Noise spectra and dark current investigations in n⁺-p-type Hg₁₋xCdxTe (x ≈ 0.22) photodiodes
The dark current and noise spectra were investigated in Hg₁₋xCdxTe (x≅0.22) photodiodes at zero and low reverse bias voltages. The photodiodes were prepared by boron implantation into LPE films. The 1/f noise is proved to be correlated with tunneling current via the deep defect states in the gap at...
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Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України
1999
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irk-123456789-1198792017-06-11T03:02:32Z Noise spectra and dark current investigations in n⁺-p-type Hg₁₋xCdxTe (x ≈ 0.22) photodiodes Ivasiv, Z.F. Sizov, F.F. Tetyorkin, V.V. The dark current and noise spectra were investigated in Hg₁₋xCdxTe (x≅0.22) photodiodes at zero and low reverse bias voltages. The photodiodes were prepared by boron implantation into LPE films. The 1/f noise is proved to be correlated with tunneling current via the deep defect states in the gap at low reverse biases U≤0.1 V. In the photodiodes, where the tunneling current is found to be dominating, the 1/f noise is observed up to frequencies 10⁴ Hz. The decrease of tunneling current results in the decrease of 1/f noise. 1999 Article Noise spectra and dark current investigations in n⁺-p-type Hg₁₋xCdxTe (x ≈ 0.22) photodiodes / Z.F. Ivasiv, F.F. Sizov, V.V. Tetyorkin // Semiconductor Physics Quantum Electronics & Optoelectronics. — 1999. — Т. 2, № 3. — С. 21-25. — Бібліогр.: 16 назв. — англ. 1560-8034 PACS: 85.60.D, 07.57.K, 85.60.G, 73.40 http://dspace.nbuv.gov.ua/handle/123456789/119879 en Semiconductor Physics Quantum Electronics & Optoelectronics Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
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The dark current and noise spectra were investigated in Hg₁₋xCdxTe (x≅0.22) photodiodes at zero and low reverse bias voltages. The photodiodes were prepared by boron implantation into LPE films. The 1/f noise is proved to be correlated with tunneling current via the deep defect states in the gap at low reverse biases U≤0.1 V. In the photodiodes, where the tunneling current is found to be dominating, the 1/f noise is observed up to frequencies 10⁴ Hz. The decrease of tunneling current results in the decrease of 1/f noise. |
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Article |
| author |
Ivasiv, Z.F. Sizov, F.F. Tetyorkin, V.V. |
| spellingShingle |
Ivasiv, Z.F. Sizov, F.F. Tetyorkin, V.V. Noise spectra and dark current investigations in n⁺-p-type Hg₁₋xCdxTe (x ≈ 0.22) photodiodes Semiconductor Physics Quantum Electronics & Optoelectronics |
| author_facet |
Ivasiv, Z.F. Sizov, F.F. Tetyorkin, V.V. |
| author_sort |
Ivasiv, Z.F. |
| title |
Noise spectra and dark current investigations in n⁺-p-type Hg₁₋xCdxTe (x ≈ 0.22) photodiodes |
| title_short |
Noise spectra and dark current investigations in n⁺-p-type Hg₁₋xCdxTe (x ≈ 0.22) photodiodes |
| title_full |
Noise spectra and dark current investigations in n⁺-p-type Hg₁₋xCdxTe (x ≈ 0.22) photodiodes |
| title_fullStr |
Noise spectra and dark current investigations in n⁺-p-type Hg₁₋xCdxTe (x ≈ 0.22) photodiodes |
| title_full_unstemmed |
Noise spectra and dark current investigations in n⁺-p-type Hg₁₋xCdxTe (x ≈ 0.22) photodiodes |
| title_sort |
noise spectra and dark current investigations in n⁺-p-type hg₁₋xcdxte (x ≈ 0.22) photodiodes |
| publisher |
Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
| publishDate |
1999 |
| url |
http://dspace.nbuv.gov.ua/handle/123456789/119879 |
| citation_txt |
Noise spectra and dark current investigations in n⁺-p-type Hg₁₋xCdxTe (x ≈ 0.22) photodiodes / Z.F. Ivasiv, F.F. Sizov, V.V. Tetyorkin // Semiconductor Physics Quantum Electronics & Optoelectronics. — 1999. — Т. 2, № 3. — С. 21-25. — Бібліогр.: 16 назв. — англ. |
| series |
Semiconductor Physics Quantum Electronics & Optoelectronics |
| work_keys_str_mv |
AT ivasivzf noisespectraanddarkcurrentinvestigationsinnptypehg1xcdxtex022photodiodes AT sizovff noisespectraanddarkcurrentinvestigationsinnptypehg1xcdxtex022photodiodes AT tetyorkinvv noisespectraanddarkcurrentinvestigationsinnptypehg1xcdxtex022photodiodes |
| first_indexed |
2025-07-08T16:50:06Z |
| last_indexed |
2025-07-08T16:50:06Z |
| _version_ |
1837098275079979008 |
| fulltext |
21© 1999, Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
Semiconductor Physics, Quantum Electronics & Optoelectronics. 1999. V. 2, N 3. P. 21-25.
