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Photoelectric emission and work function of semiconducting diamonds
Carbon. 1915, Vol. 13, pp. 425421.
Pergamon Press.
Printed in Great Bntain
PHOTOELECTRIC EMISSION AND WORK FUNCTION OF SEMICONDUCTING DIAMONDS
w. F. WE1 Arkansas State University, State University, AK 72467,U.S.A. and w. J. LEIVO OklahomaState University, Stillwater, OK 74074,U.S.A. (Receiued 27 .Iune 1975) Abstract-Photoemission of electrons was observed in p-type semiconducting diamonds. The emitted electrons responding to photons which have the necessary threshold energy are believed to have originated from filled surface states. The surface charge densities are sufficiently large so that the Fermi level is greatly affected by the surface states. The work function of the semiconducting diamonds investigated is approximately 6.0 eV.
1. INTRODUCTION
The nature of the energy bands of diamond has been studied by many investigators. The presence of surface states which affects the height of the surface barrier has been established[l]; however, the height of the surface potential barrier remained unknown. An attempt was made by Tartakowsky [2] to determine the work function of what was probably an insulating diamond using the photoelectric method. In his experiments the light source was a quartz window mercury arc, and the light traveled through air before reaching the diamond. The electronic charge was measured with a quadrant electrometer. He observed no photoemission of electrons from the diamond. The present investigations were undertaken in order to detect photoemission of electrons and to determine the work function of semiconducting diamonds. 2. EXPERIMENTAL METHODS Two natural semiconducting diamonds were prepared for the photoemission measurements. The first one is a polished rectangular parallelepiped having dimensions 2.5 x 3.5 x 6.5 mm’ and is designated as DS-2. The large faces are nearly (Ill) planes. One end of the diamond has a distinct blue color, and the other end is colorless. The second sample, designated as DS-5, is more than twice as large. It is unpolished and has an irregular shape, but two faces are flat parallel surfaces lying close to (111) planes. The diamond has a uniform dark-blue color. Photoemission was obtained from the (111) faces of both diamonds. A schematic drawing of the electrode arrangements and circuitry is shown in Fig. 1. The diamond was placed in a photoemission cell which has silvered inner walls which serve as the collecting electrodes. The other electrode was connected to the diamond. The photoemission cell has a LiF window and was maintained at a pressure of lo-’ torr by continuous pumping with a three-stage oil diffusion pump equipped with a liquid nitrogen trap. The light source was a carbon arc operated by a high voltage
Fig. 1. Schematic of electrode arrangements and circuitry for measuringphotoemissionfromdiamonds.
transformer. Both the photoemission cell and the carbon arc were placed in a vacuum chamber. The chamber was evacuated and then dry nitrogen gas which is transparent to the ultraviolet light was intmduced. Light from the carbon arc which operated in a partial nitrogen atmosphere first passed through a crystal filter and a LiF window before it was incident on the (111) face of the diamond. Electrons excited by the light and emitted from the diamond were collected by the silver surface of the photoemission cell. The photocurrent was measured with a Cary Model 31 vibrating reed electrometer. The photoemission cell, carbon arc, electrometer, battery, and circuits were electrically shielded. Electron-emission which was produced by the filtered incident light was observed at various collecting potentials. The potential was varied until a saturation emission current was obtained. Light filters used included crystals of LiF, BaF>, CaF*, NaCl, KCl, KBr, CsI, A1203and SiOZ; also some Corning glass filters and liquid filters were used. Each filter provided a definite short wavelength limit for the incident light. Measurements were made at room temperature. 3. EWERIMENTAL RESULTS In Fig. 2 are presented typical curves of the photoemis-
sion current as a function of the applied voltage. Each curve was obtained with incident light filtered through the crystal filter indicated. The saturation currents were 425
W. F. WEIand W. J. L~xvo
426
4. DISCUSSlONOF RESULTS
160 d 140 DS-2 N ‘0 120 Light filter $100 2 3 -- 80
OLiF Al3oF2 aSapphire 0Quartz
-2 0 2 Collector Bios,volts
4
Fig. 2. Photoemission current-potential characteristics for diamond DS-2.
