Energy Band Diagram of a H-Terminated P-Doped n-type Diamond (111) Surface

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1 New Diamond and Frontier Carbon Technology Vol. 17, No MYU Tokyo NDFCT 543 Energy Band Diagram of a H-Terminated P-Doped n-type Diamond (111) Surface Shozo Kono 1,2,*, Kenji Mizuochi 1, Go Takyo 1, Tadahiko Goto 1, Tadashi Abukawa 1 and Tomohiro Aoyama 1,** 1 IMRAM, Tohoku University, Sendai , Japan 2 CREST, JST, Dept. Engineering, Tohoku University, Sendai , Japan (Received 8 May 2007; accepted 11 September 2007) The energy band diagram of a hydrogen-terminated phosphorous-doped n-type diamond (111) surface has been studied by X-ray photoelectron spectroscopy and He-I excited secondary electron spectroscopy (SES). The resulting surface energy band diagram showed an upward band bending of ~3.2 ev toward the surface with the Fermi level position being 1.8 ev above the valence band maximum. The cutoff energy in SES spectra turned out to be the conduction band minimum within an error of 0.1 ev; mechanism of the upward band bending for n-type diamonds is discussed in terms of the Fermi level pinning caused by surface defects such as graphite. 1. Introduction It is well known that the surfaces of chemical vapor deposition (CVD) diamonds are are generally negative. Surface energy bands of CVD diamonds depend on the Fermi level (E F ) positions both at the surface and in the bulk. With the advancement of doping technology in diamond CVD, n-type as well as p-type CVD diamonds are now available. The E F positions in the bulk are determined by the type and quantity of the dopants. However, it is not clear how the surface Fermi levels are determined by the type and quantity of bulk dopants as well as by the nature of surface termination. Since boron-doped p-type CVD diamonds are rather conductive, the surface energy bands, studied. (1,2) The ionization energy of acceptor B is 0.36 ev; thus, E F is located around 0.3 ev above the valence band maximum (VBM) in the bulk at room temperature. The surface E F position of H-terminated B-doped diamonds is reported to be 0.5~1.3 ev above VBM; (3,4) thus, there are slight downward energy band bendings toward the surface * Corresponding author: kono@pop.tagen.tohoku.ac.jp ** Present address: Steel Research Laboratory, JFE Steel Corporation, Kawasaki , Japan 231

2 232 New Diamond and Frontier Carbon Technology, Vol. 17, No. 5 (2007) generally negative with a typical value of around 1 ev. (2) The presence of a surface conductive layer on H-terminated CVD diamond is an intriguing issue but is beyond the scope of the present study. (5) In contrast to the p-type diamonds, the surface electronic properties of n-type diamonds are not extensively studied. A nitrogen atom is an n-type dopant in diamonds and can be found in natural diamonds. The ionization energy of donor N in diamond is very large (~1.7 ev); thus, the conductivities of N-doped diamonds are very low. (6) Syntheses of single-crystal phosphorous-doped n-type CVD diamonds have been reported since (7,8) So far, the highest-quality n-type P-doped single-crystal diamonds have been realized by CVD on diamond (111) substrates. (9,10) The ionization energy of donor P in diamonds is known to be ~0.6 ev; (9) thus, the E F of P-doped diamond in the bulk can be estimated to be around ~0.6 ev below the conduction a H-terminated P-doped n-type diamond (111) surface has recently been reported to be negative ( 1.1 ev) by total photoyield spectroscopy (TPYS). (11) The E F position at the surface, i.e., the surface energy band diagram of the P-doped n-type H-terminated diamond (111) was not determined by TPYS although upward band bending toward the surface was predicted. (11) The E F positions and thus the surface energy band diagrams of H-terminated and O-terminated highly P-doped diamond (111) have recently been studied by electron spectroscopy, (4) which showed a large amount (~3 ev) of upward has been reported to be slightly positive for the H-terminated surface and almost zero for the O-terminated surface. (4) The purpose of this study is therefore to determine the surface energy band diagram (i.e., the E F position at the surface) of a P-doped H-terminated diamond (111) with good n-type characteristics. We used X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS) and secondary electron spectroscopy (SES) for this purpose. 2. Experimental Procedure The sample used was grown at AIST, Japan on an HTHP Ib type single crystal ( mm 3 ) (111) substrate by microwave plasma CVD with source gases of CH 4 /H 2 (0.05%) and PH 3 /CH 4 (450 ppm) at a total pressure of 75 Torr at 400 sccm. (12) The growth was carried out for 6 h at a substrate temperature of 900 C. Hall effect measurements conducted at AIST showed an n-type conductivity of the sample for a measured temperature range K with a typical mobility value of ~100 cm 2 /Vs. (12) TPY spectra measured at AIST showed similar spectra as reported previously for a P-doped n-type diamond (111) (11) with an onset of TPY at 4.6 ev. (12) Electron spectroscopy measurements were performed using an electron spectrometer (VG CLAM4) equipped with a 9-channeltron electron detector, a He I/II resonance UV source, a Mg K X-ray source and LEED optics. To probe a limited sample area in XPS, a collimator was placed at the front end of the X-ray source so that an elliptical

