Porous nanostructured InP: technology, properties, application
We prepared porous InP (100) substrates with a nanostructured surface relief on which InP epitaxial films were grown. The structure, morphological, and photoluminescence properties of nanostructured substrates and InP epilayers grown on them were studied. These InP epilayers grown on the porous and...
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Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України
2005
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| Zitieren: | Porous nanostructured InP: technology, properties, application / I. N. Arsentyev, A.B. Bobyl, S.G. Konnikov, I.S. Tarasov, V.P Ulin, M.V. Shishkov, N.S. Boltovets, V.N. Ivanov, A.E. Belyaev, R.V. Konakova, Ya.Ya. Kudryk, A.B. Kamalov, P.M. Lytvyn, E.P. Markovskiy, V.V. Milenin, R.A. Red’ko // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2005. — Т. 8, № 4. — С. 95-104. — Бібліогр.: 18 назв. — англ. |
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irk-123456789-1215722017-06-15T03:02:57Z Porous nanostructured InP: technology, properties, application Arsentyev, I. N. Bobyl, A.B. Konnikov, S.G. Tarasov, I.S. Ulin, V.P Shishkov, M.V. Boltovets, N.S. Ivanov, V.N. Belyaev, A.E. Konakova, R.V. Kudryk, Ya.Ya. Kamalov, A.B. Lytvyn, P.M. Markovskiy, E.P. Milenin, V.V. Red’ko, R.A. We prepared porous InP (100) substrates with a nanostructured surface relief on which InP epitaxial films were grown. The structure, morphological, and photoluminescence properties of nanostructured substrates and InP epilayers grown on them were studied. These InP epilayers grown on the porous and standard InP substrates were used to make microwave diodes. We showed the advantages of the diodes made on the porous substrates (over those made on the standard ones) caused by higher structural perfection of the InP epilayers grown on the porous substrates. 2005 Article Porous nanostructured InP: technology, properties, application / I. N. Arsentyev, A.B. Bobyl, S.G. Konnikov, I.S. Tarasov, V.P Ulin, M.V. Shishkov, N.S. Boltovets, V.N. Ivanov, A.E. Belyaev, R.V. Konakova, Ya.Ya. Kudryk, A.B. Kamalov, P.M. Lytvyn, E.P. Markovskiy, V.V. Milenin, R.A. Red’ko // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2005. — Т. 8, № 4. — С. 95-104. — Бібліогр.: 18 назв. — англ. 1560-8034 PACS 81.05.Rm http://dspace.nbuv.gov.ua/handle/123456789/121572 en Semiconductor Physics Quantum Electronics & Optoelectronics Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
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We prepared porous InP (100) substrates with a nanostructured surface relief on which InP epitaxial films were grown. The structure, morphological, and photoluminescence properties of nanostructured substrates and InP epilayers grown on them were studied. These InP epilayers grown on the porous and standard InP substrates were used to make microwave diodes. We showed the advantages of the diodes made on the porous substrates (over those made on the standard ones) caused by higher structural perfection of the InP epilayers grown on the porous substrates. |
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Article |
| author |
Arsentyev, I. N. Bobyl, A.B. Konnikov, S.G. Tarasov, I.S. Ulin, V.P Shishkov, M.V. Boltovets, N.S. Ivanov, V.N. Belyaev, A.E. Konakova, R.V. Kudryk, Ya.Ya. Kamalov, A.B. Lytvyn, P.M. Markovskiy, E.P. Milenin, V.V. Red’ko, R.A. |
| spellingShingle |
Arsentyev, I. N. Bobyl, A.B. Konnikov, S.G. Tarasov, I.S. Ulin, V.P Shishkov, M.V. Boltovets, N.S. Ivanov, V.N. Belyaev, A.E. Konakova, R.V. Kudryk, Ya.Ya. Kamalov, A.B. Lytvyn, P.M. Markovskiy, E.P. Milenin, V.V. Red’ko, R.A. Porous nanostructured InP: technology, properties, application Semiconductor Physics Quantum Electronics & Optoelectronics |
| author_facet |
Arsentyev, I. N. Bobyl, A.B. Konnikov, S.G. Tarasov, I.S. Ulin, V.P Shishkov, M.V. Boltovets, N.S. Ivanov, V.N. Belyaev, A.E. Konakova, R.V. Kudryk, Ya.Ya. Kamalov, A.B. Lytvyn, P.M. Markovskiy, E.P. Milenin, V.V. Red’ko, R.A. |
