Ion implantation doping is the primary method for forming p–n junctions in HgCdTe. However, the doping and activation in HgCdTe are influenced by various complex factors, leading to inconsistencies between the actual carriers and the distribution of impurities. Currently, there are few experimental reports on nanoscale carrier distribution in HgCdTe photovoltaic structures. In this study, we employed scanning capacitance microscopy (SCM) to obtain the nanoscale dC/dV profiles on the cross-section of HgCdTe diodes, which refer to the distributions of electrons and holes in the junction region. The depletion area of the p–n junction was then identified precisely according to the measurement. For the arsenic-implanted p-on-n structure, the electrical distribution is highly consistent with that of the dopants. In contrast, for the n-on-p structure, the SCM study reveals the formation of n–p and p–p regions instead of the simple n+-p junction by heat treatment after boron-ion implantation; both are believed to play key roles in achieving optimal performance of HgCdTe photodetectors. Our study provides a direct approach to uncover the spatial distribution of carriers in the HgCdTe p–n junction, which is crucial in determining the electrical and photoelectric properties of the diodes.

The mercury cadmium telluride (HgCdTe) material is widely used in high-sensitivity infrared detectors due to its adjustable bandgap and wide infrared spectral response range.1–4 Currently, thermal treating after ion implantation is commonly utilized to establish p–n junctions in HgCdTe and is one of the key processes that affect the performance of photovoltaic detectors.5–7 While Hall-effect measurements and mobility spectrum analysis are capable of investigating carrier species,8,9 their spatial resolution is limited, making it difficult to accurately resolve the n-to-p transition in the p–n junctions formed by ion implantation. To assess the consequences of the doping process, secondary ion mass spectrometry (SIMS) can be used to obtain the distribution of elemental concentrations. However, the activation of the donor or acceptor in HgCdTe is influenced by various complex factors, such as the location of impurities, Hg vacancy defects, local dislocation configurations, etc. Hence, it is particularly desirable to unveil the actual carrier distribution in the ion implanted HgCdTe.

Scanning capacitance microscopy (SCM) has been extensively applied to characterize the carrier distribution in various materials and devices,10–14 especially those of silicon and III–V group semiconductors.15–18 The studies of Yin Hao and Hui Xia et al. on InGaAs/InP photodiodes demonstrate that the SCM can provide microscopic insights directly related to the performance of p–n junctions.19–22 However, for HgCdTe, due to the inherently unstable physical and chemical properties, coupled with the complex surface states, it has been challenging to form metal-oxide-semiconductor capacitors (MOSCAPs) that are detectable by the SCM method on this material. There have been few reports on the effective characterization of HgCdTe p–n junctions using this approach.

In this study, by precisely controlling the surface status of HgCdTe and the tip–sample contact force, we realized SCM characterization on the cross-sections of two representative ion-implanted HgCdTe materials: the p-on-n structure formed by arsenic doping and the n-on-p structure formed by boron-ion implantation. A comparative study has been carried out on the carrier and dopant distributions in two types of HgCdTe p–n junctions.

The H1−xCdxTe samples used in the SCM (Bruker Nanoscope IV) study were short-wavelength thin-film material grown on CdZnTe substrates by liquid phase epitaxy, with an average cadmium fraction x = 0.455. Boron-ion implantation was performed on the p-type HgCdTe formed by intrinsic Hg vacancies (VHg) and then annealed at 180 °C for 150 min to prepare the n-on-p structure. The p-on-n structure was prepared by arsenic implantation on the n-type HgCdTe with a low concentration of indium doping. Subsequently, a two-step activation heat treatment is applied at 420 °C, followed by 240 °C. Before the SCM study, the samples were first carefully cleaved along the HgCdTe crystal orientation in a dry environment to obtain flat cross-sections that could minimize the influence of morphology on tip-surface electrical contact. The measurements were then conducted at room temperature using diamond-coated conductive silicon probes.