3 S. Kono et al. 233 Fig. 1. (a) Gray-scale photograph of the sample. The periphery of the sample is indicated with dashed white lines, which is a 2 2 mm 2 square in real size. (b) Schematic illustration of the sample. The peripheral paste and central patch paste of Aquadag are shown schematically in black. The two pastes are not in direct electric contact with each other. Large and small dashed line ellipses indicate areas of X-ray and UV light irradiation, respectively. (c) LEED pattern of the sample at an electron energy of 230 ev. area of ~1.0 (width) 0.8 (height) mm 2 was irradiated by the Mg K line (c.f. Fig. 1). A collimator was also placed at the front end of the He I/II resonance source for UPS/ SES and an elliptical area of ~0.5 (width) 0.7 (height) mm 2 was irradiated by He I/II resonance lines (c.f. Fig. 1). In an electron spectroscopy measurement of a sample with a low conductivity, charging-up of the sample by photoelectron emission occurs, which prevents precise determination of electronic structures. Indeed, the n-type diamond sample was chargedup if the sample was irradiated by X-ray or UV lines in a usual way as was performed for highly P-doped samples. (4) The difference in conductivity between the present n-type and highly P-doped samples is of the order of 3. (4) To circumvent the charging-up to be unsuccessful. Heating-up of the sample as was performed by Diederich et al. (1) for low-conductivity diamond samples was not applied in this study. Instead, we pasted a colloidal graphite patch on the sample surface so that charged-up potential (and possible built-up potential) can be monitored by measuring a C 1s XPS peak and a so-called E A SES peak (4) of the graphite patch. A photograph of the sample and its schematic illustration are shown in Figs. 1(a) and 1(b), respectively. In the gray-scale photograph of the sample of Fig. 1(a), the boundary of the sample is indicated with dashed white lines, which is a 2 2 mm 2 square in real size. The periphery was covered with a colloidal graphite (Aquadag) paste in order to form an electric contact between the sample surface and a Ta sample holder. The charged-up/built-up potential indicator Aquadag patch was pasted on the bottom center of the sample surface opening. Scratches appearing on the opening of the sample have nothing to do with the sample surface but are scratches on the front surface of the Ta sample holder. The Aquadag peripheral paste and center patch paste are shown schematically in

