A quantitative study of the boron acceptor in diamond by Fourier-transform photocurrent spectroscopy

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Abstract

Fourier-transform photocurrent spectroscopy (FTPS) was used as a sensitive spectroscopic method to study boron doping in diamond layers, deposited by microwave plasma enhanced chemical vapor deposition (CVD). Results were compared to boron doped single crystals, synthesized at high pressure and high temperature. Introduction of boron increases the photosensitivity of diamond; the measured spectrum of excited states of the boron acceptor depends both on the crystal perfection and the boron concentration. The spectrum of excited states is a clear fingerprint of the presence of substitutional boron (up to 31 absorption lines have been identified). A quantitative assessment of the substitutional boron concentration is possible and the sensitivity of FTPS in combination with photothermal ionization spectroscopy (PTIS) for boron detection in diamond is estimated to be better then 1 part per billion (ppb).

Introduction

Semiconductor quality diamond is now being developed for electronic applications [1], [2]. To accomplish this task, technology of chemical vapor deposition (CVD) of diamond now needs a technique for the sensitive detection of electronic defects and electrically active dopants. We have recently developed Fourier transform photocurrent spectroscopy (FTPS) [3] as a very sensitive spectroscopic study of defects and dopants in intrinsic polycrystalline heteroepitaxial CVD diamond wafers and in homoepitaxial doped single crystal thin layers [4], [5].

Boron is an acceptor in diamond with thermal ionization energy of about 373 meV. It is sometimes found in natural diamonds (blue diamonds) and as a residual impurity in CVD diamond [4]. The challenge is to detect even a very low boron concentration in thin diamond films and to make a quantitative assessment of active dopant boron concentration in CVD homoepitaxial layers or polycrystalline heteroepitaxial wafers.

Photothermal ionization spectroscopy (PTIS) [6] proved to be an extremely sensitive method for detection of shallow dopants in silicon. Here we will describe our results of FTPS (which combines ac photoconductivity measurement under bias light with the principles of PTIS) on samples with a wide range of boron concentration and crystal quality. We present the conditions under which the sensitivity of this method can reach the part per billion (ppb) range for boron doping and demonstrate how to use the photocurrent measurements for a quantitative determination of the boron acceptor concentration.

Section snippets

Experimental

Boron doped diamond layers, grown in a microwave plasma-enhanced (MWPE) CVD system and high-pressure high-temperature (HPHT) synthetized single crystals were investigated; boron doping was of the order 1016–1018 cm−3, as determined by SIMS and IR absorption. The surface of the samples was either oxidized or samples that originally had an hydrogen-terminated surface were annealed in air to reduce surface conductivity [4]. Samples are listed in Table 1.

Coplanar electrodes 1–3 mm long with typical

Results

Boron doped samples at 77–160 K are typically very photosensitive (meaning that they have a low dark current and very high photocurrent) and give a good interferogram—the necessary condition for FTIR measurement. When approaching room temperature, the dark current rises and also the current noise level. Typically, we measure the boron doped samples with an oxidized or partly oxidized (by annealing in air, as described in Ref. [4]) surface. For the latter, the Dx defect [7] dominates the

Discussion

Fourier transform photocurrent spectroscopy (FTPS) has three advantages of a traditional FTIR spectroscopy (transmittance/reflectance measurement): short acquisition time, high resolution and high light throughput giving rise to a good signal/noise ratio [13]. Simultaneously, it can have the additional advantage of the high sensitivity of photocurrent spectroscopy, which can surpass by many orders of magnitude the conventional transmittance/reflectance spectroscopy for the case of

Conclusions

We have demonstrated that Fourier-transform photocurrent spectroscopy (FTPS) is a sensitive spectroscopic method to detect boron electronic states in the bandgap of diamond, capable of detecting boron concentrations well below 1 ppb. Because the boron excited states are a clear fingerprint of the presence of substitutional boron, we can claim that FTPS surpasses other techniques for the detection of electrically active boron. Up to 31 spectral lines have been observed in good quality single

Acknowledgements

We would like to thank for CVD and HPHT diamond samples, kindly supplied by Dr. M. Nesladek, Dr. S. Koizumi, Dr. J.E. Butler and De Beers Industrial Diamonds (UK). This work was supported by the EU RTN, Doped Diamond Devices and Sensors (DoDDS) project, contract HPRN-CT-1999-00139 and by the Grant Agency of the Czech Republic, contract no. 202/02/0218.

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