In contrast to Si and III–V semiconductors, the natural oxide layer formed on HgCdTe is highly unstable and susceptible to destruction during the scanning process, leading to failure of capacitance response. To address this concern, two improvement measures were implemented. First, the cleaved samples were placed in a low vacuum environment for 48 h; this was found to be beneficial to form an oxide layer applicable for the SCM study. Second, accurate calibration and adjustment of the tip–sample contact force during measurements were carried out. Figure 1 exhibits the relationship between the tip–sample contact force and the setpoint of cantilever deflection after probe engagement. The contact force was calculated from Hooke’s law F = kx, in which k is the elastic coefficient of the probe and x is the vertical displacement of the tip determined from the force curve. It is shown that the contact force can be tuned within a wide range, from an attraction of nearly −600 nN to a positive value. According to our experience, repulsive force will generally scratch the HgCdTe surface. Therefore, all the SCM measurements were performed under attractive force in this work.

FIG. 1.

Relationship between the tip–sample contact force and cantilever deflection setpoint. The inset is a schematic diagram of the boron-ion-implanted HgCdTe cross-section measured by SCM.

FIG. 1.

Relationship between the tip–sample contact force and cantilever deflection setpoint. The inset is a schematic diagram of the boron-ion-implanted HgCdTe cross-section measured by SCM.

Close modal

For the p-on-n junction formed by arsenic doping, the tip–sample attractive force was kept at a very small value to ensure a reliable SCM study. Figures 2(a) and 2(b) show the morphology and SCM image obtained on the cross-section of the HgCdTe with the dC/dV profile plotted in Fig. 2(c). The SCM signal changes from positive to negative, which refers to the responses of holes and electrons, respectively. This is consistent with the decrease in arsenic concentration away from the sample surface plotted in Fig. 2(d).

FIG. 2.

The SCM and SIMS results of the arsenic-implanted HgCdTe. (a) Morphology of the HgCdTe cross-section, (b) SCM image, (c) the dC/dV profile of the sample along the growth direction, and (d) arsenic concentration distribution measured by SIMS.

FIG. 2.

The SCM and SIMS results of the arsenic-implanted HgCdTe. (a) Morphology of the HgCdTe cross-section, (b) SCM image, (c) the dC/dV profile of the sample along the growth direction, and (d) arsenic concentration distribution measured by SIMS.

Close modal
The transition point of dC/dV polarity happens at 2.13 µm, corresponding to an arsenic concentration of 3.5 × 1016 cm−3. The gradual change of the dC/dV signal from 2.13 to 0.84 µm indicates a wide depletion region of about 1.28 µm on the n-side. The depletion width of the pn junction can be calculated by the following equation:
W=2εs(VbiV)eNa+NdNaNd1/2,
(1)
where Vbi is the built-in potential of the junction, V is the applied bias, Na and Nd are the concentrations of acceptors and donors, respectively, and ɛs is the dielectric constant of the material. Taking Nd = 3 × 1014 cm−3 as electron density and hole concentration of 1 × 1017 cm−3, both measured by the Hall effect, the depletion width is estimated to be 1.29 µm, which agrees well with the SCM result.

Figures 3(b) and 3(c) present the SCM images on boron-ion-implanted HgCdTe under two different tip–sample contact forces. In both conditions, the differential capacitance (dC/dV) signal varies from positive to negative along the growth direction, which indicates the change of the majority carrier between holes and electrons inside HgCdTe. Moreover, instead of a simple transition from n+ to p, the SCM study reveals a distinct n+–n–p–p electrical layout in the transitional region of the junction.

FIG. 3.

The SCM and SIMS results of the boron-ion-implanted HgCdTe. (a) Morphology of the HgCdTe cross-section, (b) SCM image obtained at an attractive force of −320 nN, (c) SCM image obtained at an attractive force of −533 nN, (d) the dC/dV profiles along the growth direction under two different attractive forces, and (e) the distribution of boron concentration measured by SIMS.

FIG. 3.

The SCM and SIMS results of the boron-ion-implanted HgCdTe. (a) Morphology of the HgCdTe cross-section, (b) SCM image obtained at an attractive force of −320 nN, (c) SCM image obtained at an attractive force of −533 nN, (d) the dC/dV profiles along the growth direction under two different attractive forces, and (e) the distribution of boron concentration measured by SIMS.