4 234 New Diamond and Frontier Carbon Technology, Vol. 17, No. 5 (2007) black in Fig. 1(b): the pastes are not in direct electric contact with each other. Large and small dashed-line ellipses indicate areas of X-ray and UV light irradiation, respectively, and the x and y axes are the coordinates of the center irradiation areas as explained from another diamond sample. Since the present n-type diamond sample did not show behavior of C 1s XPS peaks as a function of sample position corresponding to the x and y coordinates. The size and position of UV irradiation were judged from the trace of visible light on the sample surface. In Fig. 1(c), a low-energy electron diffraction (LEED) pattern of the sample at an incident electron energy of 230 ev is shown. The LEED pattern indicates that the surface is ordered into the (111)1 1 periodicity but rather large areas of the surface are disordered as indicated by the high background intensity of LEED. In addition to the present n-type sample, a B-doped p-type H-terminated diamond (001) sample was used as a reference of the surface energy band. This reference sample was the same sample used as a reference in ref Experimental Results Figure 2 shows XPS peaks of the C 1s core level from the n-type sample along the surface normal emission direction as a function of the y-coordinate (at x=0 mm) of Fig. 1(b) together with the C 1s XPS peak (thin solid line) of a graphite (Aquadag) pasted on a different place of the Ta sample holder. Two peaks corresponding to diamond C 1s and graphite C 1s core levels can be recognized in Fig. 2. It can be seen in Fig. 2 that the Fig. 2. XPS spectra for C 1s core level from the n-type sample as a function of y-coordinate (at x=0 mm) of Fig. 1(b) together with C 1s XPS spectrum (thin solid line) of an Aquadag paste on a different place of the Ta sample holder.

5 S. Kono et al. 235 energy position of the diamond C 1s peak changes as a function of y-coordinate while the energy position of the graphite C 1s peak does not change. The change in the diamond C 1s peak position indicates the existence of the charge-up effect. At the X-ray position of x=y=0 mm, the C 1s peak of graphite is from the Aquadag patch on the bottom center, the energy position of which is equivalent to that of graphite pasted on the Ta sample holder. This proves two things: one is that there is effectively no charge-up of the graphite patch and the other is that the electric contact between graphite and the diamond surface is not a Schottky type, which causes a built-up potential, but an ohmic type. To reinforce the ohmic conductivity of graphite and the n-type diamond surface, we performed an I-V measurement similar to the one of Fig. 2 in ref. 4. For applied DC voltages up to 50 mv, a linear current rise up to ~2 na was obtained for both polarities, justifying the ohmic contact between graphite and the n-type diamond surface. Therefore, the C 1s peak corresponding to the diamond sample at x=y=0 mm represents the C 1s peak without charge-up except for a possible small amount of variation in charge-up potential around the graphite patch. To assess the possible charge-up potential around the graphite patch, on the diamond surface around the graphite patch. In the analysis, it was assumed that the diamond surface had a uniform sheet resistance and that the sheet resistance for the graphite was 1000 times larger than that of diamond. The analysis showed that the maximum charge-up potential on the diamond was 0.1 ev when the charge-up of the graphite patch was 0.05 ev. This shows that the effect of possible charge-up is indeed negligible within an accuracy of 0.1 ev. The C 1s XPS component of the diamond sample can be deduced by subtracting the C 1s component of graphite, the result of which is shown in Fig. 3. Fig. 3. C 1s XPS component of the n-type sample (square dotted) deduced from Fig. 2 and C 1s XPS peak (dashed) of the reference H-terminated p-type diamond sample.

6 236 New Diamond and Frontier Carbon Technology, Vol. 17, No. 5 (2007) The square dotted curve in Fig. 3 shows the C 1s component of the n-type diamond sample deduced from Fig. 2 as described above. The dashed curve shows the C 1s peak of the H-terminated p-type diamond sample used as a reference of C 1s peak position as explained previously. (4) Figure 3 shows that the present n-type P-doped diamond gives a C 1s peak position shifted by 1.6 ev from that of the reference sample and that the fullwidth at half maximum (FWHM) of the C 1s peak of n-type diamond is 1.4 ev, which is 0.3 ev wider than that of the reference sample. Figure 4 shows SES spectra as excited by a He I resonance line as a function of x, y-coordinates on the sample surface as indicated. The energy is referenced to the E F of graphite pasted on the sample holder as determined from E A peak position. The SES spectra may contain a contribution from the graphite patch or the peripheral graphite, which is not large and has no effect on the determination of low cutoff energy (E cutoff ). E cutoff of the four SES spectra are all the same at 3.65±0.05 ev. This means that the charged-up potential produced by the He I resonance line is negligible probably because of the weakness of the He I line passing through the small (0.4 mm inner diameter) collimator placed at the front end of the He resonance source. E cutoff =3.65±0.05 ev gives the work function value of this n-type diamond (111). It is not certain in Fig. 4 alone whether E cutoff corresponds to CBM or unoccupied surface states (USSs) of the H-terminated diamond (111) surface. For this purpose, the He II excited valence band edge can be used for a (001) surface Fig. 4. SES spectra as excited by a He I resonance line as a function of x, y-coordinates on the sample surface as indicated. The cutoff energy of the four SES spectra is marked by a vertical line.

