Pergamon Solid State Communications, Vol. 102, No. 12, pp. 861-865. 1997 0 195’7 Elsevier Science Ltd

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ELECTRONIC TRANSITIONS OF HOLES BOUND TO BORON ACCEPTORS IN ISOTOPICALLY CONTROLLED DIAMONDS

Hyunjung Kim,“‘* A.K. Ramdas,” S. Rodriguez” and T.R. Anthonyb

“Department of Physics, Purdue University, West Lafayette, IN 47907, U.S.A. bGE Corporate Research and Development, Schenectady, NY 12309, U.S.A.

(Received 24 February 1997; accepted 7 March 1997 by M. Cardona)

The excitation spectrum of holes bound to substitutional boron acceptors introduced into a 13C diamond is compared with that of boron acceptors in natural Type IIb diamond. While the two spectra are remarkably alike in their general features, the spectrum of the acceptors in 13C diamond is shifted to higher energies by 0.4-l 5 meV. In addition, the presence of substitutional boron relaxes the selection rule associated with translational symmetry, thus allowing the observation of the one phonon spectrum reflecting the density of states of the optical branch as well as the zone center optical phonon. 0 1997 Elsevier Science Ltd

Keywords: A. semiconductors, C. impurities in semiconductors, D. elec- tronic states (localized), D. phonons, E. light absorption and reflection.

1. INTRODUCTION

Close to four decades ago, Custers [l, 21 made the startling observation that a small number of nitrogen- free, i.e. Type II diamonds display a remarkably low resistivity (as low as 25 Q-cm) in contrast to resistivities in the range 5 X 1014 Q-cm typical of Type I and Type IIa diamonds. Austin and Wolfe [3] and Wedepohl [4] demonstrated that these low-resistivity diamonds exhibit p-type conductivity caused by the presence of acceptors as deduced from Hall effect measurements, the activation energy being -0.37 eV. Further, they reported numerous excitation lines of the acceptor-bound holes in the spectral range 2400-5500 cm-‘. These very rare, p-type, diamonds have been designated as Type IIb, whereas the somewhat more common, high resistivity, specimens are classified as Type IIa. The excitation spectrum of the acceptor-bound holes have been extensively studied with natural p-type diamonds [5-71 and synthetic diamonds [8, 91. Chrenko [lo] has concluded, on the basis of a critical examination of the literature and his own investigation, that the “acceptor” in semiconducting diamond is substitutional boron. In all the p-type diamonds studied in this manner, natural as well as synthetic, the isotopic composition of the host is

* To whom correspondence should be addressed.

98.9% ‘*C and 1 .l% 13C. In the present paper we report novel observations on the excitation spectrum of neutral boron acceptors deliberately incorporated in a 13C diamond and compare and contrast it with our similar studies of natural Type IIb diamonds.

2. EXPERIMENTAL PROCEDURE

Two Type IIb specimens of natural diamond were examined along with a 13C diamond grown by chemical vapor deposition (CVD) followed by the high pressure- high temperature technique [ Ill. The boron doping of the 13C diamond was accomplished by adding 99.999% pure amorphous, submicron, boron powder [12] to the diamond growth cell mixture of powdered Fe-3% Al metal solvent and 13C CVD diamond feedstock.

The infrared transmission of the diamond specimens were measured with a BOMEM DA.3 Fourier transform spectrometer, a variable temperature cryostat and a cooled mercury cadmium telluride detector operating at 77 K. A resolution of 1.0 cm-’ was adequate to record the spectra in view of the half-widths of the excitation lines.

3. EXPERIMENTAL RESULTS AND DISCUSSION

The absorption spectra of natural Type IIa and Type IIb diamonds, both recorded at 5 K in the range

861

862 HOLES BOUND TO BORON ACCEPTORS IN DIAMONDS Vol. 102, No. 12

EnwMmeW 250 300 350 400 450 500 550 600 650 700

I ’ I ’

T=5.OK

, I I 1 I , 1 , I , a

Natural Diamond I I

15

02 10

5 I.- 0’ - 2750 2800 2850

------- Type IIa

w

__----.___/ .,

B--

Fig. 1. The absorption spectrum of a natural, Type IIa (D18) and of a Type IIb (Dl) diamond. The spectra were recorded at a temperature T = 5.0 K. For clarity, the strong lines in the range 2750-2850 cm-’ are shown in the inset for a natural Type IIb (D2) diamond having a smaller boron concentration.

