Crystal Polarity Effects on Magnesium Implantation ...

1 downloads 0 Views 254KB Size Report
Jul 20, 2010 - plantation into Ga- and N-polarities GaN layers have ..... 19) D. J. Dewsnip, A. V. Andrianov, I. Harrison, J. W. Orton, D. E. Lacklison,. G. B. Ren ...
Japanese Journal of Applied Physics 49 (2010) 071001

REGULAR PAPER

Crystal Polarity Effects on Magnesium Implantation into GaN Layer Kuan-Ting Liu, Shoou-Jinn Chang1 , Sean Wu2 , and Yoshiji Horikoshi3 Department of Electronic Engineering, Cheng Shiu University, No. 840, Chengcing Road, Kaohsiung County 833, Taiwan 1 Institute of Microelectronics, Department of Electrical Engineering Center for Micro/Nano Science and Technology Advanced Optoelectronic Technology Center, National Cheng Kung University, 1 University Road, Tainan 701, Taiwan 2 Department of Electronics Engineering and Computer Sciences, Tung-Fang Institute of Technology, 110 Tung-Fung Road, Hunei Shiang, Kaohsiung 829, Taiwan 3 School of Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku, Tokyo 169-8555, Japan Received January 14, 2010; accepted April 21, 2010; published online July 20, 2010 Crystal polarity effects on Mg implantation into GaN layers for p-type doping have been systematically investigated. It is found that we can observe a smaller X-ray diffraction full-width at half-maximum and a stronger Mg-acceptor bound exciton emission for the Mg implantation into Npolarity GaN layer than Ga-polarity one after a proper post-implantation annealing treatment. Raman experiment demonstrates that the tensile stress occurs on the as-grown N-polarity GaN layer, which can be resulting from the Ga vacancy. Hall measurement results further indicate that the p-type conductivity can be successfully obtained for N-polarity GaN compared with Ga-polarity one after Mg implantation regardless of under the identical implantation and post-implantation annealing conditions. These phenomena can all be attributed to the more Ga vacancies in the asgrown N-polarity GaN layer that enhances Mg acceptor substitution and eventually achieve p-type conductive characteristics by an appropriate postimplantation annealing treatment. # 2010 The Japan Society of Applied Physics DOI: 10.1143/JJAP.49.071001

1.

Introduction

Gallium nitride (GaN) and its related compounds grown by metalorganic chemical vapor deposition (MOCVD) have been applied in commercial products such as light-emitting diodes (LEDs) and laser diodes.1,2) One of the main characteristics of GaN is its polarity. The polarity of GaN controlled has been demonstrated by molecular beam epitaxy (MBE) using different nucleation steps or buffer layers,3–7) however, only few reported on the different polarity growth of GaN by MOCVD.8–11) Conventionally, the MOCVD grown GaN layers always shows Ga-polarity regardless of the type of buffer layers. To explore fully the potential functions and applications of III–nitrides, the growth on various polarities of GaN by MOCVD is imperative. Controllable p-type doping is one of the most important issues for GaN-based devices. In general, p-type doping during epitaxial growth is typically achieved using Mg as the dopant. However, the large ionization energy12) of Mg acceptor and self-compensation13) limits the number of hole at room temperature. Compared with doping during epitaxial growth, doping by implantation can selectively dope a large number of elements into certain areas on the sample with a precise control of dopant concentration and depth distribution. It has been reported that the ion implantation of Mg into GaN would result in a large amount of N vacancies which act as compensating defects and cannot be repaired completely even with a high temperature annealing process.14) Different polarity can not only impact growth kinetics but possibly affecting the p-type doping characteristics of GaN since these planes have different structures, composition and chemistry. Experimental report for polarity effects on p-type doping in GaN by ion implantation technique is not ever to be seen so far. In this study, the crystal polarity effects on Mg implantation into Ga- and N-polarities GaN layers have been systematically investigated. The growth of Gaand N-polarities GaN layers by MOCVD is prepared through introducing Al intermediate layer process during 

E-mail address: [email protected]

growth. A more detailed study on the p-type doping properties of the GaN layer with various polarities will be reported. 2.

