The electrical isolation of a n-type δ-doped layer embedded into undoped GaAs was studied using proton or helium ion bombardment. The threshold dose for isolation Dth of the δ-doped layer was found to be ≈2 times higher than that predicted for thick doped layers of similar carrier concentration. The thermal stability of the isolation, i.e., the persistence of sheet resistance Rs at values &gt; 109Ω/□ after subsequent thermal annealing, is limited to temperatures below 400°C. This temperature limit for the thermal stability Tsm is markedly lower than those observed in wider doped layers in which Tsm is ≅650°C. A previously isolated δ-doped layer presents p-type conductivity after annealing at temperatures &gt;600°C . © 1999 American Institute of Physics.751319171919Shubert, E.F., (1990) J. Vac. Sci. Technol. A, 8, p. 2980Daniltsev, V.M., Irin, I.V., Murel, A.V., Khrykin, O.I., Shashkin, V.I., (1994) Inorg. Mater. (Transl. of Neorg. Mater.), 30, p. 948Zrenner, A., Koch, F., Ploog, K., (1987) Inst. Phys. Conf. Ser., 91, p. 171Van Der Pauw, L.J., (1958) Philips Res. Rep., 13, p. 1De Souza, J.P., Danilov, I., Boudinov, H., (1997) J. Appl. Phys., 81, p. 650Ziegler, J.F., Biersack, J.P., Littmark, U., (1985) The Stopping and Range of Ions in Solids, 1. , Pergamon, OxfordDe Souza, J.P., Danilov, I., Boudinov, H., (1998) J. Appl. Phys., 84, p. 4757De Souza, J.P., Danilov, I., Boudinov, H., (1996) Appl. Phys. Lett., 68, p. 535De Souza, J.P., Danilov, I., Boudinov, H., (1998) Radiat. Eff. Defects Solids, 147, p. 109Grandidier, B., Stiévenard, D., Nys, J.P., Wallart, X., (1998) Appl. Phys. Lett., 72, p. 245

Boudinov H.

Danilov I.

Daniltsev V.M.

De Souza J.P.

Murel A.V.

Shashkin V.I.

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Electrical isolation of a silicon δ-doped layer in GaAs by ion irradiation
I. Danilov, J. P. de Souza, H. Boudinov, A. V. Murel, V. M. Daniltsev, and V. I. Shashkin 
 
Citation: Applied Physics Letters 75, 1917 (1999); doi: 10.1063/1.124870 
View online: http://dx.doi.org/10.1063/1.124870 
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/75/13?ver=pdfcov 
Published by the AIP Publishing 
 
