The phosphorous level fine structure in homoepitaxial and polycrystalline n-type CVD diamond

Diamond & Related Materials 13 (2004) 2041 – 2045 www.elsevier.com/locate/diamond

The phosphorous level fine structure in homoepitaxial and polycrystalline n-type CVD diamond K. Haenena,b,*, M. Nesla´deka,c, L. De Scheppera,c, R. Kravetsd, M. Vaneˇcˇekd, S. Koizumib a

Institute for Materials Research (IMO), Limburgs Universitair Centrum, Wetenschapspark 1, B-3590 Diepenbeek, Belgium b Advanced Materials Laboratory, National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba 305-0044, Japan c Division IMOMEC, IMEC vzw, Wetenschapspark 1, B-3590 Diepenbeek, Belgium d Institute of Physics, Academy of Sciences of the Czech Republic, Cukrovarnicka 10, CZ-16253 Prague 6, Czech Republic Available online 25 July 2004

Abstract The application of very sensitive photocurrent-based spectroscopic techniques have led to the detection of new levels for the electronic structure of the phosphorous donor in n-type CVD diamond. By combining quasi-steady-state photocurrent measurements (PC), photothermal ionisation spectroscopy (PTIS) and the highly sensitive Fourier transform photocurrent spectroscopy (FTPS) technique at different temperatures, ranging from liquid nitrogen temperature to 170 K, the resulting spectra point to a richer structure than assumed up to now. This is the consequence of the improved sample quality over the last years, opening up to a much larger attainable doping window. By using doping levels, ranging from 1019 cm 3 down to 1016 cm 3 on {111}-oriented Ib HPHT substrates, still giving rise to measurable n-type conductivity, spectra showed less line broadening and more fine structure. Finally, the results will be compared with spectra measured on active P-doped polycrystalline n-type films. D 2004 Elsevier B.V. All rights reserved. Keywords: n-type doping; Phosphorous; Opto-electronic characterisation; Electronic structure

1. Introduction Notwithstanding different reports on attaining n-type CVD diamond using sulphur [1], with or without intentional co-doping with boron [2], or a combination of boron-doping followed by a deuterium treatment [3], phosphorous-doping remains the only n-type dopant up to date that leads to reproducible results and is stable to high temperatures [4]. Not only from a fundamental point of view is n-type doping an intriguing issue, it is also of crucial importance if diamond is ever going to make the step from a research material with demonstrators in the lab to devices fabricated by electronic industry. Although the

* Corresponding author. Institute for Materials Research (IMO), Limburgs Universitair Centrum, Wetenschapspark 1, B-3590 Diepenbeek, Belgium. Tel.: +32 11 268875; fax: +32 11 268899. E-mail address: [email protected] (K. Haenen). 0925-9635/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2004.06.016

position of the donor level, determined at 0.60 eV below the conduction band [5], is considered to be too deep for the more common applications, it opens up perspectives to be used as an UV-LED [6], a solar blind UV detector or high-temperature devices that can be used in harsh environments. A prerequisite for devices is a good reproducibility of high quality n-type material. In-situ n-type doping with phosphorous using phosphine (PH3) is currently quite well established by a number of groups in Japan [7] and Europe [8,9] but from the spectroscopic point of view, however, the knowledge of this donor remains limited compared to its well-known p-type counterpart, the much studied boron acceptor. Part of the reason is the fact that the n-type films prepared to date had a rather high P concentration in the range of 1018–1019 cm 3 in order to compensate for various deep defects. To overcome this drawback, we have decreased the impurity level of our single crystalline homoepitaxial layers, allowing also to reduce the concen-

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tration of phosphorous and to grow samples with Pconcentrations as low as 10 16 cm 3 while still retaining the active n-type behaviour, as confirmed by Hall-measurements. In these samples we can obtain well-defined lines associated with different excited states of the P-atom. By using a combination of quasi-steady-state photocurrent measurements (PC), photothermal ionisation spectroscopy (PTIS) and the novel highly sensitive Fourier transform photocurrent spectroscopy (FTPS), we propose an enhanced model for the P-electronic structure, richer than previously assumed [10]. Finally, these data will be compared with PC spectra taken from the first active n-type polycrystalline P-doped CVD diamond films [9].