PACS: 85.60.D, 07.57.K, 85.60.G, 73.40
Noise spectra and dark current investigations
in n+-p-type Hg1-xCdxTe (x ≅≅≅≅≅ 0.22) photodiodes
Z.F. Ivasiv, F.F. Sizov, V.V. Tetyorkin
Institute of Semiconductor Physics, NAS of Ukraine
45, prospect Nauki, 252650 Kyiv, Ukraine
Abstract. The dark current and noise spectra were investigated in Hg1-xCdxTe (x≅0.22) photodiodes at
zero and low reverse bias voltages. The photodiodes were prepared by boron implantation into LPE
films. The 1/f noise is proved to be correlated with tunneling current via the deep defect states in the gap
at low reverse biases U≤0.1 V. In the photodiodes, where the tunneling current is found to be dominat-
ing, the 1/f noise is observed up to frequencies 104 Hz. The decrease of tunneling current results in the
decrease of 1/f noise.
Keywords: long wavelength intra-red photodiode, trap assisted tunneling, 1/f noise.
Paper received 07.09.99; revised manuscript received 30.09.99; accepted for publication 11.10.99.
1. Introduction
Hg1-xCdxTe ternary compounds are recognized to be the
most important materials for fabrication of long wavelength
infrared (LWIR) photodiodes for 3-5 and 8-12 µm wave-
length regions [1]. Ion implantation seems to be the most
important method for manufacturing single n+-p-type
photodiodes as well as multi-element focal plane arrays [1].
Carrier transport mechanisms in implanted LWIR photo-
diodes has been investigated both theoretically and experi-
mentally by many authors [1-9]. It has been found that in
general case at temperatures close to 77 K total dark cur-
rent are composed of several components: bulk diffusion,
generation-recombination (GR) current in the depletion re-
gion, trap assisted tunneling (TAT) current, band-to-band
(BTB) tunneling current and surface leakage current. GR
and TAT currents dominate the total dark current at zero
and low reverse bias voltage as well as BTB current is domi-
nant at large reverse bias voltage. In the photodiodes with
improved characteristics dark current is dominated by the
diffusion current. Due to the fact that carrier transport
mechanisms listed above are influenced by deep defect states
in the gap, fitting procedure can be used for estimation of
several important parameters of these states such as con-
centration Nt, energy Et and capture rates [4-9]. Because of
some discrepancies exist in the models used for the fitting
calculation the values of above mentioned parameters are
changed in a rather wide range (e.g., Nt is changed from
~1012 cm-3 up to ~1017 cm-3 [6,7,8,14]).
