氮化鎵族光電材料與元件之研發---子計畫III：鋁銦鎵氮化物微結構及光電特性分析(I)

Meta-stable Blue Emission of Mg-Doped GaN by Mg

N

Diffusion

C. Y. Fang
, S. Y. Chen

(),

M. S. Feng


Department of Materials Science and Engineering,

National Chiao Tung University,

Hsin-Chu, 300, Taiwan, R.O.C.

Tel: +886-3-5712121 Ext: 52964

Fax:

+886-3-5728486

e-mail :

[email protected]

a)

_{Department of Materails Science and Engineering,}

National Cheng Kung University,

Tainan, Taiwan, R.O.C.

 
I

 !"#$%&'()*+

 ,- (NSC88-2218-E-009-051)

./0187.8.1~88.7.31

Abstract

Mg was incorporated into un-doped GaN by diffusion method under 900

2 and 1000 2

for 3 hours. 325.5 nm He-Cd Laser was applied on both samples to perform PL measurement. There

exhibited decaying blue emission and D-A pair emission at different temperatures. The decay

curves were second-order with very long time-constants. No obvious free exciton decay and

yellow luminescence decay were observed. A Trapping –level mechanism was proposed to explain

the emission spectrum.

Meta-stable Blue Emission of Mg-Doped GaN by Mg

N

Diffusion

C. Y. Fang
, S. Y. Chen

(),

M. S. Feng


Department of Materials Science and Engineering,

National Chiao Tung University,

Hsin-Chu, 300, Taiwan, R.O.C.

Tel: +886-3-5712121 Ext: 52964

Fax:

+886-3-5728486

e-mail :

[email protected]

a)

_{Department of Materails Science and Engineering,}

National Cheng Kung University,

Tainan, Taiwan, R.O.C.

 
I

 !"#$%&'()*+

 ,- (NSC88-2218-E-009-051)

./0187.8.1~88.7.31



    

Summary

GaN and related III-V nitride semiconductors have attracted considerable attention because of its

potential for use in optoelectronics in UV-band and high-temperature electronic devices [1].One of the

important issues in fabricating LED is to accomplish high quality p-type GaN. In general, the p-type GaN is

obtained by Cp₂Mg incorporated in the process of metalorganic chemical vapor deposition (MOCVD). The

formation of Mg-H complexes during MOCVD growth has usually been s ug gested to be responsible for the

high resistivity of as-grown Mg-doped GaN [2,3,4]. It is believed that an activation process by thermal

annealing or low energy electron beam irradiation which results in the decomposition of Mg-H complex can

be use d to get low resistivity p-type GaN.5 In this article, we doped the Mg into GaN by a diffusion method

and observed the decay phenomenon of blue emission. The Mg-related transition at different temperatures was traced to explore the origin of Mg-related transition in GaN film.

2Ƌ Experiment

The undoped GaN films were grown on the c-face sapphire substrates by MOCVD with a horizontal

In this study, Mg₃N₂ was diffused into the undoped GaN at 900
and 1000°C, respectively, which we labeled them as sample A and sample B. The time-resolved PL spectra for the undoped GaN film and

Mg-doped GaN films at different temperatures, i.e. 7.5 K , 10 K, 15 K and 35 K, were measured. There

existed temporary blue emission at these temperatures for the sample A and B, but no blue emission was observed for the undoped GaN..

3Ƌ Results

Figure 1 shows the time-dependent intensity curves of PL spectra at different temperature using 325.5 nm He-Cd laser irradiation (55 mW) for the sample A. Both samples A and B have the same phenomenon. According to the wavelength-dependent PL spectra obtained, but not shown here, we found that both samples have the same band-edge emission near 356 nm (3.483 eV) and an exiton emission at 364 nm (3.407 eV). When sample was kept at the same temperature, e.g. 10 K, after irradiation, it would not recover its intensity in the next irradiation. On the other hand, when it was back to R.T. to achieve thermal equilibrium for a while (30 min in this study) and then cooled down to 10 K, it would recover its intensity and then decay again.











 −

+











 −

+

=

exp

exp

)

(

τ

τ

t

A

t

A

I

t

I

Figure 3 is shown the AFM morphology of the samples A and B. The surfaces are rather rough; even pits appear on the surface. The appearance of nodules implies the presence of many defects in the films. The roughness for sample A is more serious than sample B, because two pits can be clearly seen on the scanned surface of sample A.

4Ƌ Discussion

In this study, a trapping-level mechanism is proposed to explain the degradation and enhancement in

the sample. Similar phenomenon on un-intentionally doped GaN had been discussed in elsewhere6. However,

in our case, it is thought that the electron-trapping states on shallow Mg-related acceptor-level are easy to trap electrons. The trapping states are induced by Mg-incorporation during diffusion process. From the morphology obtained by AFM, it indicates that there must be a number of defects in the films.