7 S. Kono et al. 237 that used for the highly P-doped diamond previously. (4) the surface energy band diagram for an n-type P-doped diamond as a function of donor E F position, and then, to evaluate the C 1s XPS peak and to compare it with that of reference p-type diamond. In the following section, we describe the method and the results of simulations. 4. Simulation of Surface Energy Band Diagram and C 1s XPS Peak Using critical values in XPS and SES as found in experiments in 3, we have simulated surface energy band diagrams for the P-doped n-type sample. The simulation was performed using a one-dimensional band diagram calculator. (13) The acceptor ionization energy for B was set to 0.36 ev and the donor ionization energy for P was set to 0.6 ev. The simulated surface energy band diagrams were used to further simulate the C 1s XPS peaks of the sample. In the simulation of the C 1s XPS peak, we need two important parameters, namely, electron mean free path and intrinsic C 1s peak line shape. The electron mean free path in diamond is rather problematic in electron spectroscopy. (14) We used 2.8 nm as a reasonable choice in accordance with a value used in a similar analysis. (1) As a reasonable choice for C 1s peak line shape, we used a Voigt function, i.e., a Lorentzian function convoluted with a Gaussian function. A HWHM of 0.3 ev for the Lorentzian was used to be consistent with the lifetime broadening of the C 1s peak, (15) and a FWHM of 0.9 ev for the Gaussian was used to be consistent with the energy resolution of Mg K excited XPS peaks. The curves in the lower part of Fig. 5(a) show simulated valence band maximum (E V ) positions for a P-doped n-type diamond for three donor concentrations of 1, 2 and cm 3. The donor concentrations were chosen so as to have a simulated C 1s XPS peak FWHM of ~1.4 ev. The E v E F, whose precise position should be readjusted from the simulated C 1s XPS peak position. Simulated E v positions (4) for the reference sample with and without a surface conductive layer are also replotted in Fig. 5(a). The curves in Fig. 5(b) show simulated C 1s XPS peaks for the n-type diamond samples with the three donor concentrations and for the reference sample with and without a surface conductive layer. The abscissa relative electron kinetic energy is shown in such a way that the origin corresponds to the case when E v coincides with E F. It is seen in Fig. 5(b) that the C 1s XPS peak for the reference sample is located at 0.4 ev with a variation of ±0.15 ev. It is to be noted that the asymmetrical shape of the experimental C 1s XPS peak for the reference sample in Fig. 3 is due to a surface hydrocarbon component on the lower-kinetic-energy side, (16) which is not taken into account in the simulated curve. C 1s XPS peaks for the n-type sample show peak positions at 1.95, 2.05 and 2.15 ev for the donor concentrations of 1, 2 and cm 3, respectively. This gives peak shifts from the reference sample peak of 1.55, 1.65 and 1.75 ev, respectively. The FWHMs of C 1s peaks for the n-type samples are 1.2, 1.4 and 1.6 ev for the donor concentrations of 1, 2 and cm 3, respectively. The experimental values in Fig. 3 are 1.6±0.05 and 1.4±0.05 ev for the peak shift and FWHM, respectively. Thus, the case for the donor concentration of cm 3 gives

