Investigation of GaN LED with Be-implanted Mg-doped GaN layer

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Investigation of GaN LED with Be-implanted

Mg-doped GaN layer

Hung-Wen Huang

a

, C.C. Kao

a

, J.T. Chu

a

, H.C. Kuo

a

,

S.C. Wang

a,

*

, C.C. Yu

b

, C.F. Lin

c

a_{Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu, Taiwan, ROC} b_{Global Union Technology Corporation, Hsinchu 300, Taiwan, ROC}

c

Department of Material Engineering, National Chung Hsing University, Taichung, 400, Taiwan, ROC Received 9 April 2004; accepted 26 May 2004

Abstract

We report the electrical and optical characteristics of GaN light emitting diode (LED) with beryllium (Be) implanted Mg-doped GaN layer. The p-type layer of Be-implanted GaN LED showed a higher hole carrier concentration of 2.3 1018

cm 3and low specific contact resistance value of 2.0 10 4_Vcm2_{than as-grown p-GaN LED samples without Be-implantation. The Be-implanted GaN LEDs with}

InGaN/GaN MQW show slightly lower light output (about 10%) than the as-grown GaN LEDs, caused by the high RTA temperature annealing process.

Keywords: Gallium nitride (GaN); Light emitting diode (LED)

The Gallium nitride (GaN)-based semiconductors have been successfully employed to realize blue laser diodes

and light-emitting diodes (LED) [1–5]. In order to

fab-ricate these devices, it is necessary to implement con-trollable doping of GaN to realize both n-type and p-type GaN. Generally, n-type GaN has been achieved with the

doping of Si[6]and p-type GaN is typically achieved by

doping magnesium (Mg) in metal-organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE). However, the performance of such LEDs and lasers remains limited by several problems related to the high resistance ohmic contact to the p-type GaN. Most of the reported methods for reducing p-contact resistance rely on the optimization of the contact annealing temperature

and improvement of metal-semiconductor interface[7–9]

and have shown improvement in the LEDs performance

[10–12]. Beryllium (Be) was considered as a promising

candidate for p-type doping in GaN because of its lower

activation energy [13]. We adopt the ion implantation

procedure for doping Be into GaN, because it can provide precise control of dopant concentration and depth dis-tribution. A number of studies on the Be implantation of

p-GaN film have been reported earlier [14–16].

Pre-viously we reported the result of obtaining high carrier concentration and low specific resistance ohmic contact

based on Be-implanted p-GaN [17–19]. But, so far no

reports of the implementation of Be-implanted p-GaN film on LED structure. In this paper, we report the first experimental results including the optical, electrical prop-erties of Be implantation in GaN LED structure and comparison with the non-implanted GaN LED. The Be implanted GaN LED shows improvement in the electrical property performance.

The GaN LED was grown by metal-organic chemical vapor deposition (MOCVD) on a c-axis sapphire substrate. The LED structure consists of a 30-nm-thick GaN low temperature buffer layer, a 2.0-mm-thick undoped GaN layer, a 1.5-mm-thick highly conductive n-type GaN layer, a multiple quantum wells (MQW) region consisting of five www.elsevier.com/locate/mseb

* Corresponding author. Tel.: +886 3 5712121; fax: +886 3 5716631. E-mail address: [email protected] (S.C. Wang).

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period 2/5-nm-thick InGaN/GaN multiple quantum wells, and a 1-mm-thick p-type GaN. For the implantation of Be

ions, 1-mm-thick p-type GaN layer is necessary [17–20].

Trimethlygallium, ammonia, and CP2Mg were used as Ga, N

and Mg source, respectively. The LEDs were then implanted with Be ions at a energy of 50 keV and dose of about 1013cm 2. The ion projection ranges in p-type layer for energy 50 keV was estimated to be about 0.25 mm by the

TRansport of Ions in Matter (TRIM) stimulation [19,20].

These implanted LEDs were subsequently rapidly thermal annealed (RTA) at 1100 8C in N2ambient for various periods

of 15, 30 and 45 s, to repair the implantation-induced damages. The LEDs were fabricated using the standard

GaN LED fabrication process [21] including lithography

patterning, inductively coupled plasma reactive-ion-etching dry etching and electrode deposition, For comparison LED samples without Be-implantation were also fabricated.

