Photocurrent and optical absorption spectroscopic study of n-type phosphorus-doped CVD diamond

Diamond and Related Materials 8 (1999) 882–885

Photocurrent and optical absorption spectroscopic study of n-type phosphorus-doped CVD diamond M. Nesla´dek a,*, K. Meykens a, K Haenen a, L.M. Stals a, T. Teraji b, S. Koizumi b a Materials Physics Division, Institute for Materials Research, Limburgs Universitair Centrum, Universitaire Campus, Wetenschapspark 1, B-3590 Diepenbeek, Belgium b Research Center for the Advanced Materials, National Institute for Research in Inorganic Materials (NIRIM), 1-1 Namiki, Tsukuba 305, Japan Received 22 July 1998; accepted 15 September 1998

Abstract A low-temperature spectroscopic study of epitaxial phosphorus-doped n-type CVD diamond films deposited on a type-Ia diamond crystal, was been carried out using the constant photocurrent method (CPM ). Two new defect levels in the gap of CVD diamond were detected. A numerical fitting of the optical cross-section data positions the first level, here denoted X , at an P1 optical excitation energy of 0.55 eV. The second level, denoted X , is positioned at about 0.8 eV. The 0.55 eV energy obtained P2 from the fitting procedure is in good agreement with Hall measurements of the activation energy of the carrier concentration for low P-doped samples. SIMS measurement shows a concentration of about 1 part per million (ppm) of phosphorus in the film. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Defects; Doping; Phosphorus; Spectroscopy

1. Introduction n-Type conductivity in CVD diamond films remains one of the most challenging topics in the field of diamond research. Recently, n-type conductivity has been reported by the group at NIRIM for CVD diamond films doped with phosphorus [1]. Nevertheless, so far there is no spectroscopic information available about the phosphorus level in the gap of diamond. Theoretical calculations suggested an excitation energy of about 0.2 eV for a substitutional phosphorus defect [2]. The first Hall measurements of the carrier concentration activation energy for highly doped NIRIM samples pointed towards a donor level around 0.43 eV below the conduction band. A parallel study on n-type doping of CVD diamond [3] has shown an activation energy of about 0.1 eV for phosphorus-doped samples using a doping from an organometallic compound. Spectroscopic information about the defect level position is necessary for an exact diagnosis of the defect and for an understanding of the P-incorporation mechanism. In the present study we have used for the first time a gap * Corresponding author. Tel: +32 11 268826; fax: +32 11 268899; e-mail: [email protected]

spectroscopic tool, i.e. the constant photocurrent method (CPM ) [4,5], to investigate the samples prepared at NIRIM [1].

2. Experimental The sample investigated was a thin epitaxial CVD diamond layer (thickness about 3 mm) deposited on a top {111} facet of type-Ia natural diamond crystal (for details, see Ref. [1]). The crystal shape was triangular with two parallel optically polished {111} facets. The larger (back surface) facet was embedded during diamond deposition and thus left intact. This also allowed us to carry out comparative CPM measurements on the larger rear surface facet only (without intercepting the diamond film). The substrate CPM measurement has permitted us to subtract any changes in the photocoductivity of the Ia substrate crystal, such as those due to heating of the substrate at the deposition temperature. Constant photocurrent measurements (CPM ) were carried out at liquid-nitrogen (LNT ) and room temperature (RT ). In our quasi steady-state (7–13 Hz modulation) CPM set-up, we used a W–halogen lamp (or a Xe-arc lamp) focused on a 30 cm ARC monochromator