8 238 New Diamond and Frontier Carbon Technology, Vol. 17, No. 5 (2007) Fig. 5. (a) Simulated valence band maximum (E V ) positions for a P-doped n-type diamond for three donor concentrations of 1, 2 and cm 3 and for the reference p-type sample with and without surface conductive layer (SCL). (b) Simulated C 1s XPS peaks for the n-type diamond samples with the three donor concentrations and for the reference sample with and without SCL. the right FWHM and peak shift values with a large margin in peak shift of ±0.15 ev. This donor concentration is consistent with values estimated from Hall effect and SIMS measurements. (12,17) Figure 6 schematically shows a summary of the surface energy band diagram for the n-type sample together with that of the reference sample without a surface conductive layer. In Fig. 6, the thick solid lines marked E C, E V and C 1s are the CBM, VBM and C 1s core level, respectively. The values in bold are those determined experimentally and the values in italics are those estimated from the simulation of surface energy bands ) is a lower bound as determined in SES, since SES does not give cutoff features if USSs are absent at the vacuum level. = 0.42 ev for the reference sample is slightly smaller than that determined by SES for a lower-concentration ( /cm 3 ) B-doped C(001)H sample. (1) This difference may be related to the difference in dopant concentration or in

9 S. Kono et al. 239 Fig. 6. Summary of the surface energy band diagram for the n-type sample together with that of reference sample. Thick solid lines marked as E C, E V and C 1s are the CBM, VBM and C 1s core level, respectively. The values in bold are those determined experimentally and the values in italics are those estimated from the simulation of surface energy bands and C 1s XPS peaks. surface cleanness condition. Since we have an error of ±0.15 ev in the determination of C 1s peak energy for the reference sample due to a possible surface conductive layer, the surface energy band diagram for n-type may vary accordingly. The value for the n-type diamond is 0.1 ev, which indicates that the E cutoff in Fig. 4 corresponds to CBM within the energy band for the P-doped n-type sample bent upward toward the surface by an amount of ~3.2 ev. 5. Discussion For a B-doped (N A =10 16 cm 3 ) p-type H-terminated diamond (111) surface, it was ) value of 0.9 ev. (1) For another B-doped (N A =10 16 cm 3 ) p-type H-terminated diamond (111) surface, it was reported that = 1.27 ev. (18) In the former case, SES E cutoff and C 1s XPS peak position were used for the evaluation of. In the latter case, a work function value derived from a Kelvin probe and C 1s XPS peak position were used for the evaluation. As mentioned already, the electron affinity of a H-terminated P-doped n-type diamond (111) surface has recently been reported to be negative ( 1.1 ev) by total

10 240 New Diamond and Frontier Carbon Technology, Vol. 17, No. 5 (2007) photoyield spectroscopy (TPYS). (11) (111) of both p-type and n-type appeared to be negative with a typical value of ~ 1 ev. However, the present result showed that the SES E cutoff of the P-doped diamond (111) sample appeared to be close to CBM, suggesting that there are no USSs near the vacuum level. As mentioned already, the SES study of H-terminated and O-terminated highly P-doped diamond (111) also showed that E cutoff is present near CBM. (4) To obtain a certain ab initio local-density functional calculations showed USSs at ~1 ev below CBM near (19) It is therefore wondered why these USSs are not seen by SES for n-type diamond (111). In fact, very weak (~3 orders of magnitude weaker than a peak just above E cutoff ) structures were present below E cutoff. However, these structures were mixed with electrons scattered by electrode elements in the electron spectrometer. Thus, an unambiguous assignment of the structures was not possible. This is in sharp contrast to the SES study of the p-type diamond (111)H surface. (1) This may be related to the difference in band bending between p-type and n-type diamonds, which is the subject of the next paragraph. Surface band bending as found in p-type diamonds is always downward to the surface. (1,18) An upward band bending toward the surface for an n-type diamond (111)H sample was predicted by TPYS although a quantitative estimation of band bending was not carried out. (11) It was shown that large amounts (~3 ev) of upward band bending toward the surface exist for highly P-doped diamond (111) surfaces. (4) This was elucidated by the fact that the surface E F positions are found at 1.9 and 1.6 ev from VBM for the H-terminated and O-terminated highly P-doped diamonds, respectively. (4) The surface E F position found for the present n-type diamond (111)H surface is 1.8 ev above VBM. The surface E F positions found for N-doped (N D =10 20 cm 3 ) diamond (100)H was 2.2 ev above VBM. (1) Therefore, all the n-type including highly P-doped and N-doped diamonds show upward band bendings. This can be inferred on the basis of the fact that the E F positions in the bulk are ~0.6 and ~1.7 ev from CBM for P-doped and N-doped diamonds, respectively, and the surface E F position would be lower than these levels, which are determined by characteristic surface states. Key factors characteristic to the surface states are surface defects for H-terminated surfaces since an ideally H-terminated diamond surface has no surface states around this energy position. (19) The most probable defect at H-terminated CVD diamond surfaces has been inferred to be graphite. (2) The charge neutrality level (CNL) of defect graphite on the diamond surface is estimated to be 1.4 ev above VBM. (2) A representative value of surface E F position as found for n-type diamond samples is ~2 ev; thus, the surface E F position is shifted upward by ~0.6 ev from the CNL of probable defect graphite. Charges that are accumulated at the defect are dependent on the defect graphite coverage. On the other hand, surface defect total charges can be precisely estimated from the energy band diagram as in Fig. 5(a), which appeared to be ~ q/cm 2 for the donor concentration of cm 3, where q is the elemental charge of an electron. According to Fig. 2 of ref. 2, the ~0.6 ev shift from the CNL with the surface defect charge density of ~ q/cm 2 may be accommodated by ~0.05 ML of defect graphite. Figure 2 of ref. 2 actually refers to p-type diamonds but would be valid for n-type diamonds since the density of states of