As to investigations of noise mechanisms in HgCdTe
photodiodes the origin of the 1/f noise still seems to be the
most important and unsolved problem. Several sources of
the 1/f noise have been proposed in application to these
photodiodes [10-15]. One of the possible source is lattice
defects of different types unintentionally or intentionally
introduced during growth of starting material. Fluctuation
of deep point defects, shallow etch pits, dislocation multi-
plication and clustering are recognized to be possible for
1/f noise source. As shown in [12], 1/f noise in LWIR photo-
diodes is a bulk phenomenon associated with defects in the
depletion region. However, the states at the interface of
HgCdTe and passivating layer can also contribute to 1/f noise
[11,13,14]. Finally, the dark current is suspected to be a
possible source of 1/f noise, too. In several papers [4, 9] the
correlation between 1/f noise and bulk BTB tunneling cur-
rent was observed. Quite the contrary, the correlation be-
tween 1/f noise and surface BTB tunneling current has been
found in the papers published earlier [15]. The role of trap
assisted tunneling current at zero and low reverse bias
voltages seems to be not investigated yet.
The aim of this work is to investigate dark current and
1/f noise in LWIR HgCdTe photodiodes in order to find
possible correlation between these phenomena. More than
thirty diodes were investigated to obtain statistical results.
2. Experimental results and discussion.
The starting materials were undoped Hg1-xCdxTe (x≅0.22)
films of p - type conductivity. The films were used for p-n-
junction fabrication by boron ion implantation. The hole
concentration in the films was close to 1⋅1016 cm-3 at tem-
perature 77 K.
Z.F. Ivasiv et al.: Noise spectra and dark current investigations...
22 SQO, 2(3), 1999
The dark current in the photodiodes has been measured
at 77 K. By differentiation of I-U curves the dynamic re-
sistance R characteristics versus voltage bias U were ob-
tained. To estimate the cut-off wavelength λc values the
photoresponse spectra were measured at 82 K. The noise
spectra were investigated in the frequency range 0.07÷
104 Hz at 77 K using CK4-73 spectrum analyzer.
Typical normalized photoresponse spectra of the
photodiodes are shown in Fig.1. The cutoff wavelength λc
for the photodiodes investigated are shown in Table 1 to-
gether with intrinsic carrier concentration, ni, and energy
band gap, Eg, values. The last two parameters were calcu-
lated using appropriate empirical expressions from [2].
Table 1.Fitting parameters used for modeling calculations
1. Band gap, eV, T=77 K 0.130
2. Cutoff wavelength, λñ, µm, T=82K 10.3±0.2
3. Intrinsic carrier concentration,
ni, cm-3, T =77 K 4.87⋅1012
4. Trap density, Nt, cm-3 1013-1016
5. Capture rate, Cp, cm3/s 10-7-10-6
6. Trap energy, Et, eV 0.72 Eg
Measured and calculated current-voltage characteristics
are shown in Fig.2 (a,b). In calculations several components
were taken into account: bulk diffusion current, generation
current in the depletion region and TAT current. The for-
mulae used for calculations of each current component are
briefly discussed in Appendix. In calculation of TAT cur-
rent thermal and tunnel transitions from the valence band
to deep defect states in the gap followed by tunnel transi-
tions to the conduction band were taken into account. The
dark current caused by band-to-band tunneling was not taken
into account because of investigations were mainly per-
formed at zero and low reverse bias voltage (U ≤ 0.1 V).
Surface leakage current seems to be negligibly small in these
photodiodes (see Appendix).
It is seen from Fig.2 that the photodiodes with different
type of current-voltage I-U characteristics were investigated.
In the photodiode with the lowest values of the reverse cur-
rent (curve 1in Fig.2(a)) the tendency to its saturation at
reverse bias is clearly seen. With increasing the reverse cur-
rent this tendency diminishes (curve 2). Further increasing
of the current is accompanied by appearance of the soft
breakdown characteristic (curve 3). Sharp soft breakdown
characteristics were observed in several photodiodes, Fig.2(b).
Typical dependencies of the R0A vs. voltage bias U (R0
is the differential resistance at zero bias, A is the junction
area) for the photodiodes investigated are shown in
2 4 6 8 10 12
1.0
0.8
0.6
0.4
0.2
0.0
W ave leng th , m icro n s
S
en
si
ti
vi
ty
, a
rb
.u
n
.
I,
x
10
A-6
Fig.1. Normalized photoresponse spectra of implanted n+-p photodiodes
Hg1-xCdxTe (x ≅ 0.22) at 82 K.