| author_sort |
Arsentyev, I. N. |
| title |
Porous nanostructured InP: technology, properties, application |
| title_short |
Porous nanostructured InP: technology, properties, application |
| title_full |
Porous nanostructured InP: technology, properties, application |
| title_fullStr |
Porous nanostructured InP: technology, properties, application |
| title_full_unstemmed |
Porous nanostructured InP: technology, properties, application |
| title_sort |
porous nanostructured inp: technology, properties, application |
| publisher |
Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
| publishDate |
2005 |
| url |
http://dspace.nbuv.gov.ua/handle/123456789/121572 |
| citation_txt |
Porous nanostructured InP: technology, properties, application / I. N. Arsentyev, A.B. Bobyl, S.G. Konnikov, I.S. Tarasov, V.P Ulin, M.V. Shishkov, N.S. Boltovets, V.N. Ivanov, A.E. Belyaev, R.V. Konakova, Ya.Ya. Kudryk, A.B. Kamalov, P.M. Lytvyn, E.P. Markovskiy, V.V. Milenin, R.A. Red’ko // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2005. — Т. 8, № 4. — С. 95-104. — Бібліогр.: 18 назв. — англ. |
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Semiconductor Physics Quantum Electronics & Optoelectronics |
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Semiconductor Physics, Quantum Electronics & Optoelectronics, 2005. V. 8, N 4. P. 95-104.
© 2005, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
95
PACS 81.05.Rm
Porous nanostructured InP: technology, properties, application
I.N. Arsentyev1, A.B. Bobyl1, S.G. Konnikov1, I.S. Tarasov1, V.P. Ulin1, M.V. Shishkov1, N.S. Boltovets2,
V.N. Ivanov2, A.E. Belyaev3, R.V. Konakova3, Ya.Ya. Kudryk3, A.B. Kamalov3, P.M. Lytvyn3,
E.P. Markovskiy3, V.V. Milenin3, R.A. Red’ko3
1A.F. Ioffe Physico-Technical Institute of RAN, 26, Politekhnicheskaya str., St.-Petersburg, 194021, Russia
E-mail: arsentyev@mail.ioffe.ru; phone: +7-812-247-91-34
2State Enterprise Scientific & Research Institute “Orion”,
8a, Eugene Pottier str.,03057 Kyiv, Ukraine
E-mail: bms@i.kiev.ua; phone: +380-44-456-05-48
3V. Lashkaryov Institute of Semiconductor Physics, NAS of Ukraine,
45, prospect Nauky, 03028 Kyiv, Ukraine
E-mail: konakova@isp.kiev.ua; phone: +380-44-525-61-82
Abstract. We prepared porous InP (100) substrates with a nanostructured surface relief
on which InP epitaxial films were grown. The structure, morphological, and
photoluminescence properties of nanostructured substrates and InP epilayers grown on
them were studied. These InP epilayers grown on the porous and standard InP substrates
were used to make microwave diodes. We showed the advantages of the diodes made on
the porous substrates (over those made on the standard ones) caused by higher structural
perfection of the InP epilayers grown on the porous substrates.
Keywords: porous InP, InP epilayers, photoluminescence, Schottky-barrier diode, Gunn
diode.
Manuscript received 19.09.05; accepted for publication 25.10.05.
1. Introduction
Indium phosphide attracts attention of the developers of
microwave active elements because this semiconductor
material has higher capabilities than gallium arsenide [1-
3]. They become apparent in the features of its band
structure that ensure higher (than in GaAs) values of
threshold field, maximal drift velocity and saturation
velocity in strong fields. Besides, its thermal
conductivity is over that of GaAs. Just these parameters
point at the possibility of InP application for fabrication
of microwave diodes (in particular, the Gunn diodes)
that could operate over the whole millimeter wavelength
range [1].
Besides, it is known that production of such diodes
requires uniform structurally perfect semiconductor
material with minimal density of strain concentrators [4].
To obtain indium phosphide meeting the above
requirements is a difficult physical and technological
problem and remains problematic up to now.