11 S. Kono et al. 241 graphite is nearly symmetric about the CNL. The E F in the bulk is below the CNL for a p-type diamond but is above the CNL for an n-type diamond; thus, the surface E F shifts in opposite directions. Therefore, the mechanism of upward band bending toward the surface for n-type P-doped diamonds can be explained, at least qualitatively, as the surface E F shifting and pinning by surface defects such as a graphitic defect. In Fig. 2 and in the related I-V measurement, we have seen that the electric contact between the graphite and n-type diamond surface is ohmic. Figure 1 of ref. 4 showed that the electric contact between graphite and the p-type diamond surface is also ohmic. In fact, we have conducted a similar I-V measurement on the reference p-type sample to further reinforce the ohmic conductivity. Indeed, a linear current rise up to ~200 na was observed by supplying DC voltages on the graphite patch up to 5 mv for both polarities. Thus, a massive piece of graphite makes ohmic contact to both n-type and p-type diamond surfaces. This indicates that the E F at the junction between massive graphite and H-terminated diamonds is close to the E F of bulk diamonds irrespective of the type of dopant. The charge neutrality level, i.e., the E F of the defect graphite, if it can (2) This may indicate a different electronic nature of defect graphite from that of massive graphite. 6. Conclusions The surface energy band diagram of a H-terminated, P-doped n-type diamond (111) surface was studied by X-ray photoelectron spectroscopy and He-I excited secondary electron spectroscopy. The charge-up problem of the low-conductivity n-type diamond sample is circumvented by pasting a graphite patch on the opening of the sample surface to monitor the amount of charge-up/built-up potential. It turned out that the amount of charge-up near the graphite patch was negligible for the XPS measurement and that there was no charge-up for the SES measurement. It also turned out that the electric contact between graphite and the H-terminated n-type/p-type diamond surface is ohmic and thus there is no built-up potential. The resulting surface energy band diagram showed an upward band bending of ~3.2 ev toward the surface. The E F position was found to be 1.8 ev above the valence band maximum. The cutoff energy in SES spectra turned out to be present n-type sample can be said to be negative. A mechanism of upward band bending for n-type diamonds is discussed to be Fermi level pinning caused by surface defects such as low-coverage graphite. Acknowledgements This work is supported in part by the Advanced Diamond Device Project administered by NEDO, Japan. The authors are grateful to Drs. D. Takeuchi, H. Kato and S. Yamasaki of AIST, Japan for the supply of the present n-type diamond sample and the usage of their experimental data.

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