2. Experimental details The homoepitaxial diamond films were prepared at IMO and at NIMS using a commercial stainless steel chamber plasma-enhanced microwave deposition reactor from ASTeX. After a careful selection and chemical oxidation of the {111}-oriented 220.5 mm3 HPHT Ib Sumitomo substrates, an epitaxial P-doped layer was deposited using a mixture of methane, hydrogen and phosphine. Typical process parameters are a pressure of 100 Torr, a substrate temperature around 850 8C and a MW power of about 700 W. A low concentration methane to hydrogen mixture is used, usually 0.05–0.075%, while the phosphine to methane ratio was varied between 2 and 1000 ppm. A more elaborate review on the growth of Pdoped films can be found in Ref. [7]. The P-doped polycrystalline films were grown at IMO using the same conditions as for the homoepitaxial films [9]. As a substrate we made use of mechanically polished 300 Am thick undoped CVD diamond films, grown on Si that was chemically removed before the polishing took place. After the growth, the surface of the films was oxidized and electrical contacts were deposited. All spectroscopic measurements were done at low temperatures making use of an Oxford cryostat. A Keithley voltage source supplied the electric field connected to the diamond samples using conductive carbon cement. In the case of PC, the generated AC photocurrents are detected using lock-in amplifier techniques. FTPS is an innovative technique that combines the advantages of traditional FTIR, like short acquisition time, high resolution and a good signal/noise ratio, with the sensitivity for electrical active defects of normal PC measurements [11]. As PTIS is a combination of thermal and photonic excitations, these measurements are inherently a part of PC as well as FTPS spectra, so the same set-ups can be used. As FTPS is based on a normal FTIR spectrometer, bias illumination is always present with this technique, enhancing the detection limit. More information concerning the used experimental techniques is described elsewhere [12].

3. Results and discussion The generated photocurrent in P-doped diamond is usually very low. For PC this is typically a few nA when focusing the W-halogen lamp onto the sample through the monochromator in the 0th order grating position. This means that the photocurrent, when measured monochromatically, is smaller than the dark current making the use of an AC (F13 Hz) excitation beam a must so lock-in techniques can be used to detect the low generated currents. As already known, the excited states of P are only a few meV separated in energy, the stability of the current is a critical issue when searching for new levels. Therefore our first approach was to try to enhance the amount of generated photocurrent. For this purpose a 15 Am thick P-doped sample was prepared at IMO from a 1000 ppm [PH3]/[CH4] gas mixture. Fig. 1(a) shows the resulting PTIS spectra taken at three different temperatures on this sample. As we can expect from the principles of PTIS, more peaks become visible in the optimal temperature range. At higher energies (Fig. 1(b)) the P photoionisation cross-section is modulated by emission of one or more LO-phonons, giving rise to oscillatory photoconductivity (OPC). As discussed before [10], PTIS peaks and OPC minima can be correlated if they are separated by n times the energy of the LO phonon involved in the OPC, n being the number of phonons emitted. In the case of P-doped diamond films this LO

Fig. 1. (a) PTIS spectra at different temperatures measured on a 15 Am thick P-doped sample prepared at IMO from a 1000 ppm [PH3]/[CH4] mixture. (b) Oscillatory photoconductivity spectrum showing the minima related to the PTIS peaks in (a).

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phonon has an energy of 155 meV [5]. The lines in Fig. 1(b) are the result of adding this energy to the peaks in Fig. 1(a). As can be seen, they correspond exactly to the position of the minima, as expected. Alternative to the combination of PC and PTIS is the FTPS technique working in PTIS mode. The advantage of FTPS is that when using white light, the situation can be compared to the 0th order grating signal in the normal PC measurements described above. So, the total photocurrent is in the order of or even higher than the dark current making even very small concentrations of dopants detectable. Moreover, PTIS itself is already a very sensitive technique and can detect very low impurity concentrations with a high spectral resolution. Usually this yields a discrete line spectrum, as is the case for example for P in Si were a concentration of 1.21014 cm 3 gives rise to discrete lines with a FWHM of F0.1 meV [13]. Here, the PTIS peaks are rather broad because of the higher P concentration, which is about 1019 cm 3. It is known that higher concentrations of the dopant atom can broaden these lines by an overlap of the wavefunctions of the bound carrier [14]. Of course, this does not rule out the possibility that the line width is affected by the presence of other defects and/or impurities. These can broaden the line width through two mechanisms. Random electric fields associated with impurities can cause level broadening by the Stark effect. The same effect can also be caused by stress when a certain concentration of contamination in the film is reached [15]. To reduce this effect, films with a lower concentration of P were deposited at IMO and NIMS. The effect is obvious as can be seen in Fig. 2. On the other hand, a too low concentration of the dopant also reduces the total amount of generated current. For the 2 ppm sample, this causes the structure at lower energies to disappear in the noise level of our set-up. Depending on the final goal of the measurements, a suited doping strategy has to be chosen. Note also,

