We report measurements of barrier heights on

AI-AIN-Mg thin-film structures as a function of

applied voltage and insulator thickness. These results

are in disagreement with currently accepted

theories based upon image potential and/or field

penetration of the electrodes

Lewicki, G. W.

Mead, C. A.

Caltech Authors - Main

VOLTAGE DEPENDENCE OF BARRIER HEIGHT IN AlN TUNNEL JUNCTIONS
G. W. Lewicki and C. A. Mead 
 
Citation: Applied Physics Letters 8, 98 (1966); doi: 10.1063/1.1754505 
View online: http://dx.doi.org/10.1063/1.1754505 
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Volume 8, Number 4 APPLIED PHYSICS LETTERS 15 February 1966 
10C. R. Crowell, H. B. Shore, and E. E. LaBate,]. Appl. Phys. 
36,3843 (1965). 
II E. Guth and C. J. Mullin, Phys. Rev. 59, 575 (1941). 
t2C. E. Drumheller and F. E. Card, "Photoemission Through 
Thin Film Dielectrics," AFAL-TR-65-167, Wright-Patterson AFB, 
Ohio, June 1965. 
13 See, for example, T. E. Hartman, J. Appl. Phys. 35, 3283 
(1964) and references contained therein. 
VOLTAGE DEPENDENCE OF BARRIER HEIGHT IN AIN 
TUNNEL JUNCTIONS* 
(photoemission; room temperature; E) 
We report measurements of barrier heights on 
AI-AIN-Mg thin-film structures as a function of 
applied voltage and insulator thickness. These re-
sults are in disagreement with currently accepted 
theories based upon image potential and/or field 
penetration of the electrodes. 
The structures were fabricated by reacting a 
freshly evaporated semitransparent aluminum film 
in a high-purity N2 glow discharge at approximately 
0.2 torr and subsequently evaporating O.4-mm 
square Mg counterelectrodes of several thousand 
A thickness. The geometry of the discharge was ar-
ranged so as to create a gradient in the AIN thick-
ness over the I-in. length of the sample. Thus 
samples between 35- and 90-A thickness were avail-
able on a single substrate. Thicknesses were de-
termined from the sample capacitance assuming a 
dielectric constant of 8.5. The capacitance increased 
by only about 1% with dc bias up to 106V /cm in 
either direction. Photoresponse was measured with 
the beam from the monochromator incident on the 
Al electrode through the glass substrate. Values of 
the barrier height were determined from plots of 
the square root of the photoresponse vs photon 
energy, as previously reported.1.2 With no dc bias 
applied to the sample a net photocurrent was ob-
served corresponding to emission of electrons from 
the Al electrode. 
The dependence of the zero bias barrier height 
on insulator thickness is shown in Fig. 1. Except for 
a small decrease in barrier height due to image 
force effects at thicknesses much lower than those 
used here, no dependence of this magnitude is 
expected on the basis of present theoretical models. 
By applying positive bias to the Mg electrode, 
*Supported in part by the Office of Naval Research and the 
International Telephone and Telegraph Co. 
98 
G. W. Lewicki and C. A. Mead 
California Institute of Technology 
Pasadena, California 
(Received 4 November 1965; in final form 20 January 1966) 
photoemission from the Al could be enhanced and 
the barrier height could be measured as a function 
of applied dc voltage. With negative bias applied 
to the Mg, the form of the superposed photocurrents 
was such that accurate determinations of the barrier 
heights was possible only over a very limited range 
of experimental conditions. The results of the Mg 
positive measurements are shown in Fig. 2. On the 
basis of an image force model, only a very weak 
dependence on voltage is expected. It appears that 
with thinner samples at higher biases, the expected 
behavior is approached. However over most of the 
range of experimental conditions, large deviations 
are apparent. It should be noted that although the 
I-V characteristics of the samples over the entire 
range were similar to others widely reported in the 
literature, only in very thin samples «50A) was 
accurate quantitative agreement with tunneling 
