High frequency photoconductivity of CVD diamond films1

Diamond and Related Materials 7 (1998) 1338–1341

High frequency photoconductivity of CVD diamond films1 F. Torrealba-Anzola a,*, A. Chambaudet a, J.-G. The´obald b, M. Jouffroy b, C. Jany c, F. Foulon c, Ph. Bergonzo c, A. Gicquel d, A. Tardieu d a Laboratoire de Microanalyses Nucle´aires, UFR des Sciences et Techniques, 16 route de Gray, 25030 Besanc¸on, France b Laboratoire d’Electronique et de Spectroscopie Hertzienne, UFR des Sciences et Techniques, 16 route de Gray, 25030 Besanc¸on, France c LETI (CEA-Technologies Avance´es) DEIN/SPE, CEA/Saclay, F-91191 Gif-sur-Yvette, France d Laboratoire d’Inge´nierie des Mate´riaux et des Hautes Pressions, CNRS, Universite´ Paris Nord, Avenue J.B. Cle´ment, 93430 Villetaneuse, France Received 4 August 1997; accepted 7 May 1998

Abstract Photoconductivity measurements on undoped diamond material can be strongly affected by the nature of the electrical contacts, due to the wide band gap and high resistivity. We have used a contactless technique based on high frequency electrical measurements in order to probe the photoconductivity s of CVD diamond films grown by a microwave assisted technique. HF-photo Resonant methods, at 9.192 GHz are carried out in a microwave cavity by a reflection spectrometer. Samples are irradiated by modulated UV light. The photoconductivity s is deduced from the variation of the quality factor and from the frequency HF-photo shift. The samples studied were deposited under various growth conditions (temperature, gas mixture, etc.). The measured high frequency photoconductivity values were compared with the physical and electrical properties measured from conventional techniques (Raman spectroscopy, current–voltage and charged particle induced conductivity characterisation). The potential of this contactless high frequency measurement technique for CVD diamond characterisation is discussed. © 1998 Elsevier Science S.A. Keywords: Photoconductivity; CVD diamond films; Electrical conductivity; Ohmic contacts

1. Introduction The interest in chemically vapour deposited diamond films stems from the unique set of electrical, chemical and physical properties of the material. As a semiconductor it is unrivalled for high performance applications and use in harsh environments. It is well suited for the fabrication of radiation detectors operating under the hostile environments encountered in the nuclear industry [1,2]. The development of such a detector requires further study and improvement of material properties such as the conductivity s and the carrier mobility m and lifetime t. Electrical characterisation of polycrystalline CVD diamond films is of great importance for electronic device applications. Many of the difficulties associated with diamond device processing are related * Corresponding author. Fax: 0033 81 666522; E-mail: [email protected] 1This paper was presented at the ‘‘Diamond 1997’’ conference. 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 20 0 - 3

to the particular surface behaviour of diamond. The surface often displays significant electrical conductivity despite the highly resistive nature of the bulk of the CVD material [3]. Care must be taken to prepare the diamond surface and to understand how it is affected by various processing steps (cleaning, oxidation, annealing) [4]. Also, critical to device fabrication is the contact formation [5]. Most devices rely on ohmic contacts for their operation. Conventional conductivity measurements assume the use of ohmic electrical contacts, a common difficulty on high resistive materials. In order to circumvent this problem, we report the first results of a new contactless method to probe the high-frequency (HF ) photoconductivity of diamond. The technique is performed at microwave frequency (9.192 GHz) in a resonant cavity. Other film properties, such as the mt product, Raman signature and dark conductivity have also been measured by conventional techniques for comparison with the high frequency photoconductivity results.