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Electrical isolation of a silicon d-doped layer in GaAs by ion irradiation
I. Danilov,a) J. P. de Souza,b) and H. Boudinov
Instituto de Fı´sica, Universidade Federal do Rio Grande do Sul, 91501-970 Porto Alegre, R.S., Brazil
A. V. Murel, V. M. Daniltsev, and V. I. Shashkin
Institute of Microstructure Physics, Russian Academy of Sciences, 603600 Nizhny Novgorod, Russia
~Received 8 March 1999; accepted for publication 29 July 1999!
The electrical isolation of an-typed-doped layer embedded into undoped GaAs was studied using
proton or helium ion bombardment. The threshold dose for isolationD th of the d-doped layer was
found to be '2 times higher than that predicted for thick doped layers of similar carrier
concentration. The thermal stability of the isolation, i.e., the persistence of sheet resistanceRs at
values.109V/h after subsequent thermal annealing, is limited to temperatures below 400 °C. This
temperature limit for the thermal stabilityTsm is markedly lower than those observed in wider doped
layers in whichTsm is >650 °C. A previously isolated-doped layer presentsp-type conductivity
after annealing at temperatures.600 °C . © 1999 American Institute of Physics.
@S0003-6951~99!03639-6#
A semiconductor layer with a dopant distribution nar-
rower than the carrier de Broglie wavelength is known as the
d-doped layer. The use ofd doping in the semiconductor
device technology leads to significant improvements in the
ultrahigh frequency and optoelectronic performance of the
devices. Examples are the high transconductance GaAs-
metal–semiconductor field effect transistors and the tunable
GaAs laser using superlattices formed by alternatedn-type
andp-type d-doped layers.1
The d-doped layers are conveniently prepared by mo-
lecular beam epitaxy or metalorganic vapor phase epitaxy
~MOVPE!.2 The extremely sharp dopant profiles are pro-
duced by interruption of the epitaxial crystal growth, depo-
sition of dopant atoms on the semiconductor surface, and
resuming the semiconductor growth. The temperature is usu-
ally lower than 700 °C to minimize thermal redistribution of
the dopant profile. However, dopant profiles wider than a
single monolayer may result in consequence of the presence
of steps in the crystal surface during dopant deposition1 and
dopant diffusion during crystal growth. Another cause of re-
distribution is segregation of dopant atoms to the newly
grown material with a constant three-dimensional density,
closely corresponding to the solid solubility limit for the
given growth temperature.3
Considering thatd-doped structures are potentially ap-
plicable to the development of electronic and optoelectronic
devices and high performance integrated circuits, it seems to
be of interest to investigate the electrical isolation formation
in this structure. Up to now such study is lacking in the
literature.
In the present work we discuss the isolation formation in
the silicond-doped layer in GaAs, via light mass ion bom-
bardment and the behavior of the sheet resistance recovery
during postirradiation annealing cycles. We used samples
having a layered epitaxial structure deposited on a semi-
insulating ~SI! liquid encapsulated Czochralski GaAs sub-
strate of~100! orientation. The deposition method was the
MOVPE performed in a horizontal reactor with
Ga~CH3!3–AsH3–SiH4–H2 gas sources at atmospheric pres-
sure. First, an undoped GaAs epilayer with a thickness of 0.8
mm was deposited on the substrate and then the growth was
suspended. Then SiH4 was introduced in the reactor chamber
for the deposition of Si dopant atoms. After the formation of
the d-profile of Si, the epitaxial growth was resumed for the
deposition of a second undoped GaAs layer with a thickness
of 0.13 mm. The temperature of the reactor was maintained
in the range of 550–650 °C to minimize redistribution of the
Si atom concentration profile.
Figure 1 shows the carrier concentration depth profile
obtained from the capacitance-voltage (C-V) method. The
peak electron concentration is 431017 cm23 at the depth of
0.13mm. The undoped layers presented residualn-type con-
ductivity with carrier concentration of 131015 cm23, as de-
noted in Fig. 1 for depths below 0.45mm. Electrical mea-
surements in Van der Pauw devices4 provided values of sheet
resistanceRs , sheet electron concentrationns , and effective
a!Present address: Instituto de Fı´sica ‘‘Gleb Wataghin,’’ Universidade Es-
tadual de Campinas, 13081-970 Campinas, S.P., Brazil.
b!Electronic mail : souza@if.ufrgs.br
FIG. 1. Depth distribution of the electrons in the as-grown sample measured
by theC-V method.
APPLIED PHYSICS LETTERS VOLUME 75, NUMBER 13 27 SEPTEMBER 1999
19170003-6951/99/75(13)/1917/3/$15.00 © 1999 American Institute of Physics
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Hall mobility meff of 1300V/h, 1.65310
12 cm22 and 2900
cm2/V s, respectively.
The preparation and electrical characterization of the
samples were realized at the Institute of Microstructure Phys-
ics of the Russian Academy of Sciences, at Nizhny
Novgorod. The ion irradiation and annealing cycles were
performed at the Physics Institute of Porto Alegre, Brazil.
Two type of devices were used in the ion irradiation
experiments: rectangular resistors of 6 mm33 mm and Van
der Pauw devices of 6 mm36 mm. The ohmic contacts to
