Strong negative electron affinity effects have been observed on the surface of as-grown chemical vapor deposited diamond using Secondary Electron Emission. The test samples were randomly oriented and the surface was terminated with hydrogen. The effect appears as an intensive peak in the low energy part of the spectrum of the electron energy distribution and may be described in the model of effective negative electron affinity

Asnin, V. M.

Dayton, J. A., Jr.

Krainsky, I. L.

Mearini, G. T.

English

NASA Technical Reports Server

NASA-T~-11l7Z4 /IV .~ 
NEGATIVE ElECIRON AFF1NI'IY EFFECT ON 1HE SURFACE OF CHEMICAL 
VAPOR DEPO)IIED DIAMOND POLYCRYSTAIlJNE F1I..MS. 
LL.Krainsky, V.MAsnin*, G.T.Mearini*, and J.ADayton, Jr. 
NASA Lewis Research Center, Cleveland, Ohio 44135 
ABS1RACT 
/ I;· I. 
Strong negative electron affinity effects have been observed on the surface of as-grown 
chemical vapor deposited diamond using Secondary Electron Emission. The test samples were 
randomly oriented and the surface was tenninated with hydrogen. The effect appears as an 
intensive peak in the low energy part of the spectrum of the electron energy distribution and may 
be described in the model of effective negative electron affinity. 
One of the remarkable features of diamond ~s the negative electron affinity (NEA) that 
may be observed on its surface under specific conditions. l-lO For NEA the vacuwn energy level 
lies below the conduction band minimum which allows electrons to escape easily from the 
surface. 11 Semiconductors with NEA are widely used as photo- and cold-cathode emitters. 
NEA from diamond has been shown to correspond with the presence of hydrogen bounded 
to the surface. 1-5 Special surface treatment of single crystal diamond and epitaxial films is, 
therefore, usually necessary to activate NEA Recently, chemical vapor deposited (CVD) 
t 
polycrystalline diamond films have received a great deal of attention because they are promising 
for novel electronic applications due to their additional advantages in comparison with diamond 
single crystals. Their fabrication is relatively easier and cheaper, they can be grown with high 
levels of impurity doping, and they can have correspondingly high conductivity.12 CVD diamond 
has also been observed to display high secondary electron emission (SEE) properties. Total 
https://ntrs.nasa.gov/search.jsp?R=19970001818 2020-06-16T03:34:12+00:00Z
secondary yield coefficients, 0=12-50, have been measured from such films tenninated with 
hydrogen, 13-16 which is indirect evidence of a strong NEA effect in this material. 
In this Letter we present a direct proof of the NEA effect on the surface of as-grown CVD 
randomly oriented polycrystaIline diamond films. 
Since the pioneer workl a commonplace tool for determination of the presence of NEA 
2in diamond has been the method of ultraviolet photoemission electron spectroscopy (UPS). The 
idea behind the measurement is very simple. Hot photoelectrons photoexcited to the conduction 
band cool in the process of energy relaxation and accumulate in the conduction band minimum. 
At an NEA surface these electrons can be emitted into the vacuum without any energy barrier 
and appear in photoemission spectra as a sharp peak at low electron energies. Obviously, the 
existence of the same kind of peak should be expected at the low energy edge of the SEE spectra 
under excitation of an NEA surface with a beam of primary electrons. • I 
In this study, we use SEE spectroscopy to find a NEA phenomenon in CVD randomly 
oriented polycrysta1line films. The ~les were made by depositing 10-20 Jlm continuous films 
on Mo substrates by microwave-plasma CVD. During the deposition process, the films were 
heavily doped with B up to a concentration of about I ()20 cm-3• Experiments were conducted in 
an ultra high vacuum chamber with a base pressure of 2xlO-1O torr. Energy distributions of the 
secondary electrons were measured with a single pass (perkin Elmer) CMA auger analyzer . 
• Samples were biased at Vt = -1.5 to -5 V, where Vt is the bias voltage of the target, during the 
measurements to compensate the workfimction difference between the target and the analyzer and 
to make the measurements of the low energy region of the spectrum more reliable. 
Fig.1 shows the low energy part of the secondary electron spectra of an as-grown CVD 
2 
diamond polycrystalline film as a function of energy E = e (Va-VJ (where Va is the analyzer 
sweep voltage ). For this sample (J =35 measured with a primary beam of energy r;, =3 ke V. It 
can be seen that most of the secondary electrons are collected in the form of a sharp low energy 
peak with maximum position at Emax. = (0.5 - 0.6) eV and with (0.6 - 0.7) eV full width at half 
maximum (FWHM). The shape of the energy distribution curves and the peak positions had very 
little dependence on the sample bias in the measured range. Therefore, we can conclude that there 
were no significant instrumental distortions of the true electron spectrum in our experiments. 