At the same time, in recent years a comparatively
novel approach to formation of structurally perfect
semiconductor films has been developed. In this
approach, textured and porous semiconductor growth
surfaces are used as a substrate material [5-11]. It should
be noted that the first report on application of porous
substrate (GaAs) for epitaxial film formation was made
in the work [5] that was carried out at the A.F. Ioffe
Physico-Technical Institute of RAN. And in [12] the
possibility for InP epitaxial film growth on porous InP
substrate was shown. In both cases the GaAs and InP
porous substrates take over elastic strains that appear in
the course of epitaxial film growth and further cooling
process. Another feature of such substrates is that their
developed surfaces serve as a high-capacity dislocation
drain. This fact offers a real possibility for improvement
of structural perfection of epitaxial layers.
In what follows, we consider obtaining the porous
indium phosphide and investigation of some of its
properties, as well as its application as a substrate
material in the course of formation of InP epitaxial
layers using the liquid-phase epitaxy (LPE) technique
and development of microwave diodes on their basis.
2. Preparation of porous indium phosphide
We used single-crystalline n-type indium phosphide
wafers as a substrate material. They had crystallographic
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2005. V. 8, N 4. P. 95-104.
© 2005, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
96
Fig. 1. Scheme of the facility for preparation of porous
substrates.
orientation (100) (with an accuracy of 20 arc seconds)
and were doped with tin (up to the concentration of
2×1018 cm−3). After mechanical grinding and lapping,
the wafers were exposed to chemical etching (to remove
a disordered surface layer). Pre-growth treatment was
performed according to the technique described in [13].
On the whole, the process of substrate preparation before
epitaxy involved the following operations:
• wafer surface degreasing in organic solvents (such as
ethanol, acetone, and carbon tetrachloride);
• treatment in polishing etchant (a mixture of HBr and
saturated water solution of K2Cr2O7, with volume
ratio of 1:1) at room temperature (operating side
down), with intense mixing for 30 s;
• thorough washing from the etchant in cold distilled
water;
• treatment in H3PO4 for 2 min (operating side up) to
remove Br ions from the surface;
• washing in distilled water;
• centrifugal drying of the substrate.
The substrates prepared in the above way were used for
both pore formation and growing of the reference
samples on the “rigid” substrate.
The porous material interlayers were formed in the
course of anodic electrochemical process (a scheme of
the facility for preparation of porous substrates is shown
in Fig. 1). The pulsed voltage (frequency of 2 Hz) was
applied to the cell according to the technique described
in [14].
The anode was a metallic tube, with one end
(operating) treated with pure indium to improve the
adhesion and contact properties, while another end was
connected to a jet pump to ensure underpressure. The
substrate prepared in the above way was placed on the
anode operating contact, with its back side oriented to
the contact. The anode was positioned so that the
substrate operating surface was in contact with the
electrolyte meniscus.
Water solutions of fluorides (NH4F + HF), chlorides
(NH4Cl + HCl) and bromides (KBr + HBr) served as
electrolytes. The pulse voltages applied to the cell
(7−10 V) were several times higher than the threshold
voltages of pore formation; the process duration was
20−25 s.
The porous layers obtained in the fluoride electrolyte
involved 30−50 nm channels of triangular section. These
channels penetrated and branched in the crystal bulk
along the direction 〈111〉 B.
The pores obtained in the above treatment modes (in
chloride and bromide electrolytes) had a trend to
propagate along the electric field (at a normal to the
crystal surface), without any preferred crystallographic
orientation. The pore size was close to that in the
fluoride layers. Such configuration of pores growing
from the surface (100) results from superposition of their
motions along the directions 〈111〉 A and 〈111〉 B.
The pore volume is filled with the products of partial
oxidation of InP. They appear as a result of InP reaction
with the anion components of solutions according to the
mechanism of nucleophilic substitution. The initial
composition of these products is InPX2+n (with n > 2 and
X = F, Cl, Br). In the course of pore formation, the
reaction products serve as a solid electrolyte that ensures
transport of the adsorbing anions to the reaction front at
the pore bottom. One cannot exclude subsequent
decomposition of the products left in the pores with
production of atomic phosphorus and trihalides (and,
maybe, more composite mixed halides) retaining the
In–P bonds.
The substrates formed in chloride electrolyte were
used for growth processes because they demonstrated
better repeatability.