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Fig. 3. FTPS spectra taken on two different P-doped samples. (a) The high resolution spectra reveal some new oscillatory photoconductivity minima and the corresponding PTIS peaks. (b) PTIS peaks.

when comparing the OPC of these spectra with similar spectra for the B acceptor [16], only two to three sets of equidistant minima can be observed. In the case of boron, it is possible to detect much higher order minima as a result of the emission of a large number of phonons, Thus a further optimisation towards better film quality and lower doping concentrations is of utmost importance.

Fig. 2. FTPS spectra show the influence of doping concentration on PTIS peaks and oscillatory photoconductivity minima.

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Fig. 4. (a) A comparison between the previous results as published in Ref. [10] and the work of Gheeraert et al. [17], who used the effective mass approximation to calculate the position of the phosphorous excited states. (b) The electronic structure of P based on the current experimental data. (c) An expanded view of (b) shows a much richer structure compared to (a).

Fig. 3 displays two FTPS spectra taken on two medium doped samples. A closer inspection of the OPC minima and the PTIS maxima immediately show that the FTPS technique is extremely suitable for fine resolution studies of impurities. Contrary to PC, PTIS peaks become visible at much lower temperatures when using FTPS. The reason is the already mentioned bias illumination present in the system. This additional (white) light significantly enhances the transition probability at 77 K, where the thermal energy k BT is only about 7 meV, a rather small value to promote charge carriers from the lower excited levels into the bottom of the conduction band. While previous results (Fig. 4(a)) only gave rise to four levels, it was already predicted from the theoretical work of Gheeraert et al., based on the effective mass approximation,

that more levels could be present. Summarising all new results from a batch of different samples, the new model is given in Fig. 4(b) and (c). As a last point, we compared the previous results with the data taken from an active n-type polycrystalline film. The use of polished polycrystalline CVD diamond films as substrates can possibly open up a new route for electronic devices in solving the substrate size problem. Although Okano et al. [18] reported active n-type doping in films grown on silicon substrates, no P could be found in the diamond film. This created an ambiguous situation about the origin of the n-type conduction. Fig. 5 shows a comparison between two PC spectra. One was taken on a homoepitaxial film, while the other spectrum shows the photocurrent of a 300 ppm [PH3]/[CH4] doped polycrystal-

Fig. 5. Photocurrent spectra of a P-doped polycrystalline (poly) and single crystal {111} oriented sample (epi) measured at liquid nitrogen temperature. The inset shows the first spectrum on a linear scale, showing clear oscillatory photoconductivity.

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line film. The main photoionisation onset is situated at about 0.60 eV, which is consistent with the position of the P-donor in homoepitaxial n-type CVD diamond films [5]. Also, the well-defined 563 meV excited state of phosphorous is visible, while at higher energies there is unmistakable oscillatory photoconductivity present. From the inset it becomes apparent that although detectable, the structure is not so well resolved as for the homoepitaxial case. However, these results clearly show that phosphorous was successfully incorporated in the polycrystalline film.

4. Conclusions Using a combination of sensitive photoelectrical optical absorption techniques, the electronic structure of the Pdonor in homoepitaxial CVD diamond films was examined. By an optimisation of the doping concentration, new levels could be detected. A comparison with active polycrystalline n-type films confirmed the successful incorporation of the phosphorous donor.

Acknowledgements This work was financially supported by the F.W.O.Vlaanderen Research Programs No. G.0124.99 and No. G.0298.02, the IAP Phase V Network, the Institute for the Promotion of Innovation by Science and Technology in Flanders (IWT) through project No. 030219 bCVD DiamondQ, and the Bilateral Scientific and Technological Cooperation Flanders-Czech Republic BOF04B03. KH is a Postdoctoral Fellow of the Fund for Scientific ResearchFlanders (Belgium) (F.W.O.-Vlaanderen) and an AML/ NIMS Research Fellow.

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