Observation of the strong low energy peak in the secondary electron distribution is evidence of 
the NEA effect in CVD diamond films. 
To investigate the role of hydrogen tennination on the NEA effect, the influence ofbeam 
exposure on the spectrum was studied. Continuous electron beam irradiation of these films is well 
known to result in desorption of hydrogen from the surface and, in turn, a decrease in the 
secondary electron yield.16 Fig.2 demonstrates transfonnation of the secondary electron spectrum 
after exposure to the electron beam for up to 6 homs at a current density of 0.180 mNcrr?- and 
£;,=3 keY. This treatment of the surface led not only to the significant reduction of the value of 
cr (from 32 to 5) but also to the decrease of the contribution of the low energy peak to the energy 
distribution ( dotted curve). Subsequent exposure of the sample to a molecular hydrogen 
environment at a pressure of 5x10.{; torr for 1 hour restored the primary distribution function of 
the secondary electrons ( dashed curve). Therefore, we observed a direct correlation between 
hydrogen coverage and NEA effect of the diamond film surface. 
The total secondary electron emission yield of the diamond films on Mo was previously 
observed to increase with an increase in the primary beam energy in the range of ~ = 1 - 1.5 
3 
keVand to saturate at the larger beam energiesIJ-1S In this study we have investigated changes 
in the SEE spectra versus Bp and have fOlU1d that the yield increase was accompanied by the 
enhancement of the low energy peak magnitude as compared with the rest of the energy 
distribution curve (see Fig. I). The ratio, K, of the number of low energy electrons in this peak 
(the area under the peak) to the total number of secondary electrons (the area under the 
distribution curve in the measured range up to Va=I4 eV) versus Bp is shown in Fig.3. 
There are two possible ways to describe the NEA phenomenon (FigA).6.9-11 It can be 
done in terms of either true or effective negative electron affinity. The effective NEA can be 
described as a consequence of the depletion band bending at the surface of the semiconductor. 
This band bending may result in the vacuum energy level occurring at an energy below the 
minimum of the conduction band. II The magnitude of the effective NEA can be defined as 
Xefi9l>sB-X, where 4>aB is band bending and X is positive electron affinity (FigAb). In the case of 
the true NEA model the semiconductor crystal has negative electron affinity as its intrinsic 
property (FigAa). 
Data obtained in this investigation agree with the effective NEA model for CVD diamond 
films. Indeed, the increase of the efficiency of the low energy electron emission as the primary 
beam energy is increased (Fig.3) may be easily understood in the model of the effective NEA 
The thickness of the depletion layer can be evaluated from the expressionI7 Loo= ($00 Kl2neNAY''2 
~ 25 A for our samples;8 taking for the estimation <flaB = 1 eV , dielectric constant 1C=5 and 
NA~I()2o cm-J• The mean free path of electrons with E,., < 1 keY in solids does not exceed 20 -30 
A and excitation of the secondary electrons occurs mostly in the surface depletion layer, from 
where the large number of the low energy electrons can not escape due to the existence of a 
4 
potential barrier. At ~ > 1.5 ke V the secondary electrons are excited below the depletion layer 
and the low energy electrons can freely escape into the vacuwn from the bulk conduction band. 
Therefore, the contribution of the very low energy electrons into the energy distribution spectnun 
increases sharply with the increase of the primary beam energy. In the case of true NEA it should 
be expected that relative contribution of the very low energy electrons into the spectrum would 
not strongly depend on the energy of primary electrons. 
We should note also that the onset of the electron spectrum, as seen in Figs.l and 2, 
corresponds to secondary electrons having near zero kinetic energy &. It is clear from the band 
diagram shown in Fig.4 that these electrons can be observed only when the effective NEA 
condition exists on a surface (for the true NEA surface minimwn kinetic energy of electrons 
would be equal to I x-4>aa I ). 
~ It can be seen from Figs. 1 and 2 that FWHM of the low energy peak is much larger than 
kT. The same p!ak shape has been observed in all photoemission measurements independent of 
the crystal orientation of the surface and its preliminary treatment when samples were excited by 
photons with energy significantly exceeding the diamond band gap.I-IO It might be, therefore, that 
the peak width is associated with the energy distribution of electrons inside the crystal rather than 
with surface potential fluctuations. I A strong nonequilibriwn distribution ofIow energy electrons 
in the conduction band minimtun is produced during the SEE process because their escape time, 
t esc , t is smaller than the energy relaxation time, which is mainly detennined by interaction with 