reached with less than two volts. The negative current is mainly attributed to the emission of electrons from the silver surface by scattered light. The incident light was also filtered through ether alcohol, Csf, KI, and KC1 crystals separately. A small current of 2.5 x 1O--‘4 ampere was observed with a KC1 crystal filter, but no current was detected with the other above filters. The short wavelength limit of KC1 is 2063A and that of ether alcohol, CsI and KI are 2250, 2420, and 254Ow respectively. The results show that very few electrons were emitted by the incident light filtered through a KC1 crystal, and no detectable number of electrons were emitted by light filtered through ether alcohol, CsI or KI crystals. The saturation current vs the short wavelength limit of the crystal filter used is shown in Fig. 3. The phot~lect~c threshold wavelength of light for the electron-emission was determined by the interception of the curve with the abscissa. This was found to be 2070 A corresponding to a
A theoretical evaluation of surface states on the (111) face of diamond by Pugh131 shows a band of surface states located below the middle of the energy gap. In accordance with previous measurements[ l] on diamond DS-2, the energy bands bend downward from the bulk to the surface. Photoelectrons which responded to the threshold energy photons are believed to have originated at the filled surface states similar to those observed in silicon by Scheer and Van Laar [4] and in alkali antimonides by Spicer[S]. Scheer and Van Laar also found the photoelectric threshold energy of photons for the electron emission to be equal to the work function. The results of Gobeli and Allen f6] and those of Diilon and Farnsworth [7] in germanium show that the work function and the photoelectric threshold for electron emission differ very little. Similarly, the work function of the semiconducting diamonds investigated is believed to be approximately equal to the photoelectric threshold and thus be 6eOeV. In investigating the energy distribution of electrons photoemitted from surface states of the (I 11) face of low resistivity silicon, Wagner and Spicer [8] found a band of surface states containing one electron per surface atom. The surface states of the (111) face of a semiconducting diamond could be expected to be similar. The photoelectrons which responded to the threshold energy photons are believed to have originated at the filled surface states. By plotting the square root of the saturation emissioncurrent vs photon energy for DS-5, Fig. 4 was obtained. The extrapolation of the curve again gives threshold 6.0 eV. The linear relation between the square root of the emission-current and the photon energy is similar to those obtained by Gobeli and Allen[6] for low resistivity p-type silicon where the square root of emitted electrons per quantum was plotted vs photon energy. This curve characterizes the low resistivity of DS-5. The emission currents measured from a diamond surface a few weeks after cleaning are lower than those measured just after cleaning as shown in Fig. 3. The work function obtained from this curve is slightly larger. The samples investigated are p-type semiconductors. At equilibrium, the Fermi level at the surface of the crystal must be the same as in the interior. The small change in the work function with time probably arises from the N -e
a 50
N
‘0
1000
1400 1800 Wovelength, %
Fig. 3. Spectral distribution of the electron emission for di~ond DS-5.
photon energy of 6.0eV. The solid line curve was obtained from data taken from a freshly prepared sample, and the dashed line curve was from the same sample after it was stored in the photoemission cell for a period of a few weeks at a pressure of 6 x lo-‘torr. The lower curve gives a value approximately O-l eV higher for the emission threshold.
f40e 5 -3os '5 $20 % g IO s F I/ 8 OS
DS-5
/
IO 8 Photon Energy,eV
” Fig. 4. Square root of photoemission current vs photon energy of diamond DS-5.
Photoelectric emission and work function of semiconducting diamonds
adsorption of impurity atoms or ions at the surface of a type which will raise the Fermi level at the surface. Equilibrium is then attained by having the electron energy bands bend further downward as the surface of the crystal is approached until the Fermi level at the surface is the same as in the interior. The slight difference in work function when the sample is measured at different times indicates the change of the potential barrier associated with the adsorption of impurities. 5. SUMMARY
Photoemission of electrons from p-type semiconducting diamonds was obtained. The work function of the diamonds measured for emission of electrons from (Ill) faces was approximately 6.0eV. The emitted electrons are believed to have originated from surface states. The
437
surface states are of such a nature as to make the energy bands bend downward as the surface is approached from the interior. The work function increased about 0.1 eV after a few weeks of aging at reduced pressure. This is believed to be the result of adsorption of atoms which produce surface states and change the Fermi level. REFERENCES I.
2. 3. 4. 5. 6. 7. 8.
Bell M. D. and Leivo W. J.. Phys. Rev. 111, 1227(19%). Tartakowsky P., Z. Phys. 58, 394 (1929). Pugh D., Phys. Rev. Lett. 12 390 (1964). Scheer J. J. and Van Laar J., Phys. Lett. 3, 246 (1963). Spicer W. E., Phys. Rea. 112, 114 (1958). Gobeli G. W. and Allen F. G., Phys. Rec. 127. 141(1962);Allen F. G. and Gobeli G. W., Phys. Reo. 127, 150 (1962). Dillon J. A. and Farnsworth H. E., J. Appl. Phys. 28, 174(1957). Wagner L. F. and Spicer W. E.. Phys. Rer. Lett. 28,138l (1972).
