Isochronal annealing study of low energy electron

JOURNAL OF APPLIED PHYSICS 105, 063711 共2009兲

Isochronal annealing study of low energy electron irradiated Al-doped p-type 6H silicon carbide with deep level transient spectroscopy J. M. Luo,1 Z. Q. Zhong,1,2,3 M. Gong,3 S. Fung,1 and C. C. Ling1,a兲 1

Department of Physics, The University of Hong Kong, Pokfulam Road, Hong Kong, People’s Republic of China 2 State Key Laboratory of Electronic Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu, 610054 People’s Republic of China 3 Department of Microelectronics, School of Physics Science and Technology, Sichuan University, Chengdu, 610065 People’s Republic of China

共Received 20 November 2008; accepted 22 January 2009; published online 20 March 2009兲 Al doped p-type 6H silicon carbide was irradiated by low energy electrons to create primary defects. Two deep levels at EV + 0.36 eV and EV + 0.81 eV were created by this irradiation. Isochronal annealing study was carried out on the electron irradiated sample to investigate the annealing out of the two primary defects and the creation of thermal annealing-induced secondary defects. Four more deep hole traps 共0.45, 0.56, 0.74, and 0.71 eV above the valence band兲 were formed during the whole annealing process up to a temperature of 1600 ° C. All the electron irradiated deep level defects were annealed out after the 1600 ° C annealing. © 2009 American Institute of Physics. 关DOI: 10.1063/1.3087757兴 I. INTRODUCTION

Silicon carbide is a wide band gap semiconductor which can be used to fabricate electronic devices working at high frequency, high power, high temperature, and high background radiation environment.1,2 Deep level defects in semiconductors are known to have a significant effect in determining the materials electrical property by compensating the free carriers and limiting the free carrier lifetime. Deep level transient spectroscopy 共DLTS兲共Ref. 3兲 is a very useful probe for studying deep level defects in semiconductors yielding information such as energy level, concentration, and capture cross section. The microstructures of the defects formed in the material depend on a number of factors, for example, the growth conditions, the annealing dynamics of the defects, the type and energy of particles for inducing the defects, and the angle and the sample temperature of the irradiation. Although DLTS will give the positions of the energy states, determining the defect configuration is not easy and the understanding of the deep level defect microstructures is still poor. A great deal of effort has been devoted to the understanding of the deep level defects in n-type SiC induced by electron irradiation and ion implantation. A number of deep electron traps have already been identified in these n-type SiC materials, for example, the electron traps locating at 0.23 eV 共ED1兲, 0.36/0.44 eV 共E1 / E2兲, 0.43–0.46 eV 共RD5兲, 0.50 eV 共Ei兲, and 0.62/0.68 eV 共Z1 / Z2兲 below the conduction band of the 6H-SiC material 共Refs. 4–15兲. For the case of hole traps, only a few investigations have been carried out.16–21 In a previous study, Gong and co-workers18 studied the deep level defects in the Al Schottky contact formed on the 1.7 MeV electron irradiated p-type 6H-SiC epimaterial. It a兲

Author to whom correspondence should be addressed. Electronic mail: [email protected].

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was observed that the deep hole traps at EV + 0.55 eV 共H1兲 and EV + 0.78 eV 共H2兲 were induced by the electron irradiation.18 An annealing study up to a temperature of 600 ° C showed that H2 was removed after the 300 ° C annealing and the intensity of H1 was significantly reduced after the 600 ° C annealing. In the present study using the similar p-type 6H-SiC epimaterial and Al Schottky structure, we have carried out a DLTS study on the p-type samples irradiated by electrons having a low energy of 0.4 MeV and the thermal evolution of the deep traps.

II. EXPERIMENTAL

The starting material used in the present study was 共0001兲 Al-doped p-type 6H-SiC epilayer grown on the p-type 6H-SiC substrate obtained from Cree Inc. The thickness and the doping concentration of the epilayer were 10 ␮m and 9.0⫻ 1015 cm−3, respectively. The doping concentration of the p-type substrate was 6.6⫻ 1018 cm3. A large area Ohmic contact was made on the backside of the p+ substrate by thermally evaporating a 50 nm Al film, followed by 950 ° C annealing in forming gas. The electron irradiations were then carried out with a Van de Graaf electron accelerator with a fluence equal to 1016 cm−2. The electrons were incident normally to the sample episurface. Isochronal annealing was carried out in an Ar atmosphere for a period of 30 min. After the annealing, Schottky contacts were deposited onto the surface of the epifilm by thermally evaporating a 50 nm thick Au circular disk having a diameter of 0.5 mm. Capacitance-voltage 共CV兲 and current-voltage 共IV兲 measurements of the fabricated samples were carried out by the HP4145A semiconductor parameter analyzer to ensure the contact quality for the DLTS measurements. The DLTS measurements were performed by the Solu DLTS system. The activation energies 共Ea兲, the concentrations 共CD兲, and the