the devices were performed with indium, as described in
detail elsewhere.5
All the ion bombardments were performed at nominal
room temperature with1H1 at the energy of 50 keV or4He1
at 80 keV with the sample surface normal tilted 13° in re-
spect to the beam incidence direction to minimize ion chan-
neling effects. The ion energies were selected usingTRIM6
code simulation to place the peak of the deposited nuclear
energy profile ~0.39 mm! deeper than the depth of the
d-doped layer~0.13mm! .
A set of resistors was submitted to H1 or He1 irradia-
tion in accumulative dose steps. After each dose step theRs
value was measured without breaking the vacuum in the im-
plantation chamber.
The results of this experiment are depicted in curves~a!
and ~b!, respectively, of Fig. 2, for He1 and H1 irradiation
cases. As the ion dose is accumulated an increase ofRs from
its original value of 1.3 103 V/h to a maximum of>5
3109 V/h was observed. The increase of theRs results
from carrier trapping at the irradiation damage centers and
mobility degradation.5 The threshold dose for isolationD th
was determined to be 431013 cm22 for H1 and 331012 cm2
for He1.
Further dose accumulation beyondD th produces plateaus
in the curves. TheRs values of the plateaus indicate com-
plete isolation of thed-doped and matrix layers, since it
closely coincides with the sheet resistance of the SI GaAs
substrate underneath. In each curve of Fig. 2, the plateau
ends at a dose for which the damage concentration peak is
high enough for the onset of the conduction via the hopping
mechanism. Since the damage peak is located deeper than
the d-doped layer, the hopping conduction has no relation-
ship with the conduction in thed-doped layer. Further in-
crease of the dose leads to a progressive decrease ofRs , as
previously discussed.5
The pattern of the curves in Fig. 2 are quite similar to
those observed for thicker doped layers (>0.2 mm!, doped
either by ion implantation5 or during epitaxial growth.7 The
shift toward lower doses of curve~a! in respect to curve~b!
results from the higher nuclear energy deposited by He1
compared to H1.
The D th values obtained in thed-doped layer need fur-
ther analysis. As inferred in a previous work,8 the isolation
formation is governed essentially by carrier trapping at the
antisite defects and/or their related defect complexes formed
by replacement collisions in the cascades. The densities of
replacement collisions per incident ionnr at the depth of the
d-doped layer estimated byTRIM6 are 13106 cm21 for He1
of 80 keV and 73104 cm21 for H1 of 50 keV. The product
D thnr corresponds to the minimum volume concentrations of
replacement collisions just necessary for trapping all the car-
riers in thed-doped layer. Using the experimental values of
D th one obtainsD thnr of 3310
18 cm23 and 2.831018 cm23,
respectively, for He1 and H1 ion bombardments. Dividing
these values by the peak carrier concentration (431017
cm23) one obtains 7.5 for He1 and 7.0 for H1. These are
the numbers of replacement collisions required for the trap-
ping of a single electron from thed-doped layer. These val-
ues are'2 times higher than that required (3.16 .2) for the
isolation of implanted doped layers of similar peak carrier
concentration.9
The discrepancies in theD th values for thick doped
9 ~0.2
mm! andd-doped layers can be explained as follows. In the
cases of much wider doped layers the carrier distribution
closely coincides with the dopant distribution and there is
charge neutrality along all the doped region. In the present
case, the donor ions are confined within a layer thinner than
the full width at the half maximum of theC-V profile ~22
nm! while the electron profile spreads over a much wider
region ~see Fig. 1!. Consequently, a space charge is present
in the layer. Thus, even after trapping of all the electrons
from the donors the space charge is still present, since the
donor ions and the trapped electron distributions do not spa-
tially coincide. Electron–hole pairs thermally generated
within the space charge region should contribute to the re-
duction of the space charge. Those generated carriers which
are not captured by the traps should contribute to the electri-
cal conduction. Consequently, an additional dose has to be
accumulated in the sample to enhance the trap concentration
in order to attain the complete electrical isolation.
In another experiment resistors and Van der Pauw de-
vices were irradiated with H1 to doses of 2.031013 cm22
(0.5D th), 4.0310
13 cm22 (D th), 8.0310
13 cm22 (2D th),
and 4.031014 cm22 (10D th). Subsequently, they were an-
nealed in the temperature range from 100 to 700 °C in a
halogen lamp furnace. The annealing cycles were conducted
in argon atmosphere for a fixed time of 60 s. The experimen-
tal setup and annealing details were described elsewhere.5
Figure 3 presents the evolution ofRs in the resistors with
the annealing temperature. Thed-doped structure irradiated
to a dose 0.5D th presents a monotone decrease ofRs with the
annealing temperature in the range of 200–300 °C. A mini-
FIG. 2. Sheet resistance vs accumulated He1 dose at the energy of 80 keV
@curve ~a!# and H1 dose at the energy of 50 keV@curve ~b!#.
1918 Appl. Phys. Lett., Vol. 75, No. 13, 27 September 1999 Danilov et al.
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143.106.108.174 On: Wed, 17 Jun 2015 20:27:20
mumRs value of;10