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equipped with three gratings [5]. The photocurrent was kept constant by regulating the stabilized W–halogen lamp power supply. The quasi steady-state photocurrent set-up is a modification of our photothermal deflection spectroscopy (PDS ) set-up [6 ], which was used here to measure the optical absorption. The photocurrent was analysed by an EG&G 5240 lock-in amplifier, with typically six decades of dynamic range detection. The spectral resolution of the system was typically 4 nm at 2000 nm. Graphite/Au coplanar interdigitated contacts with 50 mm spacing (30 fingers) were deposited by liftoff photolithography on top of the diamond film. The dark and photo I–V characteristics were linear up to an electric field of 4000 V cm−1. The shape of the photocurrent spectra was independent of the electric field. Samples were measured after an oxidation treatment [1]. By using CPM one fixes the occupation of the deep states, and thus the CPM measurement directly yields information about the spectral dependence of the photoionization cross-sections related to the various impurity levels [4,5]. One thus directly obtains for the optical absorption coefficient a [4,5]: a=N s#1/N (1) P ph where N is the concentration of defects associated with P the investigated level, s is the photoionization crosssection and N is the number of incident photons. ph P-doped samples with various P concentrations have previously been examined by Hall measurements, showing n-type conductivity and an activation energy of the carrier concentration of about 0.5 eV, depending on the doping level [7]. The activation energy of the dark conductivity for the CPM-measured sample was about 0.55–0.6 eV in the RT–600 K range. The SIMS measurement shows about 3×1017 cm−3 of phosphorus atoms incorporated in the CVD layer investigated.

3. Results and discussion In Fig. 1 the RT photocurrent (PC ) and optical absorption spectra of the diamond substrate only (see Section 2) and the PC of the CVD diamond film and the substrate are plotted. The optical absorption spectra show at RT typical unresolved vibronic absorption bands characteristic of type-Ia diamond. The onset of the band is at about 2.8 eV, with a small additional shoulder at 3 eV ( ZPL of the N3 line is 2.99 eV ). Additional absorption begins at about 3.7 eV [8]. The photocurrent spectra of the Ia crystal are similar to those reported in the literature [9], and consist of PC continuum bands (two maxima at 3 and 3.3 eV ) and an increase at about 4 eV [10]. It should be noted that while the PDS technique detects a continuum background absorption in the IR part of the spectra, the photocurrent falls off very sharply, and in the IR region

Fig. 1. Photocurrent spectra for a type-Ia natural diamond crystal compared with the spectra for a P-doped epitaxial layer deposited on the same crystal. The optical absorption spectrum is also shown (a), as is the photocurrent phase shift for the type-Ia crystal with an epitaxial diamond layer (b).

it is below the detection limit of the set-up (about 10 fA). Thus, in the part of the IR region investigated there is no absorption leading to an excitation of free carriers. The very low PDS-detected background absorption of the substrate (well below 0.1 cm−1) may have its origin in non-diamond carbon inclusions/defects, such as the continuum absorption described in Refs. [6,11]. The PC spectrum of the substrate with the CVD diamond film is quite different. At RT the PC spectrum exhibits two new shoulders which are not present in the type-Ia substrate. One shoulder (here denoted X ) has P1 an onset at about 0.5 eV. The other, which is dominant at RT (see below), shows an onset at about 0.8 eV (here denoted X ). The PDS absorption measured on both P2 facets (with and without the CVD film) was identical. The phase shift of the measured photocurrent spectra is also shown in Fig. 1. In general, when measuring the photocurrent, one also monitors changes in the mt product, which are reflected by a photocurrent phase shift [12]. These changes are related to the occupation of the gap states, and are induced by a movement of

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the quasi-Fermi levels. The phase shift in Fig. 1 for the type-Ia substrate with CVD diamond layer indicates changes in the recombination kinetics for each significant peak in the PC spectra. It can also be seen that the PC in the infrared (IR) region shows a distinct phase change. The phase changes substantially on going from 0.5 eV ( X shoulder in PC ) to about 1.0 eV ( X ). P1 P2 Thus, the phase shift analysis clearly indicates that the recombination traffic is quite different for the two levels. The LNT and RT CPM photoionization cross-section spectra are plotted in Fig. 2 on a relative scale. The measured LTN spectra are clearly sharper, and less broadened by acoustic phonons as compared to the RT measurement. This is clear supporting evidence that the X and X features correspond to the photoexcitation P1 P2 of free carriers, that and we observe a well-defined photoionization cross-section from a defect level. A numerical fitting of the optical cross-section s for the i LNT measurement is shown in Fig. 3. We use a convolution of Inkson’s formula from Ref. [14] with a Gaussian phonon broadening [5]: s(E)=A

P

{[E−(e−E )]2/2w2} +2 (E−E )3/2 i i exp− de E(E−B)2 (2pw2)1/2 −2 (2)

where A and B are constants, E, e, is the energy, E is i the photoionization threshold energy, and 2w is the full

