The influence of Cr concentration on time resolution of GaAs detectors
Investigated in this work were the influence of Cr dopant concentration and technological conditions of doping on photoconductivity (PhC) kinetics, dependence of PhC signal magnitude on voltage applied as well as the dynamic range of a photodetector based on semi-insulating GaAs:Cr. PhC relaxation w...
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Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України
2006
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| Цитувати: | The influence of Cr concentration on time resolution of GaAs detectors / L.L. Fedorenko, L.F. Linnik, L.G. Linnik, M.M. Yusupov, E.A. Solovyov, E. Sirmulis // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2006. — Т. 9, № 2. — С. 92-94. — Бібліогр.: 6 назв. — англ. |
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irk-123456789-1214252017-06-15T03:03:55Z The influence of Cr concentration on time resolution of GaAs detectors Fedorenko, L.L. Linnik, L.F. Linnik, L.G. Yusupov, M.M. Solovyov, E.A. Sirmulis, E. Investigated in this work were the influence of Cr dopant concentration and technological conditions of doping on photoconductivity (PhC) kinetics, dependence of PhC signal magnitude on voltage applied as well as the dynamic range of a photodetector based on semi-insulating GaAs:Cr. PhC relaxation was measured using a broadband system of registration in the picosecond pulse range, which is based on the oscillograph C7-19, CCD camera and personal computer. Mechanisms of recombination that influence on fast and slow components of the PhC signal were studied. The shortest time of PhC relaxation τ ~ 2.10⁻¹⁰ s was observed in GaAs:Cr samples for the chromium dopant concentration NCr ~ 3.10¹⁷ cm−3. We have found a linear increase of the fast component of PhC with the intensity of excitation as well as a weak dependence at small levels and saturation at the high ones of excitation for the PhC slow component. 2006 Article The influence of Cr concentration on time resolution of GaAs detectors / L.L. Fedorenko, L.F. Linnik, L.G. Linnik, M.M. Yusupov, E.A. Solovyov, E. Sirmulis // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2006. — Т. 9, № 2. — С. 92-94. — Бібліогр.: 6 назв. — англ. 1560-8034 PACS 73.50.Pz, 42.79.Pw, 61.72.Vv http://dspace.nbuv.gov.ua/handle/123456789/121425 en Semiconductor Physics Quantum Electronics & Optoelectronics Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
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Investigated in this work were the influence of Cr dopant concentration and technological conditions of doping on photoconductivity (PhC) kinetics, dependence of PhC signal magnitude on voltage applied as well as the dynamic range of a photodetector based on semi-insulating GaAs:Cr. PhC relaxation was measured using a broadband system of registration in the picosecond pulse range, which is based on the oscillograph C7-19, CCD camera and personal computer. Mechanisms of recombination that influence on fast and slow components of the PhC signal were studied. The shortest time of PhC relaxation τ ~ 2.10⁻¹⁰ s was observed in GaAs:Cr samples for the chromium dopant concentration NCr ~ 3.10¹⁷ cm−3. We have found a linear increase of the fast component of PhC with the intensity of excitation as well as a weak dependence at small levels and saturation at the high ones of excitation for the PhC slow component. |
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Fedorenko, L.L. Linnik, L.F. Linnik, L.G. Yusupov, M.M. Solovyov, E.A. Sirmulis, E. |
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Fedorenko, L.L. Linnik, L.F. Linnik, L.G. Yusupov, M.M. Solovyov, E.A. Sirmulis, E. The influence of Cr concentration on time resolution of GaAs detectors Semiconductor Physics Quantum Electronics & Optoelectronics |
| author_facet |
Fedorenko, L.L. Linnik, L.F. Linnik, L.G. Yusupov, M.M. Solovyov, E.A. Sirmulis, E. |
| author_sort |
Fedorenko, L.L. |
| title |
The influence of Cr concentration on time resolution of GaAs detectors |
| title_short |
The influence of Cr concentration on time resolution of GaAs detectors |
| title_full |
The influence of Cr concentration on time resolution of GaAs detectors |
| title_fullStr |
The influence of Cr concentration on time resolution of GaAs detectors |
| title_full_unstemmed |
The influence of Cr concentration on time resolution of GaAs detectors |
| title_sort |
influence of cr concentration on time resolution of gaas detectors |
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Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
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2006 |
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http://dspace.nbuv.gov.ua/handle/123456789/121425 |
| citation_txt |
The influence of Cr concentration on time resolution of GaAs detectors / L.L. Fedorenko, L.F. Linnik, L.G. Linnik, M.M. Yusupov, E.A. Solovyov, E. Sirmulis // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2006. — Т. 9, № 2. — С. 92-94. — Бібліогр.: 6 назв. — англ. |
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Semiconductor Physics Quantum Electronics & Optoelectronics |
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Semiconductor Physics, Quantum Electronics & Optoelectronics, 2006. V. 9, N 2. P. 92-94.