Close modal

First, an n layer can be clearly observed next to the n+ layer in the dC/dV profiles, which indicates that an n–p junction rather than n+–p is formed after heat treatment. This is considered to be an important factor that ultimately determines the performance of the junction. Previous research found that boron-ion implantation causes Hg interstitials (Hgi) in HgCdTe, which can be n-type activated, then forms a high electron concentration layer (n+-region) in the surface.23,24 After thermal treatment, the Hgi diffuses inward and annihilates the VHg, leads to a low concentration of the n-type region that is dominated by the donor impurities of indium. This depletion width is calculated as 0.30 µm, which aligns closely with the SCM result labeled between 1.10 and 1.41 µm in Fig. 3(d).

Next, an additional p-type layer [labeled II in Fig. 3(d)] is disclosed in the junction region. During thermal treatment, the diffusion of interstitial Hg atoms will push the original acceptor impurities into HgCdTe. In addition, as a consequence of further treatment, the impurities will diffuse toward both the surface and the interior of the material simultaneously. This, on the one hand, improves the purity of the material in the junction area; on the other hand, it helps to form a p-type region [0.69–1.10 µm in Fig. 3(d)] in the n-type region. This process is electrically verified by the emergency of “Valley II” and “Peak III” in the dC/dV profile of Fig. 3(d) obtained at −533 nN, and it has been found to be beneficial to improve the minority carrier lifetime of the materials.25 The finding of these features relies heavily on the nanoscale spatial resolution of SCM with precisely control of the tip–sample contact force.

The distribution of boron concentration measured by SIMS is plotted in Fig. 3(e) for comparison. It shows a monotonous profile along the depth direction, as expected. The high density area of about 350 nm in thickness from the top surface corresponds to the n+ damaged layer, and then the boron concentration decreases rapidly, which is consistent with the position of the p–n junction measured by SCM. Nevertheless, the n–p–p junctional region cannot be identified from the gradient change of boron concentration since boron is neither a donor nor an acceptor impurity in HgCdTe.

This study reveals the carrier distribution in two different ion-implanted HgCdTe pn junctions by measuring the local capacitance response with SCM. The conceivable electrical characterization of HgCdTe was found to highly depend on the maintenance of a reliable surface oxide layer, which can be achieved by delicately adjusting the tip–sample contact force. The depth as well as the depletion width of pn junctions can be determined with an accuracy of at least ten nm, according to the SCM study. For a p-on-n junction formed by arsenic doping, the carrier distribution is well consistent with that of the dopants; the depletion region spreads over 1.30 µm due to the low electron concentration in the n-type absorption area. In contrast, for the n-on-p structure formed by boron implantation, a distinct n+–n–p–p electrical layout in the transitional region of the junction was disclosed, which reflects the redistribution of the original impurities, the vacancies, and the doping induced interstitial impurities in HgCdTe during thermal treatment after implantation. This work demonstrates the significance of probing the carriers with high spatial resolution for assessing the consequences of the doping process and the performance of HgCdTe p–n junctions.

This work was supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant Nos. XDB0580000 and XDB43010200), the National Natural Science Foundation of China (Grant Nos. 12393833, U2241219, 11991063, 12227901, and 52072245), and the Shanghai Explorer Program (Grant Nos. 21TS1400900 and 23JC1404100).

The authors have no conflicts to disclose.

Zhaoyang Huang: Conceptualization (equal); Data curation (equal); Investigation (equal); Project administration (equal); Software (equal); Visualization (equal); Writing – original draft (equal); Writing – review & editing (equal). Chun Lin: Investigation (equal); Methodology (equal); Resources (equal). Hao Xie: Resources (equal); Validation (equal). Rui Xin: Investigation (equal). Xiang Li: Data curation (equal); Formal analysis (equal); Funding acquisition (equal); Investigation (equal); Supervision (equal). Tianxin Li: Formal analysis (equal); Funding acquisition (equal); Supervision (equal); Validation (equal); Writing – review & editing (equal).