theory obtainable. In this case the dependence upon 
thickness, voltage, and temperature were all consis-
3.o,----,---,-__ -.-__ ...-__ -.-__ .-_--, 
2.8 
2.6 
o 
2.4 
2.2 
2.0 
1.4 
1.2'-----:40=-----+...0,-----:80~-~70:-----+..80,------:OO~----,!,OO 
THICKNESS (A) 
Fig. I. Variation of zero bias barrier height (in eV) of AI-
AlN.Mg structures with AIN thickness. 
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Volume 8, Number 4 APPLIED PHYSICS LETTERS 15 February 1966 
2.8 .----r---..-----r--~-"""T--_r_-__. 
2.6 
2.4 
2.2 
2.0 
CPa 
eeV) 
1.8 
1.4 
1.2 
o .2 .4 .6 .8 1.0 1.2 1.4 
vev) 
Fig. 2. Variation of barrier height with applied bias (Mg+) 
for several AIN thicknesses. 
tent with the classical tunneling formulation indi-
cating barriers of approx. 1.7 eV. However, for 
slightly thicker samples, deviations in the I-V char-
acteristic and in the temperature dependence were 
apparent in just the range where the variations in 
barrier height become pronounced. These anoma-
lies persisted even to 4.2°K. The origin of these 
effects is not understood at this time; however it is 
clear that any serious attempt to fit experimental 
data with tunneling theory must be accompanied 
by independent experimental evaluation of all 
relevant parameters. Although these measurements 
were made on one specific system, indications are 
that similar effects exist in Al20 3 (ref. 3) and perhaps 
other materials as well. 
1 A. Braunstein, M. Braunstein, G. S. Picus, and C. A. Mead, 
"Photoemissive Determination of Barner Shape in Tunnel Junc-
tions," Phys. Rev. Letters 14, 7 (1965). 
2W. G. Spitzer and C. A. Mead, "Barner Height Studies on 
Metal-Semi-Conductor Systems,"]' Appl. Phys.'4, 3061-69 (1963). 
3 A. Braunstein et aI., previous Letter. 
RADIATIVE RECOMBINATION LIFETIMES IN LASER-EXCITED SILICON 
(photoconductivity; photoluminescence; radiative decay life-
time < 2 X 10-8 sec; possible method for study of laser pulse 
shape; E) 
Thermal broadening and the low efficiency of 
photoluminescence in silicon has prevented the 
identification of the mechanism of radiative recom-
bination at room temperature. Sinusoidal excitation 
showed that band-to-band radiative recombination 
was negligible compared to other supralinear radia-
tive processes.1 
Additional information concerning these pro-
cesses was obtained by utilizing very high excitation 
intensities, correlating the emission with photo-
conductivity, and by measuring the lifetimes for the 
decay of radiative recombination. 
The data obtained suggest that, at high excitation 
levels, the dominant donors or acceptors in n- and 
p-type silicon respectively act as efficient radiative 
recombination centers. 
Although the starting material (75 to 800 O-cm 
N. N. Winogradoff and H. K. Kessler 
Electron Devices Section 
National Bureau of Standards 
Washington, D. C. 
(Received 20 December 1965; in final form 25 January 1966) 
n-type, Sb-doped and p-type B-doped Si) had life-
times of the order of 10-3 sec as determined by the 
photoconductive decay methods,2 photolumines-
cent experiments with focused xenon flash tubes 
indicated that the radiative decay lifetimes were less 
than 10-5 sec (a limit imposed by a prolonged tail 
in the xenon tube output). 
Since the self-quenching properties of epitaxially 
formed GaAs lasers3 prorluced well-defined sharp 
cut-off light pulses having base widths of 2 x 10-8 
sec and peak powers of 7.5 W when energized with 
50-A, 100-nsec current pulses at room temperature, 
such a diode was used to excite the silicon. 
To permit the direct comparison of photocon-
ductance and photoluminescent decay properties, 
the Si samples were mounted in an electrolytic 
cell of the type described in ref. 1. 
99 
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English

Voltage Dependence of Barrier Height in AIN Tunnel Junctions

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Voltage Dependence of Barrier Height in AIN Tunnel Junctions

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