Supplementary Information: High Performance CaS Solar-blind
UV Photodiodes Fabricated by Seed-Layer-Assisted Growth
Qing Lin He1, 2, †, Ying Hoi Lai1, 2, †, Yi Liu2, Emeline Beltjens2, Jie Qi2, Iam Keong Sou1,2,*
1
Nano Science and Nano Technology Program, The Hong Kong University of Science and
Technology, HKSAR, People’s Republic of China
2
Department of Physics and William Mong Institute of Nano Science and Technology, The Hong
Kong University of Science and Technology, HKSAR, People’s Republic of China
†
These authors contribute to this work equally.
* Author to whom correspondence should be addressed. Email: phiksou@ust.hk.
HRXRD θ-2θ scans of Sample B1, C and A
Supplementary Figure 1 displays the HRXRD θ-2θ (200) scans of Sample B1, C and A. It can be
seen that the scan of Sample C, which is a seed layer for the seed-layer-assisted growth of
Sample B1, displays a lower layer peak intensity and slightly broader peak width than those of
Sample B1. These differences can be explained by the thinner thickness of Sample C due to thin
film broadening effect as well as the fact that Sample B1 contains an additional high-quality top
epitaxial layer. The θ-2θ scan of Sample A, which is a sample grown using the conventional
method, displays a much broader layer peak than those of Sample B1 and C, confirming the
contrast of the RHEED observations and photoresponse characteristics as presented in the main
text. It is worth mentioning that the “differential dip” within the layer peak near 2 θ = 31.6° is
also observable for another sample grown using the conventional method. Such a “differential
dip” feature has also been observed in Ref [3]. To the best of our knowledge, the cause of this
feature has not been understood yet. The narrower peaks with lower intensity on both sides of the
layer peaks in this figure are the thickness fringes due to thin film interference.
Sample B1
Sample C
Sample A
11
10
9
Intensity (A.U.)
10
GaAs (200)
CaS(200)
7
10
5
10
3
10
1
10
-1
10
30.0
30.5
31.0
31.5
32.0
32.5
2 (Degree)
Supplementary Figure 1 θ-2θ scans of Sample B1, C and A
Data fitting for saturation current I0 and ideality factor n
For a Schottky barrier diode with assumption that the current is due to thermionic emission, the
relation between the applied forward bias and current can be expressed as [1-2],
𝐼 = 𝐼0 exp (
𝑞𝑉
−𝑞𝑉
) [1 − 𝑒𝑥𝑝 (
)]
𝑛𝑘𝑇
𝑘𝑇
(S1)
that (S1) for 𝑉 > 3𝑘𝑇/𝑞 can be written as
𝐼 = 𝐼0 exp (
𝑞𝑉
)
𝑛𝑘𝑇
(S2)
where n is the ideality factor, T is the temperature in Kelvin, q the electronic charge, k is the
Boltzmann constant and I0 the reverse saturation current which can be extracted by extrapolating
the straight line of lnI to intercept the axis at zero voltage.
Supplementary Figure 2 displays the I-V characteristics of Sample B1 and its data fitting result
based on the above approach, yielding I0 =5.5nA and n =3.3.
2500
2000
I-V Data for Sample B1
Fitted Curve
Current (nA)
1500
1000
500
0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
Applied Voltage (V)
Supplementary Figure 2 I-V characteristics of Sample B1 and its data fitting
Results of response speed measurement
Supplementary Figure 3 displays the results of the response speed measurements performed on
Sample B1.
Decay time:
31.9ms
-100
Voltage (A.U.)
Voltage (A.U.)
Rise time:
30.4ms
-50
0
Time (mSec)
50
100
-100
-50
0
Time (mSec)
50
100
Supplementary Figure 3 (Left) Rise-time for Sample B1. (Right) Decay time of Sample B1
Seed-layer-assisted growth of Rocksalt CaTe thin film and a CaTe/SnTe/CaTe quantum
well structure
The seed-layer-assisted growth approach developed in this work can be applicable to the growth
of other materials in rocksalt phase with high crystalline quality. Rocksalt CaTe has a lattice
constant of 6.36Å (PDF#39-1494), resulting an even larger lattice misfit of around 12.6% with
zinc-blende GaAs, as compared with the CaS/GaAs system. The growth of a CaTe thin film on a
GaAs (100) substrate was carried out with similar procedures as those used for Sample B1. Two
effusion cells of elemental Ca and Te were used with optimized cell temperatures at 485℃and
290℃ respectively. The seed layer was firstly grown at substrate temperature of 200℃. The
sample was then subjected to thermal annealing under the flux of both Ca and Te to the target
temperature of 300℃. The growth of the CaTe thin film on the resulting seed layer was then
proceeded by lowering down the substrate temperature to 230℃. RHEED patterns of the
resulting CaTe thin film show bright and streaky lines, demonstrating that successful application
of the seed-layer-assisted growth approach can be extended to other rocksalt/zincblende systems
besides the CaS/GaAs system.
A high crystalline quality CaTe thin film can find an important application as being the buffer
layer for the growth of rocksalt SnTe and Pb1-xSnxTe thin films and related nano-structures,
which have been recently demonstrated to be a class of exotic materials named topological
crystalline insulator [4, 5]. In this study, the seed-layer-assisted growth approach was also
successfully applied to fabricate a CaTe/SnTe/CaTe quantum well structure, which enjoys a small
lattice mismatch of only 0.47% between CaTe and SnTe, the latter has a lattice constant of 6.33
Å (PDF#46-1210). The growth of this quantum well structure started with the growth of the
bottom CaTe layer using the seed-layer-assisted growth approach followed by epitaxial growths
of the SnTe quantum well and the CaTe cap layer. Figure 5 shows the RHEED patterns of the
three layers of which display bright and streaky lines, indicating the quantum well indeed
exhibits high crystalline perfection.
Supplementary Figure 4 RHEED patterns of a CaTe/SnTe/CaTe quantum well structure recorded
in [110] (upper) and [100] (lower) directions: (a) The bottom CaTe layer, (b) the middle SnTe
layer, and (c) the top CaTe layer.
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