F. Torrealba-Anzola et al. / Diamond and Related Materials 7 (1998) 1338–1341

2. Microwave resonance technique The theory of microwave conductivity measurement has been reported previously [6 ]. The schematic experimental set-up used for the HF photoconductivity measurements is shown in Fig. 1. The sample is placed at an antinode of the electric field in the microwave cavity. A crystal detector associated with a reflection microwave spectrometer and a directional coupler are used to analyse the wave reflected by the cavity. An orifice in the cavity facilitates the irradiation of the sample using the focused UV light of a mercury lamp. The light is modulated using a sector wheel controlled by a stepper motor. The modulation allows for phase detection, thus increasing the sensitivity of the spectrometer. In the high-frequency range, the hyperfrequency conductivity s and the susceptibility x are related by: =jve x (1) HF 0 where v is the pulsation of the AC signal and e the 0 vacuum permitivity. Thus, the HF conductivity of a material can be determined by the measurement of its susceptibility. If a sample is set in a microwave cavity, the quality factor Q of the cavity changes according to:

s

D(1/Q)=gx◊

(2)

where g is the filling factor of the cavity and x◊ the imaginary part of the susceptibility. When the change in the width of the resonance peak, which results from the introduction of the sample in the experimental set-up, is small, the impedance of the low frequency equivalent circuit is given by: Z=R(1+2jQ y+jQ gx) (3) 0 0 where R is the equivalent resistance, Q the unloaded 0

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frequency determining the value of the cavity, and y is characteristic of the detuning of the cavity. Q and y are respectively given by: 0 Lv 0 Q = (4) 0 R where L is the equivalent inductance, and y=

v−v

0

v

(5)

where (v/2p) is the resonance frequency of the cavity and (v /2p) is the frequency of the klystron. 0 Due to the low dark conductivity of diamond (<10−10 S.m−1), the product gx◊ in Eq. (2) is too low to be measured directly on the signal given by the spectrometer. Moreover, by irradiating the CVD diamond samples with modulated UV light we can inject carriers into the conduction band and thus increase the output signal. When y=0, the resulting photoconductivity signal, obtained from the phase detector, is given by: P

signal

=

4aQ g(1−a)x◊ 0 P i (1+a)3

(6)

where a is the coupling factor, g is the filling factor and is the incident power of the klystron. Thus, the Q and a values can be deduced directly 0 from the output signal of the spectrometer (see Fig. 2), using the following relationship: Dn (1+a) Q = 0 n

(7)

where n is the microwave frequency. Finally, we determine the real part of the HF photoconductivity s from Eq. (1) and Eq. (6). HF-photo

Fig. 1. Schematic of the experimental set-up used for the high-frequency photoconductivity measurements.

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Fig. 2. Schematic of the output signal observed on the high-frequency spectrometer.

3. Sample description Polycrystalline diamond films were grown on (100) silicon substrates using microwave plasma enhanced chemical vapour deposition in a bell jar reactor. The physical properties of the films were investigated by Raman spectroscopy using an argon ion laser working at 514.5 nm. The full width at half maximum (FWHM ) of the diamond sp3 line at 1332 cm−1 were measured from the spectra and are reported in Table 1. Five samples have been studied according to their thickness and growth conditions. Samples 1, 2 and 3 have been deposited using CH /H gaseous precursors at a con4 2 stant microwave power density and differing growth parameters. In order to assess the technique on a lower electrical conductivity sample, sample 4 has been deposited with the addition of a small percentage of nitrogen into the gas mixture. The respective thickness and Raman characteristics of these films are reported in Table 1. Sample 5 is a commercial polycrystalline diamond supplied by DeBeers, which enables the comparison on thicker material. The DC electrical properties of the films were investigated by Current–Voltage (I–V ) and Alpha Charged Particle Induced Conductivity (CPIC ) measurements. The dark conductivity of the films were deduced from the linear part of the I–V characteristic. The mt product