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FIG. 1. 共Color online兲 DLTS spectra of Al-doped p-type 6H-SiC episample irradiated by 0.3 and 0.4 MeV electrons 共fluence= 1016 cm−2兲 and the control as-grown sample. The parameters for the DLTS measurements are shown at the left hand corner.

capture cross sections 共␴兲 of the identified deep level defects were calculated from the Arrhenius plot assuming a temperature independent capture cross section. III. RESULTS AND DISCUSSION

As the Al/ p-SiC Schottky junction was in reverse bias, the DLTS signals were originated from the hole traps in the p-type 6H-SiC epilayer. The DLTS spectra measured with the reverse bias voltage, rate window, filling pulse voltage, and filling period width of VR = −6 V, ⌬t = 6.82 ms, V p = 0 V, and t p = 1 ms are shown in Fig. 1. The spectrum of the as-grown p-type 6H-SiC episample had a weak peak 共labeled as D1兲, having an activation energy and capture cross section of Ea共D1兲 = 0.61 eV and ␴共D1兲 ⬃ 10−18 − 10−17 cm2, respectively. After the 0.3 MeV electron irradiation, two more peaks were formed at temperatures of ⬃220 K 共D2兲 and ⬃390 K 共D3兲 which had Ea共D2兲 = 0.36 eV, ␴共D2兲 ⬃ 10−15 cm2 and Ea共D3兲 = 0.81 eV, ␴共D1兲 ⬃ 10−14 cm2, respectively. Increasing the electron energy to 0.4 MeV did not introduce extra peaks in the DLTS spectrum. However, it brought to our attention that the defect introduction rates for the deep defects were electron energy dependent. This was best illustrated by the different relative intensities of the deep level peaks as found in the DLTS spectra of samples irradiated with different electron energies 共Fig. 1兲. The physical mechanism of the observation was still not well understood and required further investigation. An isochronal annealing study was carried out on the 0.4 MeV electron irradiated sample in order to understand the thermal evolution of the deep traps. Figure 2 shows the DLTS spectra of the sample having undergone the annealing process up to a temperature of 1600 ° C. These DLTS spectra were taken with VR = −6 V, ⌬t = 8.6 ms, V p = 0 V, and t p = 1 ms. The electron irradiation induced D2 共EV + 0.36 eV兲 and D3 共EV + 0.81 eV兲 can be easily seen in the as-irradiated sample spectrum. D3 was removed after the 350 ° C annealing. As the annealing temperature reached 500 ° C, the D2 peak 共EV + 0.36 eV兲 dropped and a peak D4 共EV + 0.45 meV, ␴ ⬃ 10−12 cm2兲 appeared in the neighboring

J. Appl. Phys. 105, 063711 共2009兲

FIG. 2. 共Color online兲 DLTS spectra of the 0.4 MeV electron irradiated Al-doped p-type 6H-SiC episample 共fluence= 1016 cm−2兲 annealed at different temperatures. The parameters for the DLTS measurements are also shown in the figure.

temperature region. The D4 defect was removed after the 900 ° C annealing. After the 700 ° C annealing, two more peaks, D5 共EV + 0.56 eV, ␴ ⬃ 10−12 cm2兲 and D6 共EV + 0.74 eV, ␴ ⬃ 10−13 cm2兲, were formed and their peak intensity significantly increased at the annealing temperature of 900 ° C. After the 1400 ° C annealing, D5 and D6 vanished and a peak D7 共EV + 0.71 eV, ␴ ⬃ 10−13 cm2兲 was formed. All the deep traps were annealed out at 1600 ° C. The Arrhenius plots of the electron irradiation induced traps were shown in Fig. 3. The properties of these traps are summarized in Table I. The densities of the electron irradiation induced traps as a function of the annealing temperature are plotted in Fig. 4. All the deep level defects D2–D7 were bulk defects as increasing the VR 共with 兩VR兩 ⱕ 6 V兲 while fixing the V P would increase their intensity. We have also carried out similar annealing study on the nonirradiated p-type 6HSiC samples. It was found that these deep level defects were not thermally generated in the nonirradiated samples and thus the thermal creation of D4–D7 was associated with the electron irradiation. There have been a few DLTS studies on p-type 6H-SiC and they were conducted on the alpha particle irradiated p-n junction ultraviolet photodiodes 共Rybicki16兲, the nitrogen im-

FIG. 3. 共Color online兲 Arrhenius plot of the electron irradiation induced deep traps.


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TABLE I. Capture cross section and annealing behavior of defects identified in 0.4 MeV electron-irradiated Al-doped p-type 6H-SiC samples.

Defect

Capture cross section 共␴兲

D1, EV + 0.61 eV

⬃5 ⫻ 10

D2, EV + 0.36 eV

⬃2 ⫻ 10−15 cm2

The dominant trap in as-irradiated samples. Disappeared after 500 ° C annealing.

D3, EV + 0.81 eV

⬃2 ⫻ 10−14 cm2

Found in as-irradiated samples. Removed after 350 ° C annealing.