3. Properties of porous indium phosphide
When studying porous sublayer configuration, we
applied scanning electron microscopy (SEM) and atomic
force microscopy (AFM). Shown in Fig. 2 is a SEM
pattern of a chip of porous InP (100) substrate obtained
using the above technology. One can see a clearly
pronounced structure of porous sublayer (which is a
system of pores and partition walls). The preferred
direction of pore formation is normal to the surface. A
branchy pore structure is distinct. One can see that the
density of channels is distributed very uniformly over
the whole formed sublayer, and the sublayer−crystal
bulk interface is practically flat. The pores make a
regular network whose horizontal (vertical) period is
130 (200) nm (Fig. 3). The pore size dispersion is rather
small.
The results of our AFM studies confirmed the above
fact and enabled us to determine an approximate law
of pore size distribution shown in Fig. 4. These facts
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2005. V. 8, N 4. P. 95-104.
© 2005, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
97
Fig. 2. A formed porous layer on the InP substrate (chip): a –
the porous sublayer-crystal bulk interface; b – pore geometry
(enlarged).
Fig. 3. A pattern of porous substrate surface.
Fig. 4. The law of pore size distribution.
(i) make it possible to state that the porous sublayer is a
practically uniform system that takes strains away, and
(ii) confirm the possibility of obtaining the hetero-
structures quality of which exceeds that of the traditional
LPE-grown structures.
Our studies of the substrate surface with a formed
porous sublayer showed that the initial singular face
surface was practically lost. It was replaced by a system
of vicinal hillocks. A microrelief of porous substrate is
shown in Fig. 5. Due to such surface structure (with
plenty of elementary steps), growth of epitaxial layer at
initial stages occurs according to the mechanism of
embedding of adsorbed components in the growth steps
available at the surface (without formation of individual
nuclei of a new phase). This reduces the activation
energy of nucleation. At the same time, this results in
increase of the defect density in the layer grown
immediately on the porous substrate, thus leading to the
necessity of InP buffer layer growth directly on the
porous substrate to ensure synthesis of a high-quality
heterostructure.
To reveal the features of the surface recombination
processes at the surfaces that restricted pore channels,
we performed photoluminescence (PL) investigations of
porous substrates without epitaxial layers. Fig. 6
demonstrates PL spectra of porous and continuous
indium phosphide. The spectral curve corresponding to
non-annealed InP sample with a formed porous sublayer
is of noise character. This fact, along with the SEM data
(Fig. 2), makes it possible to state that both the sample
surface and porous sublayer bulk are passivated with the
products of electrochemical oxidation.
The spectral curve of the sample exposed to low-
temperature annealing (2 h in the hydrogen atmosphere
at T = 250 °C) has a peak at lower energies (about 1.26–
1.27 eV) than the reference sample (continuous
substrate, bandgap Eg = 1.4 eV). This rather intense peak
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2005. V. 8, N 4. P. 95-104.
© 2005, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
98
Fig. 5. A porous substrate microrelief.
has the big (106 meV) half-width. A considerable peak
energy decrease can be explained (in the case of its
interband origin) by presence of a high density of
localized states in the bandgap (over the valence band
top Ev). This decreases the porous InP bandgap value.
Absence of the principal band in the PL spectrum of
porous InP may be due to absorption of radiation in the
porous sublayer bulk and re-emission via the formed
levels in the bandgap [15].
The X-ray experiments also revealed some features
inherent to the porous material itself. The X-ray
diffraction reflection curve of porous indium phosphide is
shown in Fig. 7. Along with a narrow peak (due to high-
degree structural perfection of the monocrystalline
substrate), the rocking curve has additional peaks. This
spectrum broadening results from interference of the main
radiation (reflected from crystallographic planes) and
radiation from the planes restricting the pore channels.
The latter planes have the properties of partially
polycrystalline material, because, while having, on the
whole, the same orientation, they are, however, the result
of pore motion along the directions 〈111〉 A and 〈111〉 B.
So, they demonstrate some misorientation relatively to
each other, as well as to the principal crystallographic
directions.
Fig. 6. PL spectra of porous InP (77 K): 1 – of the standard
“rigid” InP substrate; 2 – of porous InP (initial); 3 – of porous
InP after low-temperature annealing.