1800-6000 cm-‘, are compared in Fig. 1 showing that the intrinsic two-phonon features are identical for the two in the range 1800-2360 cm-‘. A strong absorption band, with a maximum at 2453.5 cm-‘, superimposed on the two-phonon spectral features extending from 2360 to

2665 cm-‘, is observed in the spectrum for the Type IIb specimen. Significantly sharper excitation lines, as many as twenty eight, appear for the Type IIb specimen alone in the range 2665-3050 cm-‘. Given the thickness and acceptor concentration of the Type IIb specimen (Dl), the peaks of the most intense electronic excitation could not be observed; the inset shows the relevant portion of the spectrum with a specimen of a similar thickness but distinctly lower acceptor concentration. The sharp lines have been ascribed to excitations of the acceptor-bound holes, the acceptor being substitutional boron [3-7, lo]. In addition, broad absorption bands are observed at still higher frequencies. As will be clear, those in the range 3600-4300 cm-’ can be attributed to the electronic transitions coupled with the “defect- induced-first-order (DIFO), one phonon density of states spectrum’ ’ , observed in Type IIb diamonds. Similarly, features in the range 4800-5500 cm-’ can be interpreted as arising from electronic transitions accompanied by the overtones and combinations of the DIFO. The onset of the photoionization -3000 cm-’ (372 meV) is followed by a continuum well into the visible and is responsible for the typical blue color of Type IIb specimens.

In Fig. 2, we make a similar comparison between the

absorption spectra of a single crystal of Type IIa and of a boron doped Type IIb diamond, both composed of 13C. The latter shows the unmistakable signatures of the excitations of the acceptor-bound hole. (The inset

shows a comparison between the boron excitation spectrum of the 13C Type IIb diamond and a nominally boron-free 13C Type IIa diamond, the latter spectrum

amplified a hundred fold. The presence of boron introduced inadvertently during growth can be noticed in the latter.) There is an exact correspondence between the features of the excitation spectrum of the acceptors in the natural Type IIb and those in the 13C Type IIb diamond. In Table 1, the positions of the lines for the natural and 13C Type IIb diamonds are listed. The serial

Table 1. Energies of the most prominent electronic transitions of boron acceptors in diamond (meV)

Line no.

11 12a 13 14 15 20 22

Natural 13C diamond diamond

337.38 337.76 343.60 344.65 346.60 347.8 1 347.2 1 348.37 349.29 350.54 359.56 360.96 362.64 364.10

Difference

0.38 1.05 1.21 1.16 1.25 1.40 1.46

Vol. 102, No. 12 HOLES BOUND TO BORON ACCEPTORS IN DIAMONDS

Energy(meV) 250 300 350 400 450 500 550 600 650 700

7---l-

T=5.OK

%!L

rr_ c-.

2000

8 I , 1 , I , 1 , I I ,

“C Diamond

------- Type IIa

- B-doped, Type IIb

040 . -_t__+-_* , I I , I k_

863

3000 4000 5000 6000

Wavenumber(crn-‘)

Fig. 2. The absorption spectra of a 13C Type IIa and a boron-doped 13C Type IIb diamond recorded at 5.0 K. The inset shows the comparison between the Type IIb (upper one) and the Type IIa (lower one) diamond, the latter enlarged 100 times in the range 2750-2850 cm-‘. Here the Type IIa diamond evidently contains boron introduced inadvertently during growth.