D4, EV + 0.45 eV

⬃1 ⫻ 10−12 cm2

Formed after 500 ° C annealing in electronirradiated samples. Annealed at 900 ° C.

D5, EV + 0.56 eV

⬃4 ⫻ 10−12 cm2

Formed after 700 ° C annealing. Removed after 1400 ° C annealing.

D6, EV + 0.74 eV

⬃8 ⫻ 10−14 cm2

Formed after 700 ° C annealing. Removed after 1400 ° C annealing.

D7, EV + 0.71 eV

⬃10−13 cm

Formed after 1400 ° C annealing. Removed at 1600 ° C annealing.

−18

Remarks 2

cm

planted n+-p diode 共Raynaud et al.17兲, 1.7 MeV electron irradiated Schottky contact fabricated on Al-doped p-type 6HSiC material 共Gong et al.18兲, and Al-doped p-type 6H-SiC samples 共Alfieri and Kimoto21兲. It is worthwhile comparing the present results to the previous findings. D1 共EV + 0.61 eV兲 is a weak signal identified in the present asgrown sample. A similar hole state 共EV + 0.69 eV兲 was also reported by Rybicki16 in as-grown Al-doped p-type 6H-SiC and it was attributed to an Al-defect complex. It is thus plausible to attribute D1 found here to the same Al-defect complex reported by Rybicki.16 As shown in Fig. 2, D2 共EV + 0.36 eV兲 was induced by electron irradiation. At the annealing temperature of 500 ° C, it disappeared and a peak D4 共EV + 0.45 eV兲 appeared close to the temperature of ⬃220 K. D4 then annealed at 900 ° C. D2 has energy state close to that of MZ2 observed in Aldoped p-type as-grown 6H-SiC having an activation energy of 0.3–0.4 eV.21 However, their annealing behaviors were very different. D2 annealed at 500 ° C while MZ2 annealed

FIG. 4. 共Color online兲 Concentrations of the identified deep traps as a function of the annealing temperature.

Found in as-grown samples.

at 1800 ° C. D4 had Ea similar to the EV + 0.49 eV state identified in the N-implanted n+-p structure,17 although their annealing behaviors were not the same. D4 annealed at 900 ° C and EV + 0.49 eV persisted at 1200 ° C annealing. It has also been reported that the same deep trap would anneal at different temperature if it coexists with different defects. It is thus not certain if D2 and D4 are the same deep traps as identified by Alfieri and Kimoto21 and Raynaud et al.17 In the present study, D3 was created by electron irradiation with energy of ⱖ0.3 MeV and had an activation energy of 0.81 eV. This energy level is very similar to the H2 共EV + 0.78 eV兲 deep trap introduced by the 1.7 MeV electron irradiation.18 Moreover, D3 and H2 had similar annealing temperatures 共disappearing after annealing of 350 and 300 ° C, respectively兲 and capture cross sections 共⬃10−14 cm2兲. D3 and H2 are thus assigned to be the same deep hole trap. D5 共EV + 0.55 eV兲 and D6 共EV + 0.74 eV兲 are secondary defects formed after the 900 ° C annealing and they disappeared after the 1400 ° C annealing. D5 has an energy state very close to that of the boron related D center 共EV + 0.58 eV兲, which was usually found in boron-doped SiC.22 A similar trap was also identified in Al-doped p-type 6H-SiC 共Ref. 16兲 and its presence was attributed to be probably boron contamination. However in the present study, it is unlikely that the D5 is identical to the D center because the D5 density is too high 共⬃1015 cm−3兲 for an unintentionally boron doped sample. More work is thus needed to explore the true origins of D5 and D6. D7 共EV + 0.71 eV兲 is a secondary defect formed after the 1400 ° C annealing with a low concentration 共⬃1012 – 1013 cm−3兲. Its activation energy is close to that of MZ4 共0.64 eV兲 identified in nonirradiated Al-doped p-type SiC material.21 MZ4 is also a secondary defect formed


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by the 1200 ° C annealing with similar low concentration 共⬃1012 cm−3兲 to that of D7. Thus, D7 is probably the same defect as MZ4. IV. CONCLUSION

Low energy electron irradiation was carried out on Aldoped p-type 6H-SiC material. Two hole traps at 0.36 and 0.81 eV above the valence band were induced by the electron irradiation, which annealed out at 500 and 350 ° C, respectively. A deep trap at EV + 0.45 eV was induced by thermal annealing at 500 ° C, which disappeared upon annealing at 900 ° C. Two other deep hole traps at 0.56 and 0.74 eV above the valence band were formed at the annealing of 700 ° C and they both annealed out at 1400 ° C. Another hole trap EV + 0.71 eV was created at 1400 ° C annealing and then vanished at 1600 ° C. All the electron irradiation-induced primary and secondary defects were annealed out at the temperature of 1600 ° C. ACKNOWLEDGMENTS

This work was partially financially supported by the HKSAR RGC GRF 共Project No. 7033/05P兲. 1

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