Nuclear Instruments and Methods in Physics Research B 127/128 (19977)217-220
ELSENIER
Defects in ion implanted diamond films (ESR study) Yoshiyuki Show a, Tomio Izumi a, Masahiro Deguchi b, Makoto Kitabatake b, Takashi Hirao ‘, Yusuke Morid d, Akimitsu Hatta d, Toshimichi Ito d, Akio Hi&i
d
a Department of Electronics, Tokai University. 1117 Kitakaname, Hiratsuka, Kanagawa, Japan b Central Research Laboratories, Matsushita Electric Industrial Co. Ltd., 3-4 Hikaridai, Seika, Soraku. Kyoto. Japan ’ Matsushita Technoresearch Laboratories Inc., 3-1.5 Yagumo-Nakamachi, Noriguchi, Osaka, Japan ’ Department of Electrical Engineering, Osaka University. 2-1 Yamadaoka, Suitu, Osaka 565, Japan
Abstract The defect structures in ion implanted diamond films have been studied by the electron spin resonance (ESR) method. Two kinds of paramagnetic defect centers were observed in ion implanted layers. One was a carbon dangling bond that existed in crystalline diamond (g = 2.003, AHpp = 3 Oe), and the other was a carbon dangling bond in an amorphous region of the films ( g = 2.003, A Hw = 2.6-20 Oe). The defect density in ion implanted layers decreased drastically at annealing temperatures over 4OO”C,but it remained stable even after annealing at 1000°C.
1. Introduction Formation of p- and/or n-type diamond films on nondiamond substrates is important for large scale electronic applications instead of silicon devices. However, it is difficult to drive impurities into electrically active sites (nand p-type dopants) of diamond films because of the extremely low solubility of most elements, except for nitrogen and boron [l]. Ion implantation seems to be one effective doping technique for diamond films. Recently, Zvanut and Shang [2] reported that the presence of substitutional phosphorus atoms in P ion implanted diamond films was exhibited by ESR measurements, although electrical measurements did not reveal n-type conductivity. However, damaged regions such as graphite and amorphous carbon, are formed in the diamond surface region by ion implantation [3]. The graphitization and amorphorization of the diamond surface lead to dramatic changes in certain physical properties of the diamond including conductivity [4]. Therefore, it is important to understand the detailed structural mechanisms of implanted diamond layers and their annealing properties. The ESR method is an effective technique for characterization of defect structures produced by ion implantations in diamond films [5-71. In this paper, the defects produced by ion implantation and its annealing properties are discussed.
2. Experimental Diamond films were prepared on p-type silicon substrates after an ultrasonic treatment with diamond powders 0168-583X/97/$15.00
by the microwave plasma CVD method. The film deposition was performed at 2.45 GHz, using a 300 W microwave source, at a substrate temperature of 8OO”C, in diluted 10% CO with 110 seem gas flow and 30 Torr pressure. Diamond films of 2 pm thickness were deposited on Si substrates after 2 h. Ion implantations were carried out at room temperature with nitrogen ions at an energy of 100 keV with a dose of 1 X lOI to 1 X lOI ions/cm2. After the implantations, the samples were annealed for 10 min in a temperature range from room temperature to 1000°C in an argon atmosphere. The defect structures were investigated using the electron spin resonance (ESR) method. ESR measurements were performed at room temperature using an X-band spectrometer. The g-value, the line width (AH,,) and the signal intensity were determined using the signals of Mn2+ and DPPH as the calibration reference. Saturation behavior of the ESR signal was measured from microwave power of 0.1 to 200 mW [S].
3. Results and discussion
3.1. Defect structures
in the as-grown
and ion implanted
diamond films
Fig. la shows the ESR signal for the as-grown diamond films, which has a symmetrical and isotropic lineshape with g-value of 2.003 and AHpp = 3 Oe. The spin density in the film was = 1 X 10” spins/cm3. In general, the
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(a) ESR signal ^-_
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(b) Raman spectrum
10Oe
L 1200
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Raman Shift ( cm-’ ) Fig. I. Typical examples of ESR signal (a) and Raman spectrum (b) for as-grown diamond films.
ESR signals that were observed from natural diamond and high-temperature and high-pressure synthetic diamonds give a g-value of 2.0025-2.0030 and AHpp of 6.0-13 Oe [9,10]. Powder of colorless diamond has an ESR signal of g - 2.003 and AHpp = 5.5 Oe ill]. In addition, ESR signals from good crystalline diamond films give a narrow lineshape under 3.0 Oe [ 121.On the other hand, the Ran-ran spectrum from the as-grown films had a sharp line at 1333 cm-’ as shown in Fig. lb. Furthermore, the scanning electron microscopy (SEMI results for the as-grown films indicate that the films were composed of polycrystalline diamond film. Therefore, the ESR signal in Fig. la is originated from carbon dangling bonds with C atom neighbors in the diamond film, which are deposited on the Si substrate by CVD method. Fig. 2a shows the ESR signal for the diamond film, which was prepared by N’ ion implantation (1014 ions/cm2 at 100 keV) into the sample in Fig. la. The ESR signal was an isotropic line with a discernible shoulder, which had g = 2.003 and AHpp - 3 Oe. As shown in Figs. 2b and 2c, the ESR signals consist of two kinds of lines. That is, an ESR line from the diamond film, which is a non-damaged region in the inner part of the diamond films, and an ESR line from the amorphous carbon layer produced in the diamond surface region by the ion implantation. To investigate the properties of an implantation damaged layer, the ESR signal in Fig. la, which was observed from diamond films, was eliminated from the signal in Fig. 2a. Then the ESR signal of an isotropic Lorenzian line-
H
Fig. 2. The ESR signal for the diamond films prepared by N+ ion implantation with a dose of 1Ol4 ions/cm2 at 100 keV into the sample in Fig. 1. The observed ESR signal (a) can decompose into two kinds of ESR lines as shown in (b> and (c): (b) a carbon dangling bond in the diamond film, (c) the ESR line from amorphous carbon.
