SUPPORTING INFORMATION
Vertical conducting ultraviolet light-emitting diodes based on pZnO:As/n-GaN/n-SiC heterostructure
Zhifeng Shi,1 Yuantao Zhang,1,a) Bin Wu,1 Xupu Cai,1 Jinxiang Zhang,1 Xiaochuan Xia,2 Hui
Wang,1 Xin Dong,1 Hongwei Liang,2 Baolin Zhang,1,a) Guotong Du1,2
1State
Key Laboratory on Integrated Optoelectronics, College of Electronic Science and
Engineering, Jilin University, Qianjin Street 2699, Changchun 130012, People’s Republic of
China
2School
of Physics and Optoelectronic Technology, Dalian University of Technology, Dalian
116023, People’s Republic of China
a)Author
to whom correspondence should be addressed. Electronic mail: zhangyt@jlu.edu.cn, zbl@jlu.edu.cn
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VC-LEDs fabrication procedures
Firstly, an n-Al0.08Ga0.92N buffer layer was deposited on the pre-cleaned n-SiC substrate
(with a resistivity of 0.02 Ω.cm and carrier concentration of 4.5×1018 cm-3) using the
industrialized Thomas Swan CCS 3 × 2″ MOCVD system. In the process, the substrate
temperature and reactor pressure were kept at 1050 oC and 38 Torr. The flow rates of SiH4 and
NH3 were 1.44 nmol/min and 3 SLM, and the TMAl and TMGa flows were 3.34 and 21.99
μmol/min, respectively. Secondly, a graded n-AlxGa1-xN layer was deposited subsequently with
the TMAl flow decreased to 0 μmol/min and TMGa flow increased to 175 μmol/min.
Simultaneously, the SiH4 flow increased to 7.32 nmol/min. Thereafter, n-type GaN epitaxial
layer (1.5 μm) was grown on the AlGaN buffered n-SiC substrate at 1050 oC. For fabricating pZnO film, arsenic was used as the doping source. The As-doping p-type ZnO process had been
reported in our previous reports.1 In detail, about 20 nm GaAs interlayer was pre-coated on the nGaN/graded n-AlGaN/n-AlGaN/n-SiC template by magnetron sputtering. This was followed by
the deposition of ZnO film (~500 nm) using a custom designed II-VI MOCVD system with
diethylzinc (DEZn) and high purity oxygen as the precursors. The ZnO growth process was done
under 460 oC for half an hour, and then the ZnO layer was annealed in situ at 500 oC for 10 min
under an oxygen environment to promote the efficiency of As-doping. For device fabrications,
the circular Au electrode (~30 nm) and Ni electrode (~50 nm) were deposited on p-ZnO and the
backside of the n-SiC substrate respectively by conventional thermal evaporation method as the
p- and n-type contact electrodes. The schematic diagram of the VC-LED structure was shown
Figure S1.
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FIG. S1. The schematic diagram of the studied VC-LED heterostructure.
Characterizations and measurements
The morphological and crystallinity characterizations of ZnO:As films were performed by
FE-SEM (Jeol-7500F) and XRD (Rigaku Ultima IV). The PL spectra were recorded at RT and 10
K using a Zolix Omni-λ 500 monochromator/spectrograph with a continuous wave He-Cd laser
(325 nm, 30 mW) as the excitation source. The electrical properties of the ZnO:As and n-GaN
epitaxial films grown on sapphire substrates under the same growth conditions were examined by
Hall effect measurements using the Van der Pauw configuration (ACCENT HL5500PC). The
composition and chemical bonding of the ZnO:As films were determined using XPS
measurements. The I-V characteristic of the device was measured using a Keithley 4200
semiconductor characterization analyzer and EL spectra were measured by using a home-made
acquisition equipment including photomultiplier tube and lock-in amplifier systems.
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Hall results of GaN:Si and ZnO:As layers
The electron concentration and mobility of GaN epitaxial films grown on sapphire
substrates under the same growth conditions were 5.0×1018 cm-3 and 280 cm2V-1s-1, respectively.
And the ZnO:As film on sapphire substrate presented an p-type conduction behavior with a hole
concentration and mobility of 3.52×1017 cm-3 and 0.785 cm2V-1s-1, respectively.
Low-temperature PL spectrum of ZnO:As films at 10 K
Low-temperature PL spectrum obtained at 10 K was measured to investigate the optical
transition of the ZnO:As layer. As shown in Figure S2, five major emission peaks were found in
the As-doped ZnO films. As deduced from foregone researches, the dominant peak at 3.355 eV
was attributed to the neutral acceptor bound exciton emission (A0X).2 The shoulder on the lowenergy side, 3.313 eV, was generally assigned to the recombination emission between free
electrons and acceptor holes (FA).3 And three other peaks at 3.237, 3.164 and 3.093 eV can be
attributed to the donor-acceptor pair (DAP) transitions.4 It was believed that the observation of
A0X, FA and DAP emissions in the low-temperature PL confirmed the acceptor behavior of
arsenic in ZnO. In addition, the arsenic related acceptor level can be calculated from the
following equation:
EA  Egap  EFA  kBT / 2
(1
)
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where Egap was the intrinsic band gap, EFA was the free electron-acceptor level transition. Herein,
the value of EFA was 3.313 eV at 10 K in our work. With an intrinsic band gap of Egap=3.437
eV,5 the value of EA can be derived to be about 124 meV.
FIG. S2. Low-temperature PL results of As-doped ZnO films measured at 10 K.
Light-output-current characteristic of the VC-LED
Light-output-current (L-I) characteristic of the VC-LED was determined to evaluate its highefficiency EL performance. As shown in Figure S3, the normalized light output of the VC-LED
and reference LED as a function of injection current were depicted on a log-log scale, and the L-I
curves were fitted using the power law
LIp
(2)
where p accounts for the influence of nonradiative defects on the light emission characteristics.
The value of p in the superlinear zone for the VC-LED (1.27) was smaller than that of reference
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LED (1.53). Note that a linear relation with p~1 indicates a limited influence of nonradiative
defects, that is, radiative recombination dominates the recombination process. Such EL
improvements were mostly attributed to the optimized ZnO/GaN interface, where the interfacial
defects were decreased and the carrier injection efficiency through the nanosized junctions was
promoted accordingly. However, the value of p for the studied VC-LED, as comparied with an
ideal LED, was still in a high level. As a result, further researches are necessary to improve the
crystal quality of p-ZnO and reduce the influence of nonradiative defects, which provided a
recombination path for the electron-hole pairs.6
FIG. S3. Normalized light output intensity of the two corresponding diodes as a function of
driving currents.
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