numbers in Table 1 and in Fig. 3 emphasize the similarity even in minutest detail, the spectral lines of the acceptors based on the relative intensities. The entire two phonon in the 13C diamond are unambiguously shifted by 3.1 to spectrum experiences a shift to lower energy, scaling 11.8 cm-’ to higher energies (see Fig. 3). Since the according to the inverse square root of the average acceptor in both natural and 13C diamonds is boron, the isotopic mass (M-‘“) as expected from the virtual crystal small shifts strongly indicate that the acceptor states approximation. While the spectra are strikingly similar experience a central cell correction in 13C diamond

(a)

140L t

T=5OK 120

(b)

330 340

I 1 I I I I I

350 360 370 360

Enew(meV)

I 10

I, II

I: i! ! ;; ,-\

5

15

“I 1 15

Fig. 3. Comparison of the excitation spectra of a natural Type IIb (D2) and a 13C Type IIb (D41) diamond in the range 330-380 meV.

864

Fig. 4. Defect induced first

HOLES BOUND TO BORON ACCEPTORS IN DIAMONDS Vol. 102, No. 12

0.8 I I I I I I I

1=300K

- Natural Diamond

1000 1100 1200 1300 1400

Wavenumber(crn~‘)

order (DIFO) spectra of Type IIb ‘jC and natural diamonds recorded

larger than that in the natural diamond varying from -1.46 meV for the ground state to 0.06 meV for line

20 while the state corresponding to line 11, say, has a central cell shift of 1.08 meV. We thus have here a remarkable example of central cell corrections for the same substitutional acceptor but located in a host differing merely in its isotopic composition.

In order to account for the shift of the excitation spectrum of B acceptors in i3C diamond with respect to that in natural diamond, we have made preliminary estimates invoking several mechanisms consistent with the difference in atomic volumes. The estimated change in the dielectric constant gives rise to negligible shifts (- 10-3-10-4 meV) in the binding energies of the effective mass states. The changes in the Coulomb energy of the hole in the presence of the ionized acceptor yields a ground state shift no more than lo-* meV for the two approximations considered: the charge of the ion

was considered to be uniformly distributed over its volume in one and over its surface in the other. The

contribution of the tetrahedral potential leads to shifts of no more than 0.2 meV. A preliminary analysis taking account of the position dependence of the dielectric

constant also yielded negligible effects. Another mechanism [13] one could invoke, takes into account the self energy of the states arising from the electron- phonon coupling as in the case of the indirect gap discussed by Collins et al. [ 141. This self energy correc- tion is proportional to the square of the amplitude of the lattice vibrations and hence varies as the inverse square root of the isotopic mass. Thus the difference of such a contribution between that in 13C and in natural diamond is proportional to [ 1 - (M,&M,s)“*]. The magnitude of this shift cannot be easily estimated since it would require the temperature dependence of a specific

at 300 K.

electronic transition, say, for natural diamond. In the temperature range in which well resolved excitation lines have been observed such shifts are negligible within the temperature 5-300 K.

Figure 4 shows the spectra of a 13C and a natural Type IIb diamond in the range of one phonon excitations at 300 K. The observed spectrum can be ascribed to one phonon excitations [ 151, normally forbidden in infrared

absorption by selection rules but activated by the presence of boron acceptors. The shape of the spectrum reflects one phonon density of states of the optical branches of the phonon dispersion curves. Note also the sharp feature at wo, the zone center Raman frequency. The DIFO spectrum including 00 experiences the M-“* shift in going from the natural to the 13C diamond. At low temperatures, a few additional features above w. are observed for both the natural and the i3C diamond. The phonon assisted electronic transition spectra and the computed spectrum deduced by convoluting the DIFO at 5 K with each of the electronic excitation lines in the

range 2400-3100 cm-’ are in good agreement. This interpretation differs from that of Davies and Stedman [16] who ascribe these phonon assisted transitions, observed by them in natural diamond, as Fano resonances.

Acknowledgements-The authors acknowledge support from National Science Foundation Grant No. DMR 93-03 186.

1. 2. 3.

4. 5.

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Vol. 102, No. 12 HOLES BOUND TO BORON ACCEPTORS IN DIAMONDS 865

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