optical phonons. If the electron energy does not exceed the energy of a few optical phonons this 
relaxation time can be estimated as ~ lO -12_10 -13 S.19, 20 At the depth of penetration of the 
primary electrons in the crystal I ~ 50 ~ and velocity of the low energy secondary electrons v 
5 
~10 7 em s -I, 'tcsc = I / v < 10 -13 s. The condition tesc < 1;; is fulfilled for all electrons from the 
peak which has FWHM: approximately equal to the energy of four optical phonons in diamond 
( 11 ~ = 0.163 e V 12 ). The relaxation time of electrons with energies less than 11 ffiopt is much 
longer than lO -12 s. Therefore, we can anticipate that the maximum of the energy distribution of 
secondary electrons in the conduction band would be at least one energy of an optical phonon 
above its bottom. Consequently, assmning that the secondary electrons with zero kinetic energy 
are coming out of the solid at the vacuum level, we can estimate the value of kif for the film 
\U1der investigation as kif < ( Emax - Eo ) - 11 O\x. Using data from Fig.l and the band diagram 
( Fig.4) we obtained kff < 0.4 eV. 
In conclusion, we have found a strong NEA effect on the surface of a randomly oriented 
CVD poly crystalline diamom\film terminated with hydrogen using the SEE spectroscopy method. 
The effect appears as a strong peak at the low energy part of the electron energy distribution and 
may be described in the model of effective NEA. 
The authors would like to thank Yaxin Wang and John C.Angus of Case Western Reserve 
University for producing the diamond samples used in the investigation. 
This work was supported by the National Research Council and NASA Lewis Research 
Center. 
6 
REFERENCES 
* National Research Council Fellows. 
1 FJ.Himpsel, J.AKnapp, J.A van Vechten, and D.E.Eastman, Phys. Rev. 20, 624, (1979). 
2 B.B.Pate, W.E.Spicer, T.Ohta and I.Lindau, J. Vac. Sci. Technol., 17, 1087, (1980). 
3 B.B.Pate, Surf Sci. 165, 83, (1986). 
4 J. van der Weide and RJ.Nemanich, J. Vac. Sci. Technol., B 12, 2475, (1994). 
5 J. van der Weide, Z.Zhang, P.KBaumann, MG.Wensell, J. Bemholc, and RJ.Nemanich, 
Phys. Rev. B 50, 5803, (1994). 
6 W.E.Pickett, Phys. Rev. Lett. 73, 1664, (1994). 
7 N.Eimori, Y.Mori, A Hatta, T.lto, and AHiraki, Jpn. J. Appl. Phys. 33, 6312, (1994). 
8 C.Bandis and B.B.Pate, Phys. Rev. Lett., 74, 777, (1995). 
9 C.Bandis and B.B.Pate, Phys. Rev. B (to be publis~) 
10 RJ.Nemanich, P.KBaumann, and J.van der Weide, Proc. Applied Diamond Com., 
Gaithersburg, MD, August 1995, p.17 (NIST Special Publication 885) 
II RL.Bell, Negative Electron Affinity Devices (Clarendon Press, Oxford, 1973) 
12 Diamond: electronic properlies and applications, L.S.Pan and D.RKania, Eds. ( Kluwer 
Academic Publisher, 1990.) 
13 T.L.Bekker, J.ADayton, Jr., AS.Gilmour, Jr., I.L.Krainsky, MF. Rose, RRamashan, 
• D.File, and G.T.Mearini, IEEE IEDM Tech. Dig. 1992,949. 
14 G.T.Mearini, I.L.Krainsky, and lA.Dayton, Jr., Surf and Int. Anal. 21, 138 (1994). 
IS G.T.Mearini, I.L.Krainsky, Y.XWang, J.ADayton, Jr., RRamashan and MF. Rosem, 
Thin Solid Films 253, 151 (1994). 
7 
16 G.T.Mearini, I.L.Krainsky, IADayton, YaxinWang, Christian Zonnan, Jhon C.Angus, 
and R W.Hoffinan, Appl. Phys. Lett. 65,2702 (1994). 
17 T.Ando, AFowler, and F.Stern, Rev. Mod. Phys. 54,437, (1982) 
18 The narrow potential well is really a two-dimensional quantum well for electrons. At some 
ratio between the ~epth and the width of this nonsimmetrical well bound electron states in 
it do not exist. In this case the surface with effective NEA transfonns into the one with true 
NEA. 
19 E.MConwell, High Field Transport in Semiconductors, (Suppl. 9 to Sol. St. Phys. 
Academic Press, N.Y. ,1967.) 
20 There is an additional effective mechanism of energy relaxation of hot electrons in heavily 
p-doped semiconductors: electron - hole scattering. The estimate shows for our samples this 
relaxation time is also about 10 ·13 s. 
8 
FIGURE CAPDONS 
Fig.I. Energy distributions of secondary electrons N (E), for primary energies ~ = 200 eV 
( solid curve ), and E;, = 1500 e V (dashed curve). Data are nonnalized to the low 
energy peak maximum. 
Fig.2. Energy distributions of secondary electrons for as-grown diamond film - solid curve, after 
exposure of the film to the electron beam for up to 6 hours at the current density of 0.18 
mNcm2 and E;, = 3 keY - dotted curve, and after subsequent exposure of the same film 
to a molecular hydrodgen environment at the pressure of 5x10-6 Torr for 1 hour - dashed 
curve. Data are nonnalized to the low energy peak maximwn. 
Fig.3. The ratio, K, of the number of very low energy electrons in the peak ofN (E) to the total 
number of observed secondary electrons as a function of energy of primary electron beam.· 
FigA. Near the surface band diagram of a semiconductor for (a) true negative affinity and (b) 
effective negative affinity models. 
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Negative Electron Affinity Effect on the Surface of Chemical Vapor Deposited Diamond Polycrystalline Films

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