I,
x
10
A-6
Fig.2. Measured and calculated current-voltage characteristics of n+-p
photodiodes at 77 K. The dots represent measured data and the solid
lines represent theory. The fit was obtained for Nt = 1.2⋅1013; 2⋅1014;
4⋅1014 for curves 1-3 (a), respectively. The values Et = 0.72Eg,
Cp = 8⋅10-7 and Cn = 0.01Cp were the same for all three curves. To fit
experimental and calculated curves in (b) trap density was taken to
be Nt =3.2⋅1015; 4.8⋅1015 cm-3, for curve 1 and 2, respectively. Also, the
values Et = 0.72Eg, Cp = 1⋅10-7 and Cn = 0.01Cp were the same for these
curves.
b
a
Z.F. Ivasiv et al.: Noise spectra and dark current investigations...
23SQO, 2(3), 1999
Fig. 3 (a,b). At zero bias the R0A value less than 1.0 Ω×cm2
has been found to be typical for the photodiodes in which
soft breakdown was observed. Note that maximum in R0A
vs. U curves is shifted towards zero bias in these
photodiodes. The parameters used in calculations I-U and
R0A-U characteristics are listed in Table 1.
Noise power spectral density measurements were per-
formed in the frequency range of 0.7÷104 Hz for the same
photodiodes in which the dark current was measured. Typi-
cal noise spectra measured at 77 K are shown in Fig. 4 to-
gether with the calculated curves. As one can see, each noise
curve consist of frequency-dependent as well as frequency-
independent parts which can be attributed to 1/f noise and to
generation-recombination noise, respectively. The transition
region between them is rather narrow and it shifts towards
the higher frequency as the dark current increases.
From the measured and calculated data shown in
Fig.2(a,b) one can conclude the following. The best fit was
obtained for energy of deep defect states Et ≅ 0.72Eg (Et is
measured from the top of the valence band). This value
correlate well with previously observed one Et ≅ 0.75Eg
[7] in photodiodes prepared by boron implantation to bulk
material. Also this value is in agreement with the results of
DLTS measurements in undoped HgCdTe crystals [16]. It
seems that these centers are introduced into the bulk of
HgCdTe materials during their preparation and can be at-
tributed to different states of the Hg vacancies [7].
Some contradiction exists in the concentration of
deep defect centers with previously published values Nt=
= 0.1-10 NA [7], where NA is the shallow acceptor concentra-
tion (NA is approximately equal to the hole concentration).
The values of Nt obtained in present work range from ~1013
cm-3 to ~1016 cm-3 in photodiodes with NA ≅ 1016 cm-3.
This contradiction may be explained as follows. The con-
centration of shallow acceptors as well as deep defects in
HgCdTe implanted photodiodes is not uniform in the re-
gion adjacent to n+-p-junction. Nemirovsky et al. [4] have
found that n+-p-junctions can be formed with low concen-
tration of shallow acceptors near the junction. Since the
measurements were performed at zero and low reverse bias
voltage the concentration Nt derived from the fitting proce-
dure can be smaller than in the bulk (e.g., in implanted
Hg1-xCdxTe (x≅0.22) photodiodes investigated previously
[7] Nt reached the bulk value at reverse bias U≅0.6V).
The capture rates found at this study Cp ≈10-7÷10-6 cm3/s
and Cn = (0.1÷0.01)Cp differ much from the result previ-
ously obtained for Cp= (10-10÷10-9) and Cn = (10÷100) Cp
[8,14]. It should be pointed out that in these studies deep
defect centers in p-HgCdTe material were assumed to be
donor-like type. However, in present investigation the best
fit was obtained for acceptor-like centers in the gap. This
result are in accordance with the results of comprehensive
study performed by Rosenfeld and Bahir [7].
The correlation between 1/f noise and tunneling dark
current was previously observed by Nemirovsky et. al. [4]
at rather high reverse bias voltages U ≥ 200 mV. In present
investigation the correlation is revealed at low reverse bias
U ≤ 100 mV. The result obtained seems to be important
because of HgCdTe photodiodes are mainly used at zero
and low reverse bias voltage operating conditions [1]. The
calculation of the 1/f noise was performed by the formula
(A6) [4]. To fit experimental and calculated data the con-
stant α was assumed to lie in the range from 10-8 to 10-7 .