Pergamon Press.
Printed in Great Bntain
PHOTOELECTRIC EMISSION AND WORK FUNCTION OF SEMICONDUCTING DIAMONDS
w. F. WE1 Arkansas State University, State University, AK 72467,U.S.A. and w. J. LEIVO OklahomaState University, Stillwater, OK 74074,U.S.A. (Receiued 27 .Iune 1975) Abstract-Photoemission of electrons was observed in p-type semiconducting diamonds. The emitted electrons responding to photons which have the necessary threshold energy are believed to have originated from filled surface states. The surface charge densities are sufficiently large so that the Fermi level is greatly affected by the surface states. The work function of the semiconducting diamonds investigated is approximately 6.0 eV.
1. INTRODUCTION
The nature of the energy bands of diamond has been studied by many investigators. The presence of surface states which affects the height of the surface barrier has been established[l]; however, the height of the surface potential barrier remained unknown. An attempt was made by Tartakowsky [2] to determine the work function of what was probably an insulating diamond using the photoelectric method. In his experiments the light source was a quartz window mercury arc, and the light traveled through air before reaching the diamond. The electronic charge was measured with a quadrant electrometer. He observed no photoemission of electrons from the diamond. The present investigations were undertaken in order to detect photoemission of electrons and to determine the work function of semiconducting diamonds. 2. EXPERIMENTAL METHODS Two natural semiconducting diamonds were prepared for the photoemission measurements. The first one is a polished rectangular parallelepiped having dimensions 2.5 x 3.5 x 6.5 mm’ and is designated as DS-2. The large faces are nearly (Ill) planes. One end of the diamond has a distinct blue color, and the other end is colorless. The second sample, designated as DS-5, is more than twice as large. It is unpolished and has an irregular shape, but two faces are flat parallel surfaces lying close to (111) planes. The diamond has a uniform dark-blue color. Photoemission was obtained from the (111) faces of both diamonds. A schematic drawing of the electrode arrangements and circuitry is shown in Fig. 1. The diamond was placed in a photoemission cell which has silvered inner walls which serve as the collecting electrodes. The other electrode was connected to the diamond. The photoemission cell has a LiF window and was maintained at a pressure of lo-’ torr by continuous pumping with a three-stage oil diffusion pump equipped with a liquid nitrogen trap. The light source was a carbon arc operated by a high voltage
Fig. 1. Schematic of electrode arrangements and circuitry for measuringphotoemissionfromdiamonds.
transformer. Both the photoemission cell and the carbon arc were placed in a vacuum chamber. The chamber was evacuated and then dry nitrogen gas which is transparent to the ultraviolet light was intmduced. Light from the carbon arc which operated in a partial nitrogen atmosphere first passed through a crystal filter and a LiF window before it was incident on the (111) face of the diamond. Electrons excited by the light and emitted from the diamond were collected by the silver surface of the photoemission cell. The photocurrent was measured with a Cary Model 31 vibrating reed electrometer. The photoemission cell, carbon arc, electrometer, battery, and circuits were electrically shielded. Electron-emission which was produced by the filtered incident light was observed at various collecting potentials. The potential was varied until a saturation emission current was obtained. Light filters used included crystals of LiF, BaF>, CaF*, NaCl, KCl, KBr, CsI, A1203and SiOZ; also some Corning glass filters and liquid filters were used. Each filter provided a definite short wavelength limit for the incident light. Measurements were made at room temperature. 3. EWERIMENTAL RESULTS In Fig. 2 are presented typical curves of the photoemis-
sion current as a function of the applied voltage. Each curve was obtained with incident light filtered through the crystal filter indicated. The saturation currents were 425
W. F. WEIand W. J. L~xvo
426
4. DISCUSSlONOF RESULTS
160 d 140 DS-2 N ‘0 120 Light filter $100 2 3 -- 80
OLiF Al3oF2 aSapphire 0Quartz
-2 0 2 Collector Bios,volts
4
Fig. 2. Photoemission current-potential characteristics for diamond DS-2.