The data that support the findings of this study are available within the article.

1.
W.
Lei
,
J.
Antoszewski
, and
L.
Faraone
, “
Progress, challenges, and opportunities for HgCdTe infrared materials and detectors
,”
Appl. Phys. Rev.
2
(
4
),
041303
(
2015
).
2.
A.
Rogalski
, “
HgCdTe infrared detector material: History, status and outlook
,”
Rep. Prog. Phys.
68
(
10
),
2267
2336
(
2005
).
3.
V.
Gopal
,
W.
Qiu
, and
W.
Hu
, “
Modelling of illuminated current–voltage characteristics to evaluate leakage currents in long wavelength infrared mercury cadmium telluride photovoltaic detectors
,”
J. Appl. Phys.
116
(
18
),
184503
(
2014
).
4.
A. V.
Voitsekhovskii
,
S. N.
Nesmelov
,
S. M.
Dzyadukh
,
S. A.
Dvoretsky
,
N. N.
Mikhailov
,
G. Y.
Sidorov
, and
M. V.
Yakushev
, “
Electrical properties of nBn structures based on HgCdTe grown by molecular beam epitaxy on GaAs substrates
,”
Infrared Phys. Technol.
102
,
103035
(
2019
).
5.
J.
Michel
,
J.
Liu
, and
L. C.
Kimerling
, “
High-performance Ge-on-Si photodetectors
,”
Nat. Photon
4
(
8
),
527
534
(
2010
).
6.
A. D.
Mohite
,
D. E.
Perea
,
S.
Singh
,
S. A.
Dayeh
,
I. H.
Campbell
,
S. T.
Picraux
, and
H.
Htoon
, “
Highly efficient charge separation and collection across in situ doped axial VLS-grown Si nanowire p−n junctions
,”
Nano Lett.
12
(
4
),
1965
1971
(
2012
).
7.
M. S.
Leite
,
M.
Abashin
,
H. J.
Lezec
,
A.
Gianfrancesco
,
A. A.
Talin
, and
N. B.
Zhitenev
, “
Nanoscale imaging of photocurrent and efficiency in CdTe solar cells
,”
ACS Nano
8
(
11
),
11883
11890
(
2014
).
8.
I. I.
Izhnin
,
K. D.
Mynbaev
,
A.
Voitsekhovskii
,
A. G.
Korotaev
,
V. S.
Varavin
,
S. A.
Dvoretsky
,
N. N.
Mikhailov
,
M. V.
Yakushev
,
O. I.
Fitsych
,
Z.
Swiatek
, and
R.
Jakiela
, “
Analysis of carrier species in arsenic-implanted p-and n-type Hg0.7Cd0.3Te
,”
Infrared Phys. Technol.
114
,
103665
(
2021
).
9.
G. A.
Umana-Membreno
,
H.
Kala
,
J.
Antoszewski
,
Z.
Ye
,
W.
Hu
,
R.
Ding
,
X.
Chen
,
W.
Lu
,
L.
He
,
J.
Dell
, and
L.
Faraone
, “
Depth profiling of electronic transport parameters in n-on-p boron-ion-implanted vacancy-doped HgCdTe
,”
J. Electron. Mater.
42
,
3108
3113
(
2013
).
10.
R. N.
Kleiman
,
M. L.
O’Malley
,
F. H.
Baumann
,
J. P.
Garno
, and
G. L.
Timp
, in
International Electron Devices Meeting
(
IEDM Technical Digest
,
1997
), pp.
691
694
.
11.
J. S.
McMurray
,
J.
Kim
, and
C. C.
Williams
, “
Quantitative measurement of two-dimensional dopant profile by cross-sectional scanning capacitance microscopy
,”
J. Vac. Sci. Technol., B: Microelectron. Nanometer Struct.--Process., Meas., Phenom.
15
(
4
),
1011
1014
(
1997
).
12.
K. M.
Wong
and
W. K.
Chim
, “
Deep-depletion physics-based analytical model for scanning capacitance microscopy carrier profile extraction
,”
Appl. Phys. Lett.
91
(
1
),
013510
(
2007
).
13.