Consequently, there must have many trapping states in the films. In equation (1), the time constant ₁

represents the lifetime of the free Mg-related acceptor states , and₂ represents the lifetime of free donor

states . In the beginning, the holes occupancies of Mg-related level are low. With the sample excited by laser, electrons on the donor level and holes on the Mg-related acceptor level are then created. Therefore, the donor-level and acceptor-level are capable of trapping carriers. The sample should be in thermal equilibrium before irradiation. After the sample was excited by laser irradiation, carriers were generated on each level, and a new steady-state dynamic equilibrium should be established. Due to the electrons trapped on the Mg-related acceptor level and the donor-level, the intensity therefore decreased gradually. Finally, a steady state equilibrium was reached with stable emission intensity.

5ƋConclusion

Mg-doped GaN at 900
or 1000
has a number of defects which cause meta-stable blue emission.

At high temperature, all carriers in films are easy to stay in thermal equilibrium. On the contrary, at lower temperature, carriers are easy to trapped by defects, which cause decaying phenomenon of blue emission.

ACKNOWLEDGMENT

The authors would like to thank the National Science Council of the Republic of China for financial support on this search under Contract No. NSC88-2218-E-009-051.

1 _{S. Strite and H. Morkoc, J. Vac. Sci. Technol. B10, 1237 (1992).}

2 _{M. Rubin, N. Newman, J. S. Chan, T. C. Fu, and J. R. Ross, Appl. Phys. Lett. 64, 64 (1994).}

3 _{W. Gotz, N. M. Johnson, J. Walker, D. P. Bour, H. Amano, and I. Akasaki, Appl. Phys. Lett. 67, 2666 (1995).} 4 _{J. A. Van Vechten, J. D. Zook, R. D. Horing, and B. Goldenberg, Jpn. J. Appl. Phys., Part 1 31, 3662 (1992).} 5 _{C. H. Hong et al., J. Appl. Phys. 74, 1705 (1995).}

6_{Bosang Kim, I. Kuskovsky, and Irving P. Herman , D. Li , G. F. Neumark,J. Appl. Phys. 86, 2034 (1999)}

FIGURES AND TABLE

(a) Measured at 7.5 K 0 1 0 0 2 0 0 3 0 0 4 0 0 2 x 1 0-5 3 x 1 0-5 4 x 1 0-5 5 x 1 0-5 6 x 1 0-5 7 x 1 0-5 8 x 1 0-5 Inte nsi ty (a.u.) T i m e ( s e c) (b) _{Measured at 15 K} 0 2 0 0 4 0 0 6 0 0 8 0 0 1 0 0 0 1 .5 x 1 0-5 2 .0 x 1 0-5 2 .5 x 1 0-5 3 .0 x 1 0-5 Inte nsity (a.u.) T im e (s e c ) (c) Measured at 35 K 0 200 400 600 8 00 10 00 0.0 1.0 x10-6 2.0 x10-6 3.0 x10-6 4.0 x10-6 5.0 x10-6 Intens ity (a.u. ) T im e (s e c )

FIG. 1. The time-dependent PL intensity for blue emission measured at 450 nm at (a) 7.5 K with
₁,
₂ = 4.545 sec, 77.27 sec. (b) 15 K with
₁,
₂ = 18.16 sec, 1311.11 sec. (C) 35 K with
₁,
₂ = 72.73 sec, 20364.28 sec.

Table 1. Second order decay curve parameters.

Temp. I₀(a.u.) A₁(a.u.) A₂(a.u.) τ₁(sec) τ₂(sec) 5K 3.25x10-5 2.44x10-5 1.65x10-5 4.54 77.27 15 K 5.76x10-6 5.27x10-6 1.71x10-5 18.16 1311.30 35 K 2.63x10-6 1.03x10-6 1.96x10-7 72.73 20364.28

(a)

(b)

Fig. 3. The AFM morphology for (a) sa mple A diffused at 900 _{ and (b) sample B diffused at 1000 }

1 _{S. Strite and H. Morkoc, J. Vac. Sci. Technol. B10, 1237}

(1992).

2 _{M. Rubin, N. Newman, J. S. Chan, T. C. Fu, and J. R. Ross,}

Appl. Phys. Lett. 64, 64 (1994).

3 _{W. Gotz, N. M. Johnson, J. Walker, D. P. Bour, H. Amano,}

and I. Akasaki, Appl. Phys. Lett. 67, 2666 (1995).

4 _{J. A. Van Vechten, J. D. Zook, R. D. Horing, and B.}

Goldenberg, Jpn. J. Appl. Phys., Part 1 _{31, 3662 (1992).}

5 _{C. H. Hong et al., J. Appl. Phys. 74, 1705 (1995).}