(carrier mobility times lifetime) in each film was deduced from the CPIC measurements under ionising alpha particles emitted by a 241Am source (E =5.5 MeV ). alpha This excitation generates electron–hole pairs with a total charge q in the device, and induces a charge q in the 0 ind external circuit. According to Hecht’s theory the collection efficiency is given by the ratio: d ind = =mtE (8) q L 0 where d is the carrier drift length before trapping, L is the distance between electrodes, m and t are the carrier mobility and lifetime, respectively, and E is the applied electric field. The mean position of the Gaussian detection peak therefore leads to the mt product value as given in Table 1. In the case of sample 5, no Gaussian detection peak could be observed, and only its upper tail could be distinguished from the noise. This prevents the measurement of the exact value of the mt product and only an upper limit could be given. q

4. Results and discussion The most important criterion for radiation detection application is the mt product, as given by CPIC measure-

Table 1 Physical and electrical characteristics of the CVD diamond samples Sample designation

Thickness (mm)

Raman FWHM (cm−1)

Dark conductivity (×10−14 S.m−1)

s HF-photo (×10−6 S.m−1)

mt (×10−8 cm2/V±10%)

No. 1 No. 2 No. 3 No. 4 No. 5

19 16 21 45 680

2.2 3.2 3.2 7 N.A.

50 2000 2000 1.4 50

468±47 623±62 818±82 3.12±0.62 4.13±0.41

2.7 2.1 2.5 1.3 <3.5

F. Torrealba-Anzola et al. / Diamond and Related Materials 7 (1998) 1338–1341

ments. Table 1 shows the mt values obtained using a constant field value of 104 V/m. It can be seen that samples 1, 2, 3 exhibit a higher mt product, twice as high as that obtained on sample 4. The low value on sample 4 has been observed previously [7] and was attributed to nitrogen recombination centres. On the thicker sample 5, the measurement was unable to quantify the electronic quality of the material for radiation detection applications. Because of the thickness and the polycrystalline nature of this film, only lower electrical fields could be applied. A trend could be observed when comparing those values with that of the Raman measurements. As observed by Plano et al. [8], the higher the FWHM value of the diamond Raman peak, the lower the mt product. The dark conductivity measurements show significant discrepancies between the samples studied. Samples 2 and 3 exhibit much higher values than sample 1, even though the mt products are similar. Further, sample 4 has a dark conductivity value more than one order of magnitude below that of sample 1, while the mt product has only decreased by a factor of 2. This implies that the dark conductivity measurements alone are not sufficient to probe the electronic quality of the films. In fact, this technique is strongly affected by the surface preparation procedures and contact formation [9]. In particular, since the dark conductivity values are deduced from the linear part of the I( V ) measurements, it is clear that this behaviour does not imply the perfect ohmiticity of the contacts. Nevertheless, dark conductivity remains an important parameter to probe for some electronic applications requiring low noise levels. The contactless HF photoconductivity method generates a more precise distinction between the samples. Results for the s measurements are reported in HF-photo Table 1. The trends which had been observed on the mt products can be found on the s values. We found HF-photo up to 50% difference in the signal output between the sets of samples 1, 2, and 3 grown without the addition of nitrogen in the gas mixture. However, there are two orders of magnitude in s between this set of HF-photo samples and that of the nitrogenated sample 4. The low s values obtained on sample 4 correlate with the HF-photo presence of nitrogen impurities which affect the material electronic properties. This technique has also enabled a measurement of the electronic properties of sample 5 which appears to be of similar photoconductivity to

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that of sample 4. In this case, the sample thickness prevents determination of the mt product value by the conventional CPIC technique, whereas the contactless HF photoconductivity measurements were able to provide the information.

5. Conclusion For the first time, microwave photoconductivity measurements have been used to investigate the electrical properties of CVD diamond films. In order to assess the reliability of the technique, the study was conducted on a set of samples that exhibited differing physical characteristics and impurity levels. The results gave evidence that this original technique brings complementary insight into the electrical properties of diamond to that brought by the conventional dark conductivity and CPIC measurements. Thus, microwave photoconductivity measurements appear to have great potential for diamond or other high resistivity material characterisation.

Acknowledgement The authors wish to thank Mr Alain Gire who designed the modulation set-up.

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