Annealing-induced enhancement in the activation energy of heavily boron-doped polycrystalline diamond

Diamond and Related Materials 7 (1998) 1259–1262

Annealing-induced enhancement in the activation energy of heavily boron-doped polycrystalline diamond B.B. Li a, V. Baranauskas a,*, A. Peterlevitz a, D.C. Chang a, I. Doi a, V.J. Trava-Airoldi b, E.J. Corat b a Faculdade de Engenharia Ele´trica e Computac¸a˜o, Universidade Estadual de Campinas, Av. Albert Einstein N. 400, 13083-970 Campinas, SP, Brazil b Laborato´rio Associado de Sensores e Materiais, Instituto Nacional de Pesquisas Espaciais, 12201-970 Sa˜o Jose´ dos Campos, SP, C.P. 515, Brazil Received 22 July 1997; accepted 14 January 1998

Abstract We show that the electrical properties of heavily boron-doped polycrystalline chemical vapor-deposited diamond films are modified by post-deposition thermal annealing with slow heating and cooling rates. We found that the boron effective activation energy increased after heating and cooling cycles in the temperature range of 300–673 K. The improvement also depends on the doping level. Samples with a resistivity of 6.41 mV · cm, 3.84 mV · cm and 0.826 mV · cm showed improvements of 6.4, 11.5 and 205%, respectively. Scanning tunneling microscopy (STM ) images show that there were no exceptional changes in film morphology produced by the annealing processes, only an apparent cleaning of the surface at the nanometer scale. © 1998 Elsevier Science S.A. Keywords: Annealing-induced enhancement; Boron-doped polycrystalline diamond

1. Introduction Diamond films prepared by chemical vapor deposition (CVD) are an emerging material for electronic devices due to their advantageous characteristics over traditional electronic materials. Diamond single crystals offer a unique set of properties such as high thermal conductivity, high electron and hole mobilities, high radiation resistance, high chemical stability and wide electronic band-gap [1–3]. However, diamond CVD films are of a polycrystalline nature, and the study of the improvement of their electronic properties toward single crystal parameters is currently of prime interest. As a result, special attention has been paid to model and improve the doping procedures and related activation energies [4–10]. The preparation of in-situ doped diamond films by CVD involves extreme temperature gradients that induce deleterious mechanical stress from the heterosubstrate and from the almost disordered nucleation process. In this work, we show the influence of slow heating and slow cooling cycles in air on the improvement of the band-gap energies of boron-doped diamond * Corresponding author. Fax: +55 192 39 13 95; e-mail: [email protected] 0925-9635/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S 09 2 5 -9 6 3 5 ( 9 8 ) 0 0 15 7 - 5

films. Scanning tunneling microscopy (STM ) images showed a good correlation with changes in the surface morphology of the grains in the nanometer scale but no apparent changes in the morphology on the micrometer scale.

2. Experimental The diamond films were prepared by a hot-filament CVD system described elsewhere [11]. Substrates were made of silicon (111), n-type, with a resistivity above 1 V · cm. Deposition was carried out by passing a vapor mixture of ethyl alcohol (C H OH ) and acetone 2 5 (CH COOH ) diluted in hydrogen gas (99.5% vol.) at a 3 3 pressure of 120 Torr and a total flux of 200 sccm. A boron-doping source was made by dissolving solid B O in the ethyl alcohol/acetone liquid reservoir before 2 3 the start-up of the reactor. The boron concentration in the source was varied so that films with different doping levels could be grown. The CVD deposition temperature, measured by a thermocouple positioned on the reverse side of the substrate, was 973 K. Columnar polycrystalline films with a thickness of ~24.5 mm were obtained after 5 h. After termination of the growth run, the

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samples were cleaned by high-temperature hydrogen [12] (~1373 K, 15 min) and in addition, after cooling, were cleaned with deionized water, and acetone/ methanol mixture in an ultrasonic bath. The resistance values were measured as a function of temperature by using a thermal oven with controlled slow heating and cooling rates of +3.125 K min−1 and −1.56 K min−1, respectively.

3. Results and discussion The typical results of the two-contact probe resistance dependence on temperature for samples with different doping levels are presented in Fig. 1a–c. The absolute value of the resistivity was measured by the four-point probe method at room temperature. Each thermal cycle involved heating for 2 h, from room temperature to 673 K, and then cooling for the same period of time, i.e. a total heating/cooling time of 4 h. We can observe that all samples showed changes in annealing. For the sample with a higher degree of doping ( Fig. 1c), this variation is larger. However, all samples tend to stabilize after three or more annealing cycles (see Fig. 1a and b). In the mid-temperature range (500–673 K ), the predominance of extrinsic hole carriers generated by boron doping over the intrinsic thermal generated carriers is expected. The hole density (p) may be calculated using the following equation: p$

1 앀2

(N N )1/2e−Ea/2kT, A C

where k is the Boltzmann constant, E is the hole a activation energy, N is the concentration of acceptor A impurities, and N is the effective density of states [13], C respectively. Therefore, the resistivity can be expressed by the equation: 1

S

mq

(b)