The photoluminescence (PL) of p-type layer with and without Be-implantation on LEDs was investigated at room temperature using a 325 nm He–Cd laser as the excitation

source. Fig. 1 shows the room temperature PL spectra of

p-type layer on as-grown, as-implanted and annealed LEDs. The intensity of the PL line becomes much weaker after the implantation, indicating the occurrence of lattice damages due to the implantation. It can be seen that the annealed LEDs show a much stronger blue emission at about 440 nm corresponding to the donor-acceptor pair transition of

p-GaN [18–20]. In particular the blue emission of the

samples annealed for 30 s has the strongest intensity than that of others conditions. This suggests that the implanta-tion-induced damages could be repaired, at least in part, by the RTA process for 30 s at 1100 8C.

To investigate the effect of Be-implantation on the elec-trical characteristics of the p-GaN layer, both the as-grown

and Be-implanted LED samples were deposited with the same metallization of Ni (20 nm)/Au (100 nm) by electron

beam evaporation under a pressure of 2 10 6_{Torr and}

RTA 1100 8C annealed for 30 s. The measured hole carrier

concentration of Be-implanted samples is 2.3 1018_cm 3

(hole mobility is 4.4 cm2/V s) which is nearly five times

higher than the as-grown sample of 5.0 1017_cm 3_(hole

mobility is 10.2 cm2/V s).Fig. 2(a) shows the I–V charac-teristics of the Be-implanted GaN LED samples and the as-grown GaN LEDs measured by Circular Transmission Line Method (CTLM). It can be seen that Be-implanted GaN LED sample has better linearity than as-grown sample. Total resistance (Y) measured by CTLM has a linear relation with pad distances (X) as shown inFig. 2(b). The specific contact resistance rc was determined from the linear relationship

equation Y = 2Rc+ rs/2pR X, and the relation between the

specific resistance and the sheet resistance rc = rs LT2,

where Rcis the contact resistance, rs the sheet resistance,

and LTis the transfer length. By fitting the data we obtain a

lower specific contact resistance value of 2.0 10 4

Vcm2

for the Be-implanted samples than as-grown samples of 7.0

10 3

Vcm2. The results indicated that Be-implantation

not only enhances the carrier concentration but also facil-itates the p-type contact resulting in low specific resistance ohmic contact which is most desirable for optoelectronic devices. The higher carrier concentration of Be-implanted p-GaN could be responsible for the low resistance as earlier reported[19–20].

The Fig. 3(a) shows the room temperature

electrolumi-nescence (EL) spectra of the Be-implanted LED and the as-grown LED without implantation. The peak wavelength of the Be-implanted LEDs is around 430–470 nm which is slightly blue-shifted by 20 nm from that of the as-grown LED. The slight blue-shift might be also due to the change in

Fig. 1. PL spectra of the as-grown, as-implanted and Be-implanted Mg-doped GaN LED sample with post-annealing at annealing temperature of 1100oC for various annealing periods.

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the InGaN MQW. In content during the high temperature annealing[22–24]. The relative output luminous intensity as a function of injection current between 0 and 25 mA for the

LEDs were shown in Fig. 3(b). The Be-implanted GaN

LEDs shows slightly lower (about 10%) light output than the as-grown LED. This could be due to non-radiative defects and slight deterioration in the InGaN/GaN MQW active regions during the high temperature RTA process[22– 24].

In summary, Be-implanted GaN LEDs with InGaN/GaN MQW were fabricated and investigated. The p-layer of Be-implanted GaN LED showed a higher hole carrier concen-tration of 2.3 1018cm 3and low specific contact

resis-tance value of 2.0 10 4V cm2 than as-grown samples

(hole concentration 5 1017cm 3 and specific contact

resistance 7.0 10 3

Vcm2). The Be-implanted GaN LEDs

show slightly lower light output (about 10%) than the as-grown GaN LEDs possibly caused by the high RTA tem-perature process and implantation induced defects absorp-tion.

This work was supported in part by the National Science Council of Republic of China (ROC) in Taiwan under contract no. NSC 92-2215-E-009-015 and by the Academic Excellence Program of the ROC Ministry of Education under the contract no. 88-FA06-AB. The authors would like to thank Dr. G.C. Chi of National Central University (NCU) for the use of the ion implanta-tion facility.

Fig. 2. (a) The I–V curves of Ni/Au contacts on carrier concentration of as-grown and Be-implanted Mg-doped GaN LEDs. (b) CTLM fit results of the Ni/Au on carrier concentration of as-grown and Be-implanted Mg-doped GaN LED sample with post-annealing at annealing temperature of 1100 8C for annealing 30 s.