Experimental Procedure

The GaN layers used in this study are grown on c-face sapphire substrates by MOCVD. Ttrimethylgallium (TMGa), ammonia (NH3 ), and trimethylaluminum (TMAl) are used as the sources of gallium, nitrogen and aluminum, respectively. The carrier gas is hydrogen (H2 ), and the growth pressure is 100 Torr. The substrate is initially heated to 1100  C in H2 ambient for cleaning the surface of substrate, and then lowering to 550  C to grow a 30-nmthick low-temperature GaN buffer layer. The temperature is subsequently raised to 1000  C to grow a 2-mm-thick undoped GaN layer. Afterwards, TMAl is introduced to cover the surface with a few monolayers of Al. Finally, a 1-mm-thick undoped GaN layer is deposited on the top of the Al monolayers. Conventional MOCVD grown 2-mmthick undoped GaN layer without Al intermediate layer is also prepared under the identical growth conditions. After growth, second ion mass spectrometry (SIMS) depth profile analysis is executed to observe material composition and a NaOH solution is used for the polarity tests.15) The as-grown samples with and without Al intermediate layer are then implanted with Mg using multiple implantation technology. Mg ions are implanted with the total doses of 1:5  1015 cm2 (40 keV/1:5  1014 cm2 , 100 keV/4  1014 cm2 , and 200 keV/9:5  1014 cm2 ) to produce a uniform concentration of 5  1019 cm3 with a depth of 0:3 mm. After implantation, samples are respectively annealed in N2 ambient at 1100 and 1200  C for various times, capped with an undoped GaN wafer in a faceto-face geometry by rapid thermal annealing (RTA). X-ray diffraction (XRD) is employed to characterize the as-grown, after etching and postimplantation annealing samples. The residual strain in as-grown GaN layers is evaluated by a Raman scattering measurement. Optical and electrical properties are compared with both Mg implantation into GaN samples with different polarity by photoluminescence (PL) and Hall measurements.

071001-1

# 2010 The Japan Society of Applied Physics

Jpn. J. Appl. Phys. 49 (2010) 071001

K.-T. Liu et al.

Intensity (arb. unit)

Counts (arb. unit)

(a)

Ga N

(a)

With Al Without Al

Al

16.9

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 Depth (μm)

0.0

Intensity (arb. unit)

Counts (arb. unit)

(b) Ga N

(b)

17.2

With Al Without Al

Al

0.5

1.0 1.5 2.0 Depth (μm)

2.5

16.9

3.0

Fig. 1. SIMS depth profiles of Ga, N and Al for as-grown GaN layers (a) with Al, (b) without Al intermediate layer.

3.

17.0 17.1 ω (deg.)

17.0 17.1 ω (deg.)

Fig. 2. XRD rocking curves of (a) as-grown, and (b) after etching GaN layers with and without Al intermediate layer.

With Al intermediate layer

Results and Discussion

Figure 1 shows the measured SIMS profiles of Ga, N and Al for these two as-grown samples, respectively. Compared with the sample without Al intermediate layer, the sample with Al intermediate layer shows the presence of Al in the region of 1 mm from surface. Such a result agrees well with our initial design. It is well known that the crystal polarity can be distinguished by investigating its solubility against alkaline solution.15) When the crystal is N-polarity, the film is soluble in alkaline solution: when it is Ga-polarity, the film is insoluble. Figure 2 compares the XRD rocking curves for as-grown and after etching samples with and without Al intermediate layer. As shown in Fig. 2(a), the XRD rocking curve full-width at half-maximum (FWHM) of the as-grown samples with and without Al intermediate layer are 0.076 and 0.07 , respectively. Besides, the FWHM is invariable for the sample without Al intermediate layer before and after etching, as shown in Fig. 2(b). On the contrary, it is found that the FWHM of the sample with Al intermediate layer after etching increases to 0.086 , and XRD intensity diminishes rapidly. Furthermore, different surface micrographs of these two as-grown samples before and after etching measured by scanning electron microscopy (SEM) are shown in Fig. 3, respectively. As shown in Figs. 3(a) and 3(c), a few etched pits are observed on the surface of sample with Al intermediate layer after etching compared with that before etching. On the other hand, the surface morphology of the sample without Al intermediate layer remains the original regardless of etching in the alkaline solution. The results of the tests show that a polarity change occurs from Ga- to N-polarities by introducing Al intermediate layer during the growth of GaN in MOCVD. The GaN polarity