These values agree with the value α = 10-7 found in [4].
Because of the increase of the dark current in the investi-
gated photodiodes is related with the tunneling through deep
defect states in the gap, one can assume the origin of both
phenomena to be the same. Also it should be pointed out
that no correlation has been observed between GR current
in the depletion region and 1/f noise.
The mechanism of 1/f noise in diodes with dominant
trap-assisted tunneling current is unknown yet. One possi-
bility of its origin has been pointed out in [14]. The funda-
mental fluctuations in the process rates can exist due to the
infrared divergent coupling of free carriers to low-frequency
photons and other infraquanta.
R
A
,
cm
Ω
0
2
Fig.3. Typical resistance-area versus bias voltage characteristics for several photodiodes at 77 K. The designations are the same as in Fig.2a (curve 1)
and Fig.2b (curves 1 and 2).
a b
R
A
,
cm
Ω
0
2
Z.F. Ivasiv et al.: Noise spectra and dark current investigations...
24 SQO, 2(3), 1999
Conclusion
Dark current have been measured and calculated in n+-p-
type Hg1-xCdxTe (x ≅ 0.22) photodiodes at 77 K. It has been
found that generation in the depletion region and tunneling
through deep defect states in the gap are dominant carrier
transport mechanisms at low reverse bias voltage. From the
fitting procedure the density of the tunneling centers
Nt = 1013 –1015 cm-3 and their energy E t ≅ 0.72Eg has been
estimated. The 1/f noise measured at low reverse bias can
be attributed to the tunneling dark current through deep
defect states.
References
1. C.T.Elliot and N.T.Gordon, Infrared Detectors, in Handbook on Semi-
conductors, Vol.4, pp.841-936, Ed. by C.Hilsum, North-Holland,
Amsterdam (1993).
2. J.R.Lowney, D.G.Seiler, C.L.Littler and I.T.Yoon, Intrinsic carrier
concentration of narrow gap mercury cadmium telluride// J.Appl.Phys.
71, pp.1253-1258 (1992).
3. J.Y.Wang, Effect of trap tunneling on the performance of long-wave-
length Hg1-xCdxTe photodiodes // IEEE Trans. on Electron Devices
ED - 27 (1), pp.48-57 (1980).
4. Y.Nemirovsky, D.Rosenfeld, R.Adar, and A.Kornfeld, Tunneling and
dark currents in HgCdTe photodiodes // J.Vac.Sci.Technology A7 (2),
pp.528-535 (1989).
5. N.L.Bazhenov, S.I.Gasanov and V.I.Ivanov-Omskii, Excess currents
in narrow-gap Hg1-xCdxTe p-n junctions // Infrared Phys. 34 (1),
pp.37-41 (1993).
6. Akira Ajisawa and Naoki Oda, Improvement in HgCdTe diode char-
acteristics by low temperature post-implantation annealing //
J.Electron. Materials 24 (9), pp.1105-1111 (1995).
7. D.Rosenfeld and G.Bahir, A model for the trap-assisted tunneling
mechanism in diffused n-p and implanted n+-p HgCdTe photodiodes
// IEEE Trans. on Electron Dev. 39 (7), pp.1638-1645 (1992).
8. Y.Nemirovsky, R.Fastow, M.Meyassed, and A.Unikovsky, Trapping
effects in HgCdTe //J.Vac.Sci.Technol. B.9 (3), pp.1829-1839 (1991).
9. R.E.Dewames, J.G.Pasko, E.S.Yao, A.H.B.Vanderwyck, and
G.M.Williams, Dark current generation mechanisms and spectral noise
carrent in long-wavelength infrared photodiodes // J.Vac.Sci.Technol.
A6, p.2655 (1988).
10. Tian, Ji Zhongyun, Lei Ruoxi, Gu Shengqiong, Boqi, Measurements
of noise of photovoltaic HgCdTe and InSb infrared detector //
Proc.SPIE 2894, pp.174-179 (1996).