C.-S.
Jiang
,
J. T.
Heath
,
H. R.
Moutinho
, and
M. M.
Al-Jassim
, “
Scanning capacitance spectroscopy on n+-p asymmetrical junctions in multicrystalline Si solar cells
,”
J. Appl. Phys.
110
(
1
),
014514
(
2011
).
14.
M. N.
Chang
,
C. W.
Hu
,
T. H.
Chou
, and
Y. J.
Lee
, “
Contrast distortion induced by modulation voltage in scanning capacitance microscopy
,”
Appl. Phys. Lett.
101
(
8
),
083503
(
2012
).
15.
E.
Bussmann
and
C. C.
Williams
, “
Sub-10 nm lateral spatial resolution in scanning capacitance microscopy achieved with solid platinum probes
,”
Rev. Sci. Instrum.
75
(
2
),
422
425
(
2004
).
16.
P. A.
Rosenthal
,
Y.
Taur
, and
E. T.
Yu
, “
Direct measurement and characterization of n+ superhalo implants in a 120 nm gate-length Si metal–oxide–semiconductor field-effect transistor using cross-sectional scanning capacitance microscopy
,”
Appl. Phys. Lett.
81
(
21
),
3993
3995
(
2002
).
17.
D. M.
Schaadt
,
E. J.
Miller
,
E. T.
Yu
, and
J. M.
Redwing
, “
Lateral variations in threshold voltage of an AlxGa1−xN/GaN heterostructure field-effect transistor measured by scanning capacitance spectroscopy
,”
Appl. Phys. Lett.
78
(
1
),
88
90
(
2001
).
18.
O.
Douhéret
,
S.
Anand
,
C. A.
Barrios
, and
S.
Lourdudoss
, “
Characterization of GaAs/AlGaAs laser mesas regrown with semi-insulating GaInP by scanning capacitance microscopy
,”
Appl. Phys. Lett.
81
(
6
),
960
962
(
2002
).
19.
H.
Yin
,
T.
Li
,
W.
Hu
,
W.
Wang
,
N.
Li
,
X.
Chen
, and
W.
Lu
, “
Nonequilibrium carrier distribution in semiconductor photodetectors: Surface leakage channel under illumination
,”
Appl. Phys. Lett.
96
(
26
),
263508
(
2010
).
20.
H.
Yin
,
T.
Li
,
W.
Wang
,
W.
Hu
,
L.
Lin
, and
W.
Lu
, “
Scanning capacitance microscopy investigation on InGaAs/InP avalanche photodiode structures: Light-induced polarity reversal
,”
Appl. Phys. Lett.
95
(
9
),
093506
(
2009
).
21.
H.
Xia
,
T.-X.
Li
,
H.-J.
Tang
,
L.
Zhu
,
X.
Li
,
H.-M.
Gong
, and
W.
Lu
, “
Nanoscale imaging of the photoresponse in PN junctions of InGaAs infrared detector
,”
Sci. Rep.
6
(
1
),
21544
(
2016
).
22.
Y.
Li
,
S.
Zhang
,
H.
Xia
,
H.
Chen
,
W.
Wang
,
J.
Li
, and
T.
Li
, “
Retrieve the carrier population and built-in potential alignment in multi-quantum-well GaAs/InGaAs p-i-n photodiode
,”
Physica E
135
,
114970
(
2022
).
23.
G. L.
Destéfanis
, “
Electrical doping of HgCdTe by ion implantation and heat treatment
,”
J. Cryst. Growth
86
(
1–4
),
700
722
(
1988
).
24.
R. E.
DeWames
,
G. M.
Williams
,
J. G.
Pasko
, and
A. H. B.
Vanderwyck
, “
Current generation mechanisms in small band gap HgCdTe p-n junctions fabricated by ion implantation
,”
J. Cryst. Growth
86
(
1–4
),
849
858
(
1988
).
25.
L. O.
Bubulac
, “
Defects, diffusion and activation in ion implanted HgCdTe
,”
J. Cryst. Growth
86
(
1–4
),
723
734
(
1988
).