The effect of PL spectrum broadening is caused by
the fact that, near the InP reflection (004), the diffraction
curves for the samples grown on porous substrates are
superposition of the coherent components of diffracted
radiation from the monocrystalline part of InP substrate
and the components of the porous part of InP substrate
that is disordered by the pores. To separate the
contributions from the above components, we took the
rocking curves near the InP reflection (311). The
diffraction from porous substrates has two peaks: one
(with FWHM of about 54 arc seconds) is related to a
thin (< 100 nm) near-surface region of the porous InP
substrate, while another (with FWHM of about 300 arc
seconds) is related to the bulk part of the porous InP
substrate with a developed system of branchy pores. The
interval between the peaks is 540 arc seconds. This
corresponds to mismatch of lattice parameters of these
layers ∆a/a ~ 5×10−3 [12].
Fig. 7. A diffraction reflection curve of porous InP.
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2005. V. 8, N 4. P. 95-104.
© 2005, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
99
4. Properties of the epitaxial indium phosphide layer
It was necessary to apply a buffer layer when growing
the heterostructures. So, we shall consider briefly the
properties of these structures.
The quality of grown structures was controlled from
the PL data. Shown in Fig. 8 is PL spectrum of epitaxial
indium phosphide on the porous substrate at a high level
of excitation. It involves a broad low energy peak due to
the porous sublayer, similarly to the spectrum in Fig. 6.
Let us compare the properties of epitaxial layers
grown (under the same technological conditions) on
porous and continuous substrates. A comparison
spectrum is shown in Fig. 9. The parameters of the
corresponding layers are given in Fig. 10 and Table 1.
The principal PL band of the sample grown on the
porous substrate has a smaller half-width in comparison
with the traditional composition. This indicates higher
quality of the grown layer. The positions of the addi-
tional PL bands (caused by the impurity level) are the
same. However, a distinction in their intensities requires
further consideration. It seems that higher intensity of
the impurity peak is related to a big amount of the
background impurities in the crystallized layer [16].
Table 1. Parameters of the InP homoepitaxial layers
determined from the PL data.
Type of
substrate
Principal
PL band,
eV
Half-width of
the principal
peak, meV
Additional
PL band,
eV
Layer
thickness,
µm
continuous 1.41 16.6 1.375 1.68
porous 1.41 12.7 1.378 2.01
The difference in the layers thicknesses is due to
different growth mechanisms. For the porous layers, this
mechanism is close to the screw one that is typical of
atomically-rough faces, while for the continuous layers
the growth is proceeding layer-by-layer, because the face
is atomically-flat.
Fig. 8. PL spectrum of epitaxial InP on porous substrate with
grown buffer layer (77 K): porous substrate – hν = 1.28 eV;
∆E = 60 meV; epitaxial layer – hν = 1.40 eV; ∆E = 11 meV.
Fig. 9. PL spectra of InP epitaxial layers (77 K) grown on “rigid”
(1) and porous (2) substrate: 1 – hν = = 1.41 eV, ∆E = 16.6 meV;
2 – hν = 1.41 eV, ∆E = 12.7 meV.
Fig. 10. Chips: epitaxial InP layers on “rigid” (a) and “porous”
(b) substrates.
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2005. V. 8, N 4. P. 95-104.
© 2005, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
100
5. Properties of the Schottky-barrier diode structures
made on the porous InP substrates
The Au−TiB−n++-InP diodes (400 µm in diameter) were
made using the magnetron sputtering of TiBx and Au
followed with photolithography. The ohmic contacts
were formed using the magnetron sputtering of Ge, Au
and TiBx layers and their firing in the hydrogen
atmosphere at T = 500 °C for 1 min. We measured
I−V curves of the diode structures at room temperature;
from them, we calculated the Schottky barrier height ϕB
and ideality factor n. The diodes were made on the
standard “rigid” epiready InP (100) substrate
(concentration of tin impurity was ~ 2×1018 cm−3) and
three other substrates of the same type, but with a
formed porous layer (Table 2).
Table 2. Parameters of the porous InP layers.
Type
of substrate
Technology
of pore
formation
Porous layer
thickness,
µm
Note
InP A0 - - initial rigid
epiready
substrate
InP A1 chloride 8−10
InP A2 chloride 16−18
InP A3 fluoride 15
The histograms of ϕB and n values in each of the
above wafers obeyed the Gaussian law. It was found that
in the Schottky-barrier diodes (SBDs) made on the
porous substrates InP A2 and InP A3 (porous layer
thicknesses of which were 1.5−2 times large than those
of the InP A1 substrate) the average statistical spread of
ϕB (n) values was by 0.07 eV (0.1) less than in the
diodes made on the epiready and InP A1 substrates. This
fact confirms the SEM results that the porous sublayer is
practically uniform system.