© 2006, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
92
PACS 73.50.Pz, 42.79.Pw, 61.72.Vv
The influence of Cr concentration
on time resolution of GaAs detectors
L.L. Fedorenko1, L.F. Linnik1, L.G. Linnik1, M.M. Yusupov1, E.A. Solovyov1, E. Sirmulis2
1V. Lashkaryov Institute of Semiconductor Physics, NAS of Ukraine,
45, prospect Nauky, 03028 Kyiv, Ukraine
Phone: +380 (44) 525-64-77, +380 (44) 525-18-75, +380 (44) 525-59-39;
e-mail: lfedor@isp.kiev.ua, ny@isp.kiev.ua, slv@isp.kiev.ua
2Semiconductor Physics Institute, 11, Gostauto, Vilnius Lt-01108, Lithuania;
E-mail: sirmulis@delfi.lt
Abstract. Investigated in this work were the influence of Cr dopant concentration and
technological conditions of doping on photoconductivity (PhC) kinetics, dependence of
PhC signal magnitude on voltage applied as well as the dynamic range of a photodetector
based on semi-insulating GaAs:Cr. PhC relaxation was measured using a broadband
system of registration in the picosecond pulse range, which is based on the oscillograph
C7-19, CCD camera and personal computer. Mechanisms of recombination that
influence on fast and slow components of the PhC signal were studied. The shortest time
of PhC relaxation τ ~ 2.10−10 s was observed in GaAs:Cr samples for the chromium
dopant concentration NCr ~ 3.1017 cm−3. We have found a linear increase of the fast
component of PhC with the intensity of excitation as well as a weak dependence at small
levels and saturation at the high ones of excitation for the PhC slow component.
Keywords: GaAs, fast detector, picosecond YAG-laser, X-, γ-radiation, photo-
conductivity.
Manuscript received 07.02.06; accepted for publication 29.03.06.
1. Introduction
Diagnostic of the pulse radiation process requires the
development of ultra-fast X-ray, γ- and laser radiation
detectors with nano- and picosecond times of relaxation.
The detectors must precisely reproduce a pulse shape
and measure its magnitude with well linearity within few
orders. For such a purpose, used are semiconductor
compounds with a high mobility of intrinsic charge
carriers and high sensitivity to X-ray and γ-radiation, e.g.
GaAs, InPCdTe, CdZnTe [1, 2]. Subnanosecond time
resolution can be realized using detectors based on high
resistive GaAs, InP [3, 4]. A short drop of relaxation can
be obtained by decreasing the lifetime of photocarrier,
e.g., Cr doping, neutron irradiation, or low temperature
growing GaAs [5]. The best semi-insulating GaAs was
grown with Cr doping concentration NCr ~ 1⋅1016 cm−3
and background impurities concentration Nph ≤
≤ 5⋅1015 cm−3. Nevertheless, a short relaxation drop of
the photoresponse usually takes place in the long-term
component. The latter could be conditioned by the
influence of both type charge carriers sticking to shallow
centers, bending of the energy band in semi-insulating
GaAs, low hole mobility or carrier diffusion in the field
range. According to the well-known relation
ne ΔΔ μσ ~ , (1),
where τβkIn =Δ , σΔ − detector photoconductivi-
ty, nΔ − concentration, μ − mobility, τ − lifetime of a
photocarrier; σΔ decreases with τ . For the pulse shape
to be reproduced, the criterion rττ << (where rτ is the
characteristic time of process relaxation) should be
satisfied. At the same time, the magnitude of τ must be
sufficient to adjust the detector sensitivity. The aim of
the paper was to determine the optimal degree of crystal
doping for fast ( )ps200≤τ and highly sensitivity
detectors based on GaAs doped with chromium.
2. Experimental
To achieve the purpose mentioned above, the set-up for
photoconductivity (PhC) kinetics measurements in
semiconductor crystals under pulse laser excitation was
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2006. V. 9, N 2. P. 92-94.
© 2006, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
93
designed and adjust. The block scheme of the set-up is
given in Fig. 1. The set-up consists of the YAG:Nd+3
laser with free mode-locking (1), microstrip circuit of
photoconductivity signal detection (2), broad-band
oscillograph (3), CCD camera (4), and PC with frame
grabber (5). The train of picosecond pulses with the
duration ps37=pt (tp being much less than the
characteristic nonstationary relaxation time τnst) and
~16 ns pulse intervals was obtained using passive solid-
state shutter based on polystyrene dye. The synch-
ronization scheme of a trigger laser with oscillograph
scanning and trigger of frame grabber was developed.
We used semi-insulating GaAs samples with the various
Cr concentrations NCr ~ 1715 103...101 ⋅⋅ cm−3, which
were grown by the Czochralsky technique: using fluxing
agent and horizontally oriented crystallization. To
provide compensation, the GaAs:Cr crystals were grown
in As vapor that ensure an electron type of conductivity
when ρ ~ 109 Ohm.cm. The influence of the Cr dopant
concentration on photoconductivity drop, the
dependence of the PhC signal magnitude on the voltage
applied and the dependence of the dynamic range of
GaAs:Cr photodetectors on the radiation intensity were
investigated.