2

eEa/2kT, N N A C where m is the hole mobility, and q is the electron unit charge. Let us suppose, as demonstrated for other heavily doped semiconductors [14–16 ], that also the mobility in highly doped diamond does not depend exponentially on the temperature. Thus, using the above equation, the values of E can be extracted from the a experimental data of resistivity vs. temperature. The dependence of E on annealing time can be seen in a Fig. 2. In all samples, there was an improvement as a result of the annealing. For the sample with a resistivity r=6.41 mV · cm, the effective activation energy increased from 266.3 to 283.4 meV, and for the sample with a resistivity r=3.84 mV · cm, the effective activation energy increased from 164.3 to 183.3 meV, i.e. an improvement of 6.4 and 11.5%, respectively. However r$

(a)

(c) Fig. 1. (a,b,c) Two-contact probe resistance dependence with temperature. Only the heating cycles are shown. (&) first cycle, (Ω) second cycle, (6) third cycle, (( ) fourth cycle. (a) r=6.41 mV · cm; (b) r= 3.84 mV · cm; (c) r=0.826 mV · cm.

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Fig. 2. Dependence of E on the total annealing time. Recipe (2 h slow a heating and 2 h slow cooling).

in the more highly doped sample (r=0.826 mV · cm), the activation energy increased from 81.9 to 168.5 meV, i.e. an increase of 205%. The increase in acceptor activation energy, E , may a be understood as a phenomenon that is not linked exclusively to the boron atoms moving to substitutional positions but is due to the improvement in the polycrystalline structure. The electrical conductivity in the polycrystalline diamond depends on several parameters, such as the stress in the polycrystalline structure and the presence of non-diamond films on the boundary surfaces. Therefore, the annealing improvement can occur as a result of a better thermal accommodation of the grains, minimization of the number of traps for carriers at the grain boundaries and corresponding energy states, and minimization of leakage currents in the surface of the grains. STM of the samples made at the micron level revealed sharp individual grains of octahedral (111) faces with lateral dimensions up to ~5 mm and with a maximum roughness of ~2.4 mm. Therefore, the aspect ratio of the large columnar grains, that is, the thickness divided by their lateral size, is around 10.2. No dramatic morphological changes were observed between the as-deposited samples and the annealed samples, only an apparent surface cleaning, as shown in Fig. 3a and b. Roughness histograms of the surface of the grains at nanometer scales revealed that the annealed grains are typically less rough, as seen in Fig. 4, for the sample with a resistivity of 0.826 mV · cm. The roughness of the surface of the grains apparently decreased from typical values of ~84 nm to ~67 nm, but this result is not conclusive because these statistical parameters are very close to the STM experimental error. Although cleaning with activated hydrogen inside the reactor is very effective in removing non-diamond carbon [12], low levels

Fig. 3. Typical scanning tunneling microscopic images of the (a) as-deposited diamond and (b) annealed diamond samples.

Fig. 4. Typical roughness histograms of the surface of the grains at the nanometer scale.

of this material may still be present on the diamond surface. However, the prolonged exposure to the reactive oxygen ions [17,18] of the atmosphere during the annealing process will promote the etching of non-diamond carbon and thus alleviate the problem of surface conductivity. Fig. 5 shows the typical Raman high-resolution spectra for as-deposited (and cleaned ) samples and annealed samples, respectively. The peak at 1335.5 cm−1, corresponding to the line characteristic of

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activation energy of boron in heavily doped diamond. However, the values reported are still far from the lightly doped single-crystal diamond, in which the average experimental value of boron activation energy, E , is 370 meV [24,25]. Besides the improvement in the a value of the activation energy, annealing yields more stable electric properties in the deposited films. Thus, annealing research efforts are expected to provide important information for future electronic applications of diamond.

Acknowledgement The authors gratefully acknowledge partial support from FAPESP, CNPq and UNICAMP and INPE for this research.

References

Fig. 5. Raman spectra of as-deposited (and cleaned) and after annealing diamond, for samples with resistivities r=6.41 mV · cm, r= 3.84 mV · cm and r=0.826 mV · cm, respectively.

C–C sp3 bondings, is present in both spectra, and it is comparatively less dispersive after the annealing. The measurements were made with a resolution of 0.4 cm−1. For the sample with r=6.41 mV · cm, the change observed in the phonon line due to annealing was +1.0 cm−1, and for the sample with r= 0.826 mV · cm, the change was −0.8 cm−1. These frequency shifts were not observed for the sample with r= 3.84 mV · cm. In principle, these frequency shifts may be due to slight broadening after annealing. The slight asymmetry of the peaks can be due to the initial influence of the Fano effect [19], but nothing can be concluded definitively because it also exists as an interference from the overlap of a broad peak around 1200 cm−1. The peak around 1200 cm−1 appears on boron-doped CVD diamond [20,21] and in natural type IIb diamond [22] and is assigned to defects and nitrogen in the B form [22]. The graphitic diamond band around 1500 cm−1 is absent from all spectra. Since the graphite Raman peak is 50 times more sensitive than the Raman diamond peak [23], we can conclude that the annealing procedures used do not graphitize the samples.

4. Conclusions In this study, it has been shown that annealing at the mid-temperature range is effective in improving the

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