Shown in Fig. 11 are the histograms of ϕB values on
the initial epiready and porous InP A2 substrates. From
them, one can also see that ϕB is distributed more
uniformly over the porous substrate than over the initial
epiready one. This seems to be related to removal of the
most imperfect areas in the InP bulk in the course of
pore formation.
6. Properties of the Schottky-barrier diode structures
made on epilayers grown on the porous InP
substrates
The SBD structures Au−TiBx−n-n+-n++-InP were made
as forward mesas. An area of wafer with such structures
is shown in Fig. 12. The TiBx and Au layers were
formed using the magnetron sputtering.
We used the Auger electron spectroscopy (AES) to
obtain component concentration depth profiles in the
barrier-forming TiBx−n-n+-n++-InP contacts formed on
the device structures that were prepared on the porous
and standard “rigid” InP substrates. It was found that, in
the first case (porous substrate), the TiBx−InP interface
was more abrupt than in the standard device structure.
This is in agreement with the barrier contact parameters
calculated from the experimental I−V curves [17].
Fig. 11. Histograms of ϕB on the initial epiready InP substrate
(a) and porous InP A2 substrate (b) (inset - chips of the
corresponding substrates).
Fig. 12. An area of technological wafer with Au−TiBx−n-n+-
n++-InP SBD structures made using the basic process of the
State Enterprise SRI “Orion” (mesa diameter is 40 µm).
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2005. V. 8, N 4. P. 95-104.
© 2005, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
101
Fig. 13. X-ray diffraction patterns of Au−TiBx−Au contacts:
1 – initial sample; 2, 3 and 4 - after RTA at T = 300, 400 and
600 °C, respectively.
The ohmic contacts to the diode structures of both
types were made on the basis of Au−TiBx−Au
metallization using the magnetron sputtering of the cor-
responding contact components onto the InP substrates
heated up to 200 °C. Shown in Fig. 13 are the X-ray
diffraction patterns taken for the initial sample and those
exposed to rapid thermal annealing (RTA) at T = 300,
400 and 600 °C. One can see that in the non-annealed
sample (curve 1 in Fig. 13) the gold film crystallites are
oriented in the plane (111), the TiBx layer is quasi-
amorphous, and AuIn2 traces are also observed. RTA at
T = 300 °C (curve 2) doesn’t change the phase compo-
sition of the system contacts. After RTA at T = 400 °C
(curve 3), the gold drops abruptly (while texturing
remains the same) and the amount of AuIn2 increases.
After RTA at T = 600 °C (curve 4), the signal from
pure gold decreases abruptly, and the amount of
polycrystalline AuIn2 phase (with predominant
crystallite orientation along (220)) increases. The crystal
structure of TiBx doesn’t change: it remains quasi-
amorphous after all RTA performed. These results
correlate with the Auger concentration depth profiles of
the contact components in the studied systems, taken
before and after RTA (see Fig. 14). Indeed, the profiles
practically did not change in the initial sample and that
annealed at 300 °C (Fig. 14a, b). Contrary to this, RTA
at T = 400 (600) °C led to considerable changes at the
Au−InP interface related to the intense phase formation.
Contact resistivity ρc in the initial sample and those after
RTA at 300 °C was ~ (1…2)×10−3 Ohm⋅cm2, while after
RTA at T = 400 °C it was ~ (0.8…2)×10−4 Ohm⋅cm2.
RTA at 600 °C increased ρc by more than an order of
magnitude in comparison with the value after RTA at
T = 400 °C.
The above results concerning the ρc variations can be
explained by taking into account formation of a deve-
loped relief on the semiconductor substrate surface due
to the intense phase formation (say, as a result of RTA at
T = 400 °C). It is known that this determines the non-
polar mechanism of current flow in the contact and leads
to both increase of the generation-recombination proces-
ses at current flow and formation of ohmic contacts.
0.0 0.2 0.4 0.6 0.8 1.0 1.2
d/d0
0.0 0.2 0.4 0.6 0.8 1.0 1.2
d/d0
0.0 0.2 0.4 0.6 0.8 1.0 1.2
d/d0
0.0 0.2 0.4 0.6 0.8 1.0 1.2
d/d0
Fig. 14. Component distribution profiles in Au−TiBx−Au
contacts: a – initial sample; b, c and d – after RTA at T = 300,
400 and 600 °C, respectively.