reached with less than two volts. The negative current is mainly attributed to the emission of electrons from the silver surface by scattered light. The incident light was also filtered through ether alcohol, Csf, KI, and KC1 crystals separately. A small current of 2.5 x 1O--‘4 ampere was observed with a KC1 crystal filter, but no current was detected with the other above filters. The short wavelength limit of KC1 is 2063A and that of ether alcohol, CsI and KI are 2250, 2420, and 254Ow respectively. The results show that very few electrons were emitted by the incident light filtered through a KC1 crystal, and no detectable number of electrons were emitted by light filtered through ether alcohol, CsI or KI crystals. The saturation current vs the short wavelength limit of the crystal filter used is shown in Fig. 3. The phot~lect~c threshold wavelength of light for the electron-emission was determined by the interception of the curve with the abscissa. This was found to be 2070 A corresponding to a
A theoretical evaluation of surface states on the (111) face of diamond by Pugh131 shows a band of surface states located below the middle of the energy gap. In accordance with previous measurements[ l] on diamond DS-2, the energy bands bend downward from the bulk to the surface. Photoelectrons which responded to the threshold energy photons are believed to have originated at the filled surface states similar to those observed in silicon by Scheer and Van Laar [4] and in alkali antimonides by Spicer[S]. Scheer and Van Laar also found the photoelectric threshold energy of photons for the electron emission to be equal to the work function. The results of Gobeli and Allen f6] and those of Diilon and Farnsworth [7] in germanium show that the work function and the photoelectric threshold for electron emission differ very little. Similarly, the work function of the semiconducting diamonds investigated is believed to be approximately equal to the photoelectric threshold and thus be 6eOeV. In investigating the energy distribution of electrons photoemitted from surface states of the (I 11) face of low resistivity silicon, Wagner and Spicer [8] found a band of surface states containing one electron per surface atom. The surface states of the (111) face of a semiconducting diamond could be expected to be similar. The photoelectrons which responded to the threshold energy photons are believed to have originated at the filled surface states. By plotting the square root of the saturation emissioncurrent vs photon energy for DS-5, Fig. 4 was obtained. The extrapolation of the curve again gives threshold 6.0 eV. The linear relation between the square root of the emission-current and the photon energy is similar to those obtained by Gobeli and Allen[6] for low resistivity p-type silicon where the square root of emitted electrons per quantum was plotted vs photon energy. This curve characterizes the low resistivity of DS-5. The emission currents measured from a diamond surface a few weeks after cleaning are lower than those measured just after cleaning as shown in Fig. 3. The work function obtained from this curve is slightly larger. The samples investigated are p-type semiconductors. At equilibrium, the Fermi level at the surface of the crystal must be the same as in the interior. The small change in the work function with time probably arises from the N -e
a 50
N
‘0
1000
1400 1800 Wovelength, %
Fig. 3. Spectral distribution of the electron emission for di~ond DS-5.
photon energy of 6.0eV. The solid line curve was obtained from data taken from a freshly prepared sample, and the dashed line curve was from the same sample after it was stored in the photoemission cell for a period of a few weeks at a pressure of 6 x lo-‘torr. The lower curve gives a value approximately O-l eV higher for the emission threshold.
f40e 5 -3os '5 $20 % g IO s F I/ 8 OS
DS-5
/
IO 8 Photon Energy,eV
” Fig. 4. Square root of photoemission current vs photon energy of diamond DS-5.
Photoelectric emission and work function of semiconducting diamonds
adsorption of impurity atoms or ions at the surface of a type which will raise the Fermi level at the surface. Equilibrium is then attained by having the electron energy bands bend further downward as the surface of the crystal is approached until the Fermi level at the surface is the same as in the interior. The slight difference in work function when the sample is measured at different times indicates the change of the potential barrier associated with the adsorption of impurities. 5. SUMMARY
Photoemission of electrons from p-type semiconducting diamonds was obtained. The work function of the diamonds measured for emission of electrons from (Ill) faces was approximately 6.0eV. The emitted electrons are believed to have originated from surface states. The
437
surface states are of such a nature as to make the energy bands bend downward as the surface is approached from the interior. The work function increased about 0.1 eV after a few weeks of aging at reduced pressure. This is believed to be the result of adsorption of atoms which produce surface states and change the Fermi level. REFERENCES I.
2. 3. 4. 5. 6. 7. 8.
Bell M. D. and Leivo W. J.. Phys. Rev. 111, 1227(19%). Tartakowsky P., Z. Phys. 58, 394 (1929). Pugh D., Phys. Rev. Lett. 12 390 (1964). Scheer J. J. and Van Laar J., Phys. Lett. 3, 246 (1963). Spicer W. E., Phys. Rea. 112, 114 (1958). Gobeli G. W. and Allen F. G., Phys. Rec. 127. 141(1962);Allen F. G. and Gobeli G. W., Phys. Reo. 127, 150 (1962). Dillon J. A. and Farnsworth H. E., J. Appl. Phys. 28, 174(1957). Wagner L. F. and Spicer W. E.. Phys. Rer. Lett. 28,138l (1972).