17.2

Without Al intermediate layer

As-grown

(a)

(b)

(c)

(d)

After etching

Fig. 3. SEM surface morphologies of as-grown GaN layers (a) with Al intermediate layer, (b) without Al intermediate layer, and after etching GaN layers (c) with Al intermediate layer, and (d) without Al intermediate layer.

selection process can be explained that since the bonding energy between Al and N is higher than that between Ga and N, the polarity of GaN grown on the Al covered surface depends on bonding configuration among Al and N when Ga and N species arrive at the Al covered surface. If N-polarity occurs, the N atom needs the cooperation among three Al atoms which can form much stronger metallic bonds.10) Therefore, we consider that the N-polarity is kinetically favorable on the Al covered surface. The residual strain in as-grown GaN layers is investigated by room temperature Raman measurement using the 532

071001-2

# 2010 The Japan Society of Applied Physics

Jpn. J. Appl. Phys. 49 (2010) 071001

K.-T. Liu et al.

(a)

1100°C, 10s

Intensity (arb. unit)

Intensity (arb. unit)

N-polarity

Ga-polarity

1100°C, 20s 1200°C, 10s

16.9 550

560

570

580

-1

590

17.2

600

Intensity (arb. unit)

Raman shift (cm ) Raman spectra of the Ga- and N-polarities GaN layers.

nm line of an Arþ laser as an excitation source. Figure 4 shows Raman spectra of the Ga- and N-polarities GaN layers, respectively. The Raman signal of the GaN E2 mode is observed at 568 cm1 for Ga-polarity GaN, 564 cm1 for N-polarity GaN, respectively. Compared with 568 cm1 for GaN of free strain,16,17) it can be found that the tensile stress occurs in N-polarity GaN since Raman peak of N-polarity GaN shifts to smaller wave number. The outcome can be attributed to the more Ga vacancy clusters are promoted in N-polarity GaN layer.18) In addition, Hall measurement shows the residual carrier concentrations of as-grown samples with and without Al intermediate layer are 4:06  1016 and 1:35  1017 cm3 , respectively. In other words, the N-polarity GaN has a lower residual carrier concentration than Ga-polarity GaN. Various postimplantation annealing conditions can accomplish different material properties, especially in different polarity of compound semiconductors. Figure 5 shows the XRD rocking curves of the Mg-implanted Ga- and Npolarities GaN layers after annealing, respectively. Both samples are annealed at 1100  C for various periods of 10 s, 20 s, and at 1200  C for 10 s. It can be seen obviously that the smallest FWHMs of XRD rocking curve are all obtained by postimplantation annealing at 1200  C, 10 s for Mg-implanted Ga- and N-polarities samples. The observed results suggest that the implanted samples are to be annealed at 1200  C for 10 s to achieve a superior recovery of crystal quality. In addition, the XRD rocking curve FWHM of Mgimplanted N-polarity sample after annealing at 1200  C for 10 s is 0.075 . Such a value is much smaller than the 0.095 XRD rocking curve FWHM observed from the Mg-implanted Ga-polarity sample annealed at 1200  C for 10 s. This implies that Mg-implanted N-polarity sample after an appropriate annealing treatment has a better crystal quality than Mg-implanted Ga-polarity sample. Figure 6 shows the 10 K PL spectra of the Mg-implanted Ga- and N-polarities samples annealed at various conditions, respectively. It is found that a very weak Mg acceptor-bound exciton (I1 line) emission at 3.46 eV and a donor–acceptor-pair (DAP) transition at 3.27 eV followed by two longitudinal optical (LO) phonon replicas, as well as a notorious yellow band (YB) emission near 2.3 eV19) are observed for both samples annealing at 1100  C for 10 s, probably because the implantation-induced damages are not removed sufficiently. In contrast, the PL intensity of I1 line increases and YB

(b)

1100°C, 10s 1100°C, 20s 1200°C, 10s

16.9

17.0 17.1 ω (deg.)

