

function of 1=pND: When the carrier concentration of the specimen is larger than 8  1017_cmÿ3_{, r}

cdecreases

rapidly with increased doping concentration. The ratio of Es/E0 is 9.5 and m equals 0.19 m0 [17] for C-face

GaN. By ®tting the rcvs 1=pND curve and using Eq. (1), the fBn of W is calculated to be 0.058 eV fBnof

the semiconductor for a Schottky barrier on an n-type semiconductor is related to the metal work function (fm) and the wsby fBn=fmÿws. The Schottky barrier

height of W on GaN ®lms shows the thermal stability as the value of 0.058 eV with and without 3008C RTA treatment. The I±V curve shows the linear property of W metal with the RTA treated temperatures above 3008C on n-type GaN ®lms with the carrier concen-tration below 1  1019_cmÿ3_{. The electron mobility as}

the function of carrier density shows the linear prop-erty in Fig. 3. The Si as the n-type doping source shows the better activation eciency on the undoped GaN ®lms with 4.5  1016 _cmÿ3 _{and 560 cm}2_{/V.s of}

the background carrier concentration and mobility.

The decreasing mobility values have the traditional behavior characteristic of ionized impurity scattering with the increasing electron concentration.

In summary, low ohmic contact resistivity and ther-mal stable W metal on n+_{-type GaN ®lms were}

achieved. The dependence of speci®c contact resistivity on the doping concentration of n-type GaN is investi-gated. Good ohmic characteristics on GaN ®lms were observed with a carrier concentration higher than 8.4  1018 _cmÿ3 _{without thermal treatment, and the}

speci®c contact resistivity of 3.6  10ÿ4 _Ocm2 _is

obtained without thermal annealing on n+ _{GaN (1.8}

 1019_cmÿ3_{). The barrier height of W on n-type GaN}

is calculated to be 0.058 eV, and the W metal barrier height shows the thermal stability as a value of 0.058 eV for as-deposition and 3008C RTA treatment W metal. After rapid thermal annealing (RTA) treat-ment on W/GaN ®lms (1.8  1019_cmÿ3_{) at 10008C for}

30 s, the speci®c contact resistivity were reduced to 1  10ÿ4_Ocm2_.

Acknowledgements

This work has been supported by the National Sciences Council under the Contact No. NSC 89-2215-E-009-069. The technical support from the Semicon-ductor Research Center at National Chiao-Tung Uni-versity are also acknowledged.

References

[1] Nakamura S, Mukai T, Senoh M. Appl Phys Lett 1994;64:28.

[2] Asif Khan M, Kuznia JN, Bhattarai AR, Olson DT. Appl Phys Lett 1993;62:1786.

[3] Asif Khan M, Bhattarai AR, Kuznia JN, Olson DT. Appl Phys Lett 1993;63:1214.

[4] Asif Khan M, Kuznia JN, Olson DT, Van Hove JM, Blasingame M, Reitz LF. Appl Phys Lett 1992;60:2917. [5] Nakamura S, Senoh M, Nogahama S, Iwasa N, Yamada

T, Matsushita T, Kiyoku H, Sugimoto Y. Jpn J Appl Phys 1996;35:L74.

[6] Shen TC, Gao GB, Morkoc H. J Vac Sci Technol B 1992;10:2113.

[7] Lin ME, Ma Z, Huang FY, Fan ZF, Allen LH, Morkoc° H. Appl Phys Lett 1994;64:1003.

[8] Foresi JS, Moustakas TD. Appl Phys Lett 1993;62:2859. [9] Binari SC, Dietrich HB, Kelner G, Rowland LB,

Doverspike K, Gaskill DK. Electron Lett 1994;30:909. [10] Khan MA, Kuznia JN, Olson DT, Scha€ WJ, Burm J,

Shur MS. Appl Phys Lett 1994;65:1121.

[11] Foresi JS, Moustakas TD. Appl Phys Lett 1993;62:2859. [12] Lin ME, Ma Z, Huang FY, Fan ZF, Allen LH, Morkoc°

H. Appl Phys Lett 1994;64:1003. Fig. 3. The electron mobility as the function of electron

con-centration shows the ionized impurity scattering process at heavy doping GaN ®lms.

[13] Fan Z, Mohammad SN, Kim W, Aktas O, Botchkarev AE, Morkoc° H. Appl Phys Lett 1996;68:1672.

[14] Zolper JC, Shul RJ, Baca AG, Wilson RG, Pearton SJ, Stall RA. Appl Phys Lett 1996;68:2273.

[15] Cole MW, Eckart DW, Han WY, Pfe€er RL, Monahan

T, Ren F, Yuan C, Stall RA, Pearton SJ, Yi Y, Lu Y. J Appl Phys 1996;80:178.

[16] Sze SM, editor. Physics of Semiconductor Devices, 2nd ed. New York: Wiley, 1981. p. 849.

[17] Marlow GS, Das MB. Solid-State Electron 1982;25:91. C.F. Lin et al. / Solid-State Electronics 44 (2000) 757±760