shape with 2.003 and AHpp = 20 Oe appeared as shown in Fig. 3a. The spin density was = 1.3 X 1019 spins/cm3. Amorphous carbon films usually give a broad ESR line
i :
A$,=20 Oe S.D.=l .3x10’%m3
Fig. 3. The ESR signals observed in diamond films by N+ ion implantation at 100 keV with doses of 10’4-10’6 ions/cm’ at room temperature.
Y. Showet al./Nucl. Instr. and Meth. in Phys.Res. B 127/128 (1997) 217-220
with g-value of 2.0027 and AHpP of = 16 Oe [13]. Therefore, the above results indicate that the diamond surface is converted to an amorphous carbon layer by ion implantation. When the ion dose was increased from lOI to lOI ions/cm*, the spin density in the implanted layer also increased from 1.3 X 1OL9to 1.2 X lo*’ spins/cm3, while the AHpp decreased from 20 to 2.6 Oe as shown in Figs. 3b and 3c. The abrupt decrease of AHPP is due to the exchange narrowing for a high spin density of 1.2 X lo*’ spins/cm3 Il31. Next, we employed the saturation method to determine the ESR relaxation times. Fig. 4 shows the peak-to-peak amplitude, YA, plotted as a function of the microwave power. It clearly shows the amplitude YL for the diamond film prepared by ion implantation at 100 keV with a dose of 1014 ions/cm2 becomes linearly proportional to the microwave power. No saturation in YA is obtained up to 200 mW. Similar saturation behavior for the sample of lOi ions/cm2 was observed for the samples with dose of 1015 and lOI ions/cm*. The amplitude Yh for as-grown diamond films, on the other hand, become proportional up to 10 mW of microwave power. When the resonant line is strongly saturated above 50 mW, the Y& decreases with increasing power in the manner shown in Fig. 4. Furthermore, the spin-lattice relaxation time (r,) for the as-grown diamond films and amorphous carbon films were a magnitude of 10e5 and under 10m6 s, respectively. Even if these ESR signals have the same g-value and AHpp, we can determine the origin of center by carrying out saturation measurement of ESR lines. That is, the ESR signal due to the carbon dangling bond in the diamond layer strongly saturates with increasing microwave power, while that due
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Annealing Temperature(T) Fig. 5. The isochronal annealing curves of spin density bnnealing times = 10 min). to the carbon dangling bond in the amorphous carbon layer becomes linearly proportional to microwave power. 3.2. Annealing behavior of ion implanted layer Fig. 5 shows the isochronal annealing curves of the spin density created in the diamond surface layer by N+ ion implantation. When these samples were annealed at temperature above 400°C, the spin densities decreased. By increasing annealing temperature, the spin density in the sample of a dose of 1014 ions/cm2 disappeared at lOOO”C, while those with doses of 10” and lOI ions/cm2 did not disappear even at 1000°C. It seems that the spin density of ion implanted layer with 1014 ions/cm* after annealing at 1CQO”Cis in order of 10” spins/cm3. This means that the spin density of the ion implanted layer after annealing at 1000°C is comparable to that of as-grown diamond films. Furthermore, the resonant lines of these samples did not saturate at a microwave power of up to 200 mW. The amorphous carbon layer containing many carbon dangling bonds produced by high dose implantation, remains stable even after annealing at lOOO”C, although the a-center produced in silicon by implantation generally disappeared at temperatures above 600°C [ 141.
4. Summary 1;
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,,,,,
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,
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Fig. 4. Peak-to-Peak amplitude Yh plotted as a function of the microwave power.
We have studied defect structures introduced in diamond surface regions which were implanted with a dose of lOI to lOI N+ ions/cm2 at 100 keV, using the ESR method. The ESR analysis revealed the presence of two kinds of paramagnetic defects from as-grown and implanted diamond films. One is a paramagnetic defect cenI. FUNDAMENTALS/BASICS
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ter (g = 2.003, AHpp= 3 Oe),which arises from the carbon dangling bond in diamond films. This ESR center exists in as-grown diamond films. Moreover, the ESR center saturates at low microwave powers above 50 mW. The other is a paramagnetic defect center (g = 2.003, AHpp= 2.6- 20 Oe) with a Lorenzian lineshape, which arises from the amorphous carbon region produced by ion implantation. This ESR center did not saturate up to 200 mW. The amorphous carbon region, produced by implantation with a dose of 1014 ions/cm*, was removed after annealing at 1000°C. However, in the case of N’ ion implantation with a dose of lOI and 10’6/cm2, the ESR center remained stable even after annealing at 1OOO’C.
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