Fig. 2. Room-temperature and liquid-nitrogen temperature CPM photoionization cross-section spectra for a P-doped epitaxial layer deposited on a {111} type-Ia crystal facet. Two levels, denoted X and P1 X , are indicated. The values j and j are discussed in the text. P2 XP1 XP2

Fig. 3. Theoretical fit of the LNT photoionization cross-section spectrum using the Inskon formula [14].

width at half maximum (FWHM ) of the Gaussian phonon-broadening term. We found good agreement with our experimental data. Our fit yields 0.55±0.02 eV for the X level and 0.81±0.02 eV for the X level P1 P2 [13]. The RT FWHM term is about 0.1 eV for X and P1 0.2 eV for X . Thus, the fit of the optical cross-section P2 is in very good agreement with the Hall and activation energies of the dark-conductivity measurements, giving about the same value for the X defect as the activation P1 energy of the carrier concentration [7]. This indicates that the X defect corresponds to a P-related donor P1 level, at about 0.55 eV from the conduction band. The SIMS results indicated that around 3×x1017 cm−3 P atoms were present in the CVD film. Thus our CPM results show the high sensitivity of our experimental set-up and the excellent quality of the CVD epitaxial diamond layer. The X level observed in the photocurrent spectra P2 deserves further study. From the presented CPM data it cannot be decided directly whether the X is a donor P2 or an acceptor level. In the case of a donor level, a hypothetical intercept of the dark conductivity with activation energies of 0.55 and 0.81 eV is above 2000 K, and thus the X level is difficult to see by Hall or darkP2 conductivity measurements. In the case of an acceptor level, X can reduce the doping action of the X level, P2 P1 acting as a compensating level. (marked to j It is interesting that ratio of j XP2 XP1 maximum values in the photoionization cross-sections related to X and X , respectively [13]; see Fig. 2) are P1 P2

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different for the LNT and RT measurements. If the X level is a donor level, one should consider a moveP2 ment of the electron quasi-Fermi level (E ) towards the fn conduction band upon a reduction in temperature to explain the observed change in photocurrent at LNT and RT [13] (the occupation of the X level will P1 increase). If X is an acceptor level, a change in the P2 recombination kinetics due to a change of both E and fn E be considered. Consequently, the recombination fp traffic for the X and X levels can be different P1 P2 (supported by the phase-shift changes in Fig. 1), inducing a change in the mt product. This will be investigated in a forthcoming study. Knowledge of the origin of the X level is very P2 important for a better understanding of the doping process. Just by speculation one could suggest its possible origin. Recently, [1] we have investigated a level associated with hydrogen in CVD diamond with a photoionization onset of about 1.2 eV. A low-energy shoulder with an onset at about 0.9 eV was also observed [5] in the as-grown CVD diamond films. The appearance of this shoulder was sensitive to surface treatments such as hydrogenation of the film. Another possibility for the origin of the X level is a new extrinsic defect, such as P2 a phosphorus (complex)-related defect. In any case, while the origin of the X level is convincing, a systemP1 atic study on undoped and differently doped samples is necessary for the identification of the X level. P2

4. Conclusions For the first time, a spectroscopic study of n-type CVD diamond films has been carried out using the constant photocurrent method (CPM ). The optical cross-section spectra clearly point to the existence of two new defect levels in the gap of CVD diamond. The liquid-nitrogen temperature measurement and a theoretical optical cross-section fitting of the measured data positions the first level, denoted X , at an optical P1 exitation of about 0.55 eV. The second level, denoted

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X , is positioned at 0.8 eV. The value of 0.55 eV for P2 X is in very good agreement with Hall measurements P1 of the carrier concentration for low-phosphorus doped samples. Thus, the X defect can be attributed to the P1 electrically active phosphorus-related level, about 0.55 eV away from the conduction band. More information is necessary for a clear identification of the X level. P2 Acknowledgements This work was supported by Belgian FWO Research Program, No. G.0014.96. The authors would like to thank Dr. M. Vanecek from the Institute of Physics of the Czech Academy of Sciences for fruitful discussions.

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