11. Wenmu He and Zeynep Celik-Butler, Temperature dependence of 1/f
noise in Hg1-xCdxTe MIS infrared detectors // Trans. Electron Dev.
42 (1), pp.160-165 (1995).
12. R.Schiebel, D.Bartolomew, M.Bevan, R.S.List, and M.Ohlson, Noise
and material defects in HgCdTe diodes // Journal of Elecrtonic Ma-
terials 24 (9), pp.1299-1203 (1995).
13. S.R.Babu, K.Hu, Manthripragada et al., Improved HgCdTe detectors
with novel anti-reflection coating //Proc. SPIE 2816, pp.84-89 (1995).
14. Wenmu He and Zeynep Celik-Butler, 1/f noise and dark current com-
ponents in HgCdTe MIS infrared detectors // Solid - State Electron-
ics 1 (1), pp.127-132 (1996).
15. W.A.Radford, and C.E.Jones, 1/f noise in ion-implanted and double-
layer epitaxial HgCdTe photodiodes // J.Vac.Sci.Technol. A3 (1),
pp.183-188 (1985).
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in p-type HgCdTe // Appl. Phys. Lett. 39 (3), pp. 248-250 (1981).
Appendix
The dark current in the photodiodes investigated can be
given by the sum of several components, namely
surfBTBTATGRD JJJJJJ ++++= , (A1)
Fig. 4. Measured (dots) and calculated (solid lines) noise spectra for the photodiodes with R0A = 1.0 Ω⋅cm2 (1) and R0A = 0.3 Ω⋅cm2 (2) at 77 K. The
reference shot noise level (2qJ)1/2 at reverse bias 10 mV is 1.7⋅10-13 and 7.92⋅10-13 A/Hz-1/2, respectively.
U
,
V
/H
z
0
1000 10000100
f , H z
10
U
,
V
/H
z
c
1000100
f , H z
101
1 2
Z.F. Ivasiv et al.: Noise spectra and dark current investigations...
25SQO, 2(3), 1999
were JD is diffusion current, JGR is the depletion genera-
tion-recombination current, JTAT is the trap-assisted cur-
rent, JBTB is the band-to-band tunneling current and Jsurf is
the surface leakage current.
For asymmetric junctions the first two components are
given by expressions:
2
1
0
2
⋅⋅=
n
ni
D
q
Tk
p
nq
J
τ
µ
, (A2)
1010
2
np
Wqn
J
pn
i
RG ττ +
= , (A3)
where p0 is the hole concentration, µn and τn are the elec-
tron mobility and the minority carrier lifetime in the bulk,
where ( ) 1
0
−= tnn Ncτ , ( ) 1
0
−= tpp Ncτ , cn and cp are the
capture coefficients for electrons and holes, respectively,
Nt is the trap density, p1 = Nv⋅exp(-Et/kT), n1 = Nc⋅exp(-Eg+
+Et/kT), Nv and Nc are the effective density of states in the
valence and conduction bands, W is the depletion region
width, Et is the trap energy measured from the valence band
edge. The second and third terms in (A1) were intensively
discussed earlier [1]. In this work, the appropriate equa-
tions for JTAT current were taken from [7]. The trap-assisted
tunneling current is given by
+
+
=
ccvvp
tTAT nnpc
WqNJ
ωω
11
1
, (A4)
were ωcNc and ωvNv are the tunneling rates .
The surface leakage current caused by the fast surface
states is given by [14]
2
Snq
J i
surf = , (A5)
where S is the surface recombination velocity. In properly pre-
pared photodiodes the value of S does not exceed 100 cm/s
[14]. As one can see from the formula (5), the contribution
of the surface leakage current to the dark current can be
neglected in our case. In order this current to be dominant,
the value S should be higher than 103 cm/s. However, so
high values of S were not observed in the diodes investi-
gated.
The calculation of the 1/f noise current was carried out
with
Jn =
=
f
JTATα , (A6)
where JTAT current was calculated using (A4).
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