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2005. V. 8, N 4. P. 95-104.
© 2005, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
102
Since the Schottky barriers were made as forward mesas,
and the ohmic contact was formed to the back side of the
wafer, no essential distinctions in structure, phase
formation and ohmic contact properties were found
between the standard samples and those made on the
porous substrate. However, I−V curves of different
SBDs (made on epilayers grown on the standard “rigid”
and porous InP substrates) differ essentially. In both
cases, the main mechanism of current flow
corresponding to the forward branches of C−V curves is
thermal emission. This is evidenced by small values of
the ideality factor (n ~ 1.08…1.1 in both cases).
However, the SBDs made on the standard substrates
have an excess current component in the 0.03…0.2 V
forward bias (this component values differ considerably
in different samples). Contrary to this, no such
component exists in the samples made on the porous
substrates. This agrees with the previously published
results [11] of measurements of I−V curves for diodes
with Au−Ti Schottky barrier made on the InP epilayers
grown under similar conditions on porous substrates. An
excess current component was observed also on the back
C−V curves branch in the diode structures on the
standard substrates. It decreased essentially in the diode
structures formed on the epilayers grown on the porous
substrates. In this case, one can observe even avalanche
breakdown that corresponds to the impurity
concentration in the epistructure.
An analysis of C−V curves shows that the observed
changes of leakage currents seem to result from the
structure-impurity nonuniformities existing in the
standard device structures. However, in the course of
porous substrate formation, the most imperfect areas are
being etched-off. This increases the degree of structure
uniformity of formed InP growth surface, thus
improving the electrophysical properties of epilayers
(this is in agreement with the above PL data). The
typical forward and reverse branches of I−V curves of
the Au−TiBx−n-n+-n++-InP diode structures made on the
standard and porous InP substrates are given in Fig. 15.
7. The features of the electrical characteristics of the
Gunn diodes on epilayers formed on the porous InP
substrates
The Gunn diode models were made (according to the
basic process of the State Enterprise SRI “Orion”) on the
InP epilayers grown on the standard and porous
substrates, with ohmic contacts Au−TiBx−AuGe (the
contact resistivity was ≤10−5 Ohm⋅cm2). In both cases,
we used the epistructures in which the substrate
thickness was ~350 µm and impurity (tin) concentration
was ~ 2×1018 cm−3. The buffer layer had the thickness of
~3 µm and impurity concentration of
~ (5…6)×1017 cm−3. The active layer was 2 µm thick; its
impurity concentration was ~ 1016 cm−3. The mesa
diameter was ~ 60 µm. An area of technological wafer
with the Gunn diode chips is shown in Fig. 16.
Fig. 15. Forward (a) and reverse (b) branches of I−V curves for
Au−TiBx−n-n+-n++-InP SBDs made on the standard “rigid”
(curves 1 and 1′) and porous (curves 2 and 2′) substrates.
Curves 1 and 2 (1′ and 2′) – the forward (reverse) branches of
I−V curves.
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2005. V. 8, N 4. P. 95-104.
© 2005, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
103
Fig. 16. Part of the technological wafer with the Gunn diode
chips.
The diode chips were assembled in metal-quartz
packages. The measurement of microwave parameters in
the 118−150 GHz frequency range was made in a
rectangular (0.8×1.6 mm) waveguide oscillator. The
maximal power output at a frequency of 120 GHz (the
second harmonic) was ~ 3.5…4 mW. It was found also
that the oscillators with the diodes made on the porous
substrate began to oscillate at operating voltages close to
the threshold ones. And in the ordinary Gunn oscillators
made on the standard InP substrates, the generation of
microwave oscillations appears at operating voltages
20 % over the threshold ones.
8. Conclusion
The results of our structural, photoluminescence and
electrophysical investigations of (i) the nanostructured
InP substrates, (ii) epilayers grown on them, and (iii)
microwave diodes formed on these epilayers have
demonstrated a real possibility of development of
structurally perfect InP epitaxial layers of device quality.
Acknowledgements
The work was made in the framework of the
Russia−Ukraine Program on Nanophysics and Nano-
electronics.
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