17.2

Fig. 5. XRD rocking curves of the different annealing treatment for Mg implantation into (a) N- and (b) Ga-polarity GaN layers.

Intensity (arb. unit)

Fig. 4.

17.0 17.1 ω (deg.)

(a)

T=10K 1200 °C, 10s 1100 °C, 20s 1100 °C, 10s

2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 Photon energy (eV) (b)

1200 °C, 10s 1100 °C, 10s

1100 °C, 20s

Fig. 6. (Color online) 10 K PL spectra of the different annealing treatment for Mg implantation into (a) N- and (b) Ga-polarity GaN layers.

emission decreases as the annealing temperature up to 1200  C for both samples. Compared with Mg-implanted Ga-polarity sample, the much stronger PL intensity and the fact that no YB emissions is observed both suggest a superior recovery of crystal quality can be achieved by

071001-3

# 2010 The Japan Society of Applied Physics

Jpn. J. Appl. Phys. 49 (2010) 071001

K.-T. Liu et al.

200

Implanted Npolarity GaN

I 1/I YB

150 100

Implanted Gapolarity GaN

50 0 1100 °C,10s

1100 °C,20s

1200 °C,10s

Anneal conditions Fig. 7. The intensity ratio I1 =IYB as a function of annealing conditions for Mg implantation into Ga- and N-polarities GaN layers.

annealing at 1200  C, 10 s for the Mg-implanted N-polarity sample. To further quantify the effect of annealing on the samples, we set the intensity ratio of I1 line and YB emission, I1 =IYB as a measure of the relative repair condition of the implantation-induced damages. Figure 7 shows the variation of the intensity ratio I1 =IYB as a function of annealing conditions for both samples. As can be seen, the intensity ratio of both samples has no apparent change for annealing at 1100  C for 10 s, and 20 s. However, at annealing condition of 1200  C for 10 s, the intensity ratio of Mg-implanted N-polarity sample changes rapidly compare with Mg-implanted Ga-polarity one. This result clearly indicates that the implantation-induced damage can be repaired by RTA at 1200  C, 10 s for Mg-implanted Npolarity sample. These PL characteristics are in reasonable agreement with the observed results by XRD measurements. Hall measurement results exhibit the both Mg-implanted Ga- and N-polarities samples after annealing at 1100  C regardless of 10 s or 20 s, which demonstrate n-type conductivity. The implantation-induced damages may not be removed sufficiently at the annealing conditions. On the other hand, up to annealing at 1200  C for 10 s, the Mgimplanted N-polarity sample converts its conductivity to p-type but the Mg-implanted Ga-polarity sample still remains n-type conductivity. The experimental results show that crystal polarity can effectively affect p-type doping efficiency for Mg implantation into GaN layers. The hole concentration and mobility are 4:7  1016 cm3 and 3.47 cm2 V1 s1 , respectively, for the Mg-implanted N-polarity GaN layer annealed at 1200  C for 10 s. These phenomena can all be attributed to the as-grown N-polarity GaN layer have more Ga vacancies that enhances Mg acceptor substitution and eventually result in p-type conductivity through Mg implantation by a proper annealing treatment. Although we have successfully obtained p-type GaN by using N-polarity GaN through Mg implantation method, the hole concentration is still low. The activation efficiency of the Mg acceptor is only 0.094%. This low activation efficiency is probably caused by the compensation effect or an incomplete N-polarity GaN layer. 4.

Conclusions

Magnesium implantation characteristics in Ga- and Npolarities GaN layers for p-type doping have been system-

atically investigated. It is found that a polarity change occurs from Ga- to N-polarities by introducing Al intermediate layer during the growth of GaN in MOCVD. Raman measurement reveals that the as-grown N-polarity GaN layer promotes tensile stress, which is due to the Ga vacancy occurred. Both XRD and PL measurements exhibit that we can achieve a superior recovery of crystal quality by annealing the Mg-implanted GaN with N-polarity at 1200  C for 10 s. Hall measurement shows that p-type conductivity can be achieved for the Mg implantation into N-polarity GaN layer by an appropriate annealing treatment. The behavior can be attributed to that the more Ga vacancies in the N-polarity GaN layer can help to enhance Mg acceptor substitution, and eventually contribute to p-type conductivity. Although we have successfully obtained p-type GaN by using N-polarity GaN through Mg implantation method, the activation efficiency of Mg is still low; more research works still need to be done in order to improve p-type conductivity. Acknowledgement

The work was partially supported by the National Science Council under Contract number NSC98-2221-E-272-005.