The samples possessed the bar-like shape with
dimensions 2×2×5 mm. Their ohmic contacts were
formed on the longer opposite sides, see insert in Fig. 1.
Conductors had minimal length and were made of thin
cooper foil μm10≤δ to reduce parasitic reactive
resistance and skin-effect conjunctive. The samples were
mounted in specially developed microstrip circuit
adjusted with the impedance Ohm50=ρ .
Typical oscillograms of GaAs:Cr PhC are shown in
Fig. 2. Fig. 3 shows the dependence of GaAs
photodetector signal magnitude on the voltage applied.
Fig. 1. Block-scheme of the pulse PhC registration set-up with
the subnanosecond duration of excitation: 1 – laser with free-
mode locking, 2 – detecting semiconductor element, 3 –
oscillograph C7-19, 4 – CCD camera GBC 500E, 5 – PC with
video card Rage Fury Pro 32M, 6 – photodiode FD-256, 7 –
lens, NG – filters.
a
b
c
Fig. 2. Typical oscillograms of PhC pulses of GaAs samples
with the Cr concentration NCr: ~1.1015 cm−3 (a, b);
~ 3.1017 cm−3 (c, d).
3. Results and discussion
We found that, with increasing the dopant concentration
NCr, the magnitude of the long-term component of PhC
relaxation is decreased, which is indicative of the
extension of the fast recombination channel contribution.
The highest rate τst/τnst > 10 (τst − stationary relaxation
time) was observed at NCr ≅ 3⋅1017 cm−3 and responds
consequently to the short relaxation time of PhC τ ≅
2⋅10−10 s. The fact of increasing the Cr concentration
does not simultaneously lead to the relaxation time
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2006. V. 9, N 2. P. 92-94.
© 2006, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
94
0 1x103 2x103 3x103 4x103 5x103 6x103 7x103 8x103
0
50
100
150
200
250
300
350
400
P
ho
to
co
nd
uc
tiv
ity
, V
Electrical strengh, V/cm
Fig. 3. Dependence of GaAs:Cr photodetector signal
magnitude on voltage applied.
decrease. The latter is caused by self-compensation of
GaAs:Cr, which is specified by the system of energy
levels in the bandgap. As a result of enlargement of the
electron capture cross-section on deeper levels (placed
close to the medium of the bandgap) that appear after
doping, the probability of electron sticking on shallow
centers, responsible for nonstationary drop of PhC,
decreases but hole sticking still remains, and that is
proved by the long-term component of PhC relaxation
drop with the temperature decrease down to 150 K.
Taking into account the fact that the time resolution of
semi-insulating GaAs photoresistivity detectors is
determined by the lifetime of nonstationary electrons:
0
1
SVN
=τ , (2),
where 0N = N – N– is the quantity of free sites in
impurity centers that can capture electrons, S is the
capture cross-section of electrons by impurity center, V
is the electron velocity. As one can see [3], 0N depends
not only on the impurity concentration N but on the
degree of the dopant concentration, too. In the case of
compensation centers with the concentration Ncomp, when
almost linear growth together with the dopant
concentration NCr takes place, the following rate
dependence 1/τ ~ NCr [6] exists. In the case of pure
compensation and N – N– ≈ N ≈ 0N , when N is close to
the degree of Cr solubility in GaAs, the lifetime can
reach a few picoseconds. The detail mechanism of the
compensation effect influence on nonstationary
component of PhC has not yet understood and requires
special investigations.
The linear dependence of the PhC signal magnitude
on electric field applied (Fig. 3) testify to the possibility
to increase the detector photosensitivity, if it is necessary
up to the electric breakdown threshold Ebreak,
kV/cm30break ≤E (for some samples the value of Ebreak
was 50 kV/cm).
Our investigation of the stationary photoconductivity
magnitude ΔvPhC dependence on the laser radiation
intensity I showed a linear growth of ΔvPhC in the range of
six orders of I variation (GaAs:Cr samples with NCr ≥
1.1017 cm−3). Nonstationary component of PhC
approached to saturation for maximal I. An optimal ratio
of fast and slow components (10:1) and photosensitivity
was obtained when NCr ≅ 3.1017 cm−3 at the pulse laser
intensity starting with І ~ 3⋅1022 cm−2s−1 until 1⋅1026cm−2s−1
(tp = 37 ps, λ = 1.06 μm).
4. Conclusions
• It was found that, in semi-insulating GaAs:Cr,
NCr = 3⋅1017 cm−3 ensure the relaxation time of
photoconductivity τ ~ 2⋅10−10 s that corresponds to the
bandwidth of the registration system Δf = 5 GHz and is
optimal for fast detector development with a
corresponding time of detection.
• We found a linear growth of the fast component
with the intensity increase and approachment to
saturation of the slow component at high levels of
excitation, which is connected with a full recharge of
sticking centers at the laser intensity increase as well as
growth of the chromium dopant concentration up to
1015… 3⋅1017 cm−3.
• The possibility to control the behavior of PhC
relaxation components in GaAs:Cr crystals by adjusting
degree of doping and compensation has been shown.
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