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References

[1] S.J. Pearton, C.B. Vartuli, J.C. Zolper, C. Yuan, R.A. Stall, Appl. Phys. Lett. 67 (1995) 1435.

[2] S. Nakamura, T. Mukai, M. Senoh, Jpn. J. Appl. Phys. 30 (Part 2) (1991) L1998.

[3] F.A. Ponce, D.P. Bour, Nature (London) 386 (1997) 351.

[4] A.G. Drizhuk, M.V. Zaitev, V.G. Sidorov, D.V. Sidorov, Tech. Phys. Lett. 22 (1996) 259.

[5] S. Nakamura, G. Fasol, The blue laser diode–gallium-nitride based light emitters and laser, Springrt, Berlin, 1997.

[6] A.V. Andrianov, D.E. Lacklison, J.W. Orton, D.J. Dewsnip, S.E. Hooper, C.T. Foxon, Semicond. Sci. Technol. 11 (1996) 366.

[7] D.J. King, L. Zhang, J.C. Ramer, S.D. Hersee, L.F. Lester, Mater. Res. Soc. Symp. Proc. 468 (1997) 421.

[8] T. Mori, T. Kozawa, T. Ohwaki, Y. Taga, S. Nagai, S. Yamasaki, S. Asami, N. Shibata, M. Koike, Appl. Phys. Lett. 69 (1996) 3537.

[9] T. Kim, J. Khim, S. Chae, T. Kim, Mater. Res. Soc. Symp. Proc. 468 (1997) 427.

[10] C.H. Kuo, S.J. Chang, Y.K. Su, L.W. Wu, J.F. Chen, J.K. Sheu, J.M. Tsai, Jpn. J. Appl. Phys. 42 (2003) 2270.

[11] C.H. Chen, S.J. Chang, Y.K. Su, Jpn. J. Appl. Phys. 42 (2003) 2281. [12] R.C. Tu, C.J. Tun, S.M. Pan, H.P. Liu, C.E. Tsai, J.K. Sheu, C.C. Chuo, T.C. Wang, G.C. Chi, I.G. Chen, IEEE Photon. Technol. Lett. 15 (2003) 1050.

[13] F. Bernardini, V. Fiorentini, A. Bosin, Appl. Phys. Lett. 70 (1997) 2990.

[14] R.G. Willson, S.J. Pearton, C.R. Abernathy, J.M. Zavada, Appl. Phys. Lett. 66 (1995) 2238.

[15] R.G. Willson, C.B. Vartuli, S.J. Pearton, C.R. Abernathy, J.M. Zavada, Solid-State Electron. 38 (1995) 1329.

[16] J.C. Zolper, J. Cryst. Growth 178 (1997) 157, and references therein. [17] C.F. Chu, C.C. Yu, Y.K. Wang, J.Y. Tsai, F.I. Lai, S.C. Wang, Appl.

Phys. Lett. 77 (2000) 3423.

[18] C.C. Yu, C.F. Chu, J.Y. Tsai, C.F. Lin, W.H. Lan, C.I. Chiang, S.C. Wang, Jpn. J. Appl. Phys. 40 (2001) L417.

Fig. 3. (a) EL of as-grown and Be-implanted Mg doped GaN LED sample. (b) L–I characteristic of as-grown and Be-implanted Mg doped GaN LED sample with post-annealing at annealing temperature of 1100 8C for annealing 30 s.

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[19] C.C. Yu, C.F. Chu, J.Y. Tsai, C.F. Lin, S.C. Wang, J. Appl. Phys. 92 (2002) 1881.

[20] H.W. Huang, C.C. Kao, J.Y. Tsai, C.C. Yu, C.F. Chu, J.Y. Lee, S.Y. Kuo, C.F. Lin, H.C. Kuo, S.C. Wang, Mater. Sci. Eng. B107 (2004) 237. [21] C.F. Chu, C.C. Yu, H.C. Cheng, C.F. Lin, S.C. Wang, Jpn. J. Appl.

Phys. 42 (2003) L147.

[22] M.D. McCluskey, L.T. Romano, B.S. Krusor, D.P. Bour, Appl. Phys. Lett. 72 (1998) 1730.

[23] Y.S. Lin, K.J. Ma, C.C. Yang, T.E. Weirich, J. Cryst. Growth 242 (2002) 35.

[24] C.J. Youn, T.S. Jeong, M.S. Han, J.W. Yang, K.Y. Lim, H.W. Yu, J. Cryst. Growth 250 (2003) 331.