1) S. Nakamura, M. Senoh, N. Iwasa, and S. Nagahama: Jpn. J. Appl. Phys. 34 (1995) L797. 2) S. Nakamura, M. Senoh, S. Nagahama, N. Iwasa, T. Yamada, T. Matsushita, Y. Sugimoto, and H. Kiyoku: Jpn. J. Appl. Phys. 36 (1997) L1059. 3) D. Huang, M. A. Reshchikov, P. Visconti, F. Yun, A. A. Baski, T. King, H. Morkoc, J. Jasinski, Z. Liliental-Weber, and C. W. Litton: J. Vac. Sci. Technol. B 20 (2002) 2256. 4) S. Sonoda, S. Simizu, Y. Suzuki, K. Balakrishna, J. Shirakashi, and H. Okumura: Jpn. J. Appl. Phys. 39 (2000) L73. 5) S. Sonoda, S. Simizu, X. Q. Shen, S. Hara, and H. Okumura: Jpn. J. Appl. Phys. 39 (2000) L202. 6) K. Xu, N. Yano, A. W. Jia, A. Yoshikawa, and K. Takahashi: J. Cryst. Growth 237 (2002) 1003. 7) X. Q. Shen, T. Ide, S. H. Cho, M. Shimizu, S. Hara, H. Okumura, S. Sonoda, and S. Shimizu: J. Cryst. Growth 218 (2000) 155. 8) T. Ito, K. Ohtsuka, K. Kuwahara, M. Sumiya, Y. Takano, and S. Fuke: J. Cryst. Growth 205 (1999) 20. 9) M. Sumiya, K. Yoshimura, T. Ito, K. Ohtsuka, S. Fuke, K. Mizuno, M. Yoshimoto, H. Koinuma, A. Ohtomo, and M. Kawasaki: J. Appl. Phys. 88 (2000) 1158. 10) A. Yoshikawa and K. Xu: Thin Solid Films 412 (2002) 38. 11) M. Sumiya, N. Ogusu, Y. Yotsuda, M. Itoh, S. Fuke, T. Nakamura, S. Mochizuki, T. Sano, S. Kamiyama, H. Amano, and I. Akasaki: J. Appl. Phys. 93 (2003) 1311. 12) S. Strite and H. Morkoc: J. Vac. Sci. Technol. B 10 (1992) 1237. 13) H. Obloh, K. H. Bachem, U. Kaufmann, M. Kunzer, M. Maier, A. Ramakrishnan, and P. Schlotter: J. Cryst. Growth 195 (1998) 270. 14) S. J. Pearton, C. B. Vartuli, J. C. Zolper, and C. Yuan: Appl. Phys. Lett. 67 (1995) 1435. 15) J. L. Weyher, S. Muller, I. Grzegory, and S. Porowski: J. Cryst. Growth 182 (1997) 17. 16) P. Perlin, C. Jauberthie-Carillon, J. P. Itit, A. S. Miguel, I. Grzegroy, and A. Polian: Phys. Rev. B 45 (1992) 83. 17) T. Kozawa, T. Kachi, H. Kano, H. Nagase, N. Koide, and K. Manabe: J. Appl. Phys. 77 (1995) 4389. 18) C. X. Peng, H. M. Weng, C. F. Zhu, B. J. Ye, X. Y. Zhou, R. D. Han, W. K. Fong, and C. Surya: Physica B 391 (2007) 6. 19) D. J. Dewsnip, A. V. Andrianov, I. Harrison, J. W. Orton, D. E. Lacklison, G. B. Ren, S. E. Hooper, T. S. Cheng, and C. T. Foxon: Semicond. Sci. Technol. 13 (1998) 500.

071001-4

# 2010 The Japan Society of Applied Physics