4 V/h is reached and it is maintained
even after higher temperature annealing cycles~see curve 1
in Fig. 3!. The sheet electron concentration returned close to
the original value (ns'1.6310
12 cm22), while meff was
maintained seriously degraded~560 cm2/V s!.
The temperature limit for the thermal stabilityTsm in-
creases with the increase of the ion dose. Here,Tsm is con-
sidered as the annealing temperature for whichRs decreases
to 109 V/h. The values ofTsm are 250, 300, and 400 °C,
respectively for the doses ofD th ~curve 2!, 2D th ~curve 3!,
and 10D th ~curve 4!.
For the resistor irradiated to the dose of 10D th , a con-
tinuous increase ofRs in the temperature range of
100– 300 °C is noticed~see curve 4!. It is caused by the
decrease in the hopping conduction by virtue of the damage
annealing. The annealing of the trapping centers in the
d-doped layer manifests at temperatures above 350 °C.
An intriguing feature disclosed in the present study is the
lower thermal stability of the isolation in then-typed-doped
layer compared to that in ion-implanted or epitaxial thicker
layers, for which the dopant profile is much wider. In this
latter case, after irradiation to doses of>10D th the isolation
persists even after annealing cycles at>650 °C,5 which are
200– 300 °C higher than in thed-doped layer. This fact is
not clearly understood at present.
Another interesting phenomenon is the convection from
n-type top-type conduction observed in structures irradiated
to doses in excess ofD th and annealed at 600–700 °C. The
final Rs values are in the range of 10
5– 106 V/h with a
systematic increase ofRs with the irradiation dose~see
curves 2, 3, and 4 in Fig. 3!. The sheet hole concentration
and meff ranged from 0.35 to 1.8310
12 cm22 and 25–95
cm2/V s, respectively.
We argue that the conversion top-type conduction may
result after relocation of the Si atoms ind-doped layers10
from substitutional positions of the Ga sublattice to those of
As sublattice assisted by As vacancies. Very likely, mechani-
cal stresses present at thed-layer/crystal interfaces should
play a significant role in the stimulation of the process. One
should point out that such type conversion is absent in im-
planted Si doped GaAs layers submitted to similar irradiation
and annealing processes. Furthermore, nonirradiatedd-doped
structures retain then-type conduction up to the maximum
experimented temperature of 700 °C. In this latter case, only
a marginal increase ofRs by ;20% is denoted~see curve 0
in Fig. 3!.
In summary the electrical isolation of ann-type d layer
doped with silicon in GaAs was studied using H1 and He1
irradiation. TheD th values were found to be about two times
higher than those predicted for an implanted or epitaxial
layer of similar carrier concentration peak. This discrepancy
is explained taking into account the lack of space charge
neutrality along thed-doped layer. Besides trapping all the
carriers from the dopant atoms in thed-doped layer, the
space charge still persists. Due to the electric field present in
the layer, thermally generated carriers accumulate over the
layer and those carriers which were not trapped contribute to
the electric conduction. This fact is reflected in aD th higher
than predicted for doped layers where space charge neutrality
exists.
The thermal stability of the isolation was found to be
restricted to temperatures below 400 °C, which is'250 °C
lower than in wider doped layers. Furthermore, it was ob-
served that the irradiated-doped layers present conversion
from n-type to p-type conduction after annealing at
600– 700 °C.
This work was partially supported by Fundac¸ão de Am-
paro à Pesquisa do Estado do Rio Grande do Sul
~FAPERGS!, Fundac¸ão de Amparo a` Pesquisa do Estado de
São Paulo~FAPESP!, and Conselho Nacional de Pesquisas
~CNPq!.
1E. F. Shubert, J. Vac. Sci. Technol. A8, 2980~1990!.
2V. M. Daniltsev, I. V. Irin, A. V. Murel, O. I. Khrykin, and V. I. Shashkin,
Inorg. Mater.~Transl. of Neorg. Mater.! 30, 948 ~1994!.
3A. Zrenner, F. Koch, and K. Ploog, Inst. Phys. Conf. Ser.91, 171 ~1987!.
4L. J. Van der Pauw, Philips Res. Rep.13, 1 ~1958!.
5J. P. de Souza, I. Danilov, and H. Boudinov, J. Appl. Phys.81, 650
~1997!.
6J. F. Ziegler, J. P. Biersack, and U. Littmark,The Stopping and Range of
Ions in Solids~Pergamon, Oxford, 1985!, Vol. 1.
7J. P. de Souza, I. Danilov, and H. Boudinov, J. Appl. Phys.84, 4757
~1998!.
8J.P. de Souza, I. Danilov, and H. Boudinov, Appl. Phys. Lett.68, 535
~1996!.
9J.P. de Souza, I. Danilov, and H. Boudinov, Radiat. Eff. Defects Solids
147, 109 ~1998!.
10B. Grandidier, D. Stie´venard, J. P. Nys, and X. Wallart, Appl. Phys. Lett.
72, 2454~1998!.
FIG. 3. Evolution of the sheet resistance with annealing temperature in
samples irradiated with 50 keV protons to the doses of 0.5D th ~curve 1!, D th
~curve 2!, 2D th ~curve 3! and 10D th ~curve 4!. Curve 0 represents theRs
evolution in a non irradiated sample. In our samples,D th is 4310
13 cm22
for H1 irradiation at the energy of 50 keV.
1919Appl. Phys. Lett., Vol. 75, No. 13, 27 September 1999 Danilov et al.
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Electrical Isolation Of A Silicon δ-doped Layer In Gaas By Ion Irradiation

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Electrical Isolation Of A Silicon δ-doped Layer In Gaas By Ion Irradiation

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