COL 12(9), 092301(2014) CHINESE OPTICS LETTERS September 10, 2014
1671-7694/2014/092301(4) 092301-1 © 2014 Chinese Optics Letters
Owning to the large light absorption coeicient of the
direct bandgap, GaN-based semiconductors are ideal
photodetector (PD) candidates, especially those oper-
ating in the ultraviolet (UV) region of the spectrum.
The UV PDs have imperative applications in both the
civilian and the military elds, such as missile-treating
warning, biological agent detection, spatial optical com-
munications, and UV imaging
[1]
. In the past decade, a
variety of GaN-based PDs have been demonstrated
[2–4]
.
The simplest one is of the photo-conductive type,
whereas, the others require either a p–n junction or a
Schottky barrier
[5–7]
. Among them, PDs based on the
p–i–n structure have attracted intensive attention be-
cause they can oer high-quantum eiciency, high
detectivity, and low dark current. However, for a
conventional top-illuminated p–i–n structure, the per-
formance of GaN-based PDs is usually limited by the
following factors: 1) the strong absorption eect of the
top p-type GaN contact layer, which can signicantly
reduce the possibility of the incident light penetrating
into the absorption region, leading to a relatively low-
quantum eiciency and 2) a high density of threading
dislocations within the GaN lms grown on frequently
used sapphire substrates, which can provide additional
paths for excess leakage current, resulting in severe reli-
ability problems. Therefore, the performance of GaN-
based p–i–n PDs can be further improved by thinning
the p-type contact layer and improving the crystal
quality of the GaN epi-layer. In this work, we fabricate
the GaN p–i–n PDs on patterned sapphire substrate
GaN-based p–i–n ultraviolet photodetectors with a
thin p-type GaN layer on patterned sapphire
substrates
Hongjuan Huang (黄红娟)
1
, Dawei Yan (闫大为)
1*
, Guosheng Wang (王国胜)
2
,
Feng Xie (谢 峰)
2
, Guofeng Yang (杨国锋)
1
, Shaoqing Xiao (肖少庆)
1
,
and Xiaofeng Gu (顾晓峰)
1
1
Key Laboratory of Advanced Process Control for Light Industry (Ministry of Educa-
tion), Department of Electronic Engineering, Jiangnan University, Wuxi 214122,
Jiangsu, China
2
The 38th Research Institute of China Electronics Technology Group Corporation,
Hefei 230088, China
*
Corresponding author: daweiyan@jiangnan.edu.cn
Received April 18, 2014; accepted June 3, 2014; posted online August 11, 2014
We study the performance of GaN-based p–i–n ultraviolet (UV) photodetectors (PDs) with a 60 nm thin p-
type contact layer grown on patterned sapphire substrate (PSS). The PDs on PSS exhibit a low dark current
of ~2 pA under a bias of -5 V, a large UV/visible rejection ratio of ~7×10
3
, and a high-quantum eiciency
of ~40% at 365 nm under zero bias. The average quantum eiciency of the PDs still remains above 20% in
the deep-UV region from 280 to 360 nm. In addition, the noise characteristics of the PDs are also discussed,
and the corresponding specic detectivities limited by the thermal noise and the low-frequency 1/f noise are
calculated.
OCIS codes: 230.0250, 230.5160, 230.5440.
doi: 10.3788/COL201412.092301.
(PSS) through a lateral overgrowth-like process with
only a 60 nm thin top p-type layer. The lateral epi-
taxial overgrowth (LEO) technology has been success-
fully used to produce low dislocation density areas on
PSS
[8–11]
. It turns out that the PDs fabricated on PSS
have a low dark current, a high-UV/visible rejection
ratio, and a large quantum eiciency even in the pho-
to-sensitive deep-UV range. We also discuss the noise
characteristics of the PDs, and calculate the specic
detectivities limited by the thermal and low-frequency
noises, respectively.
Figure 1(a) shows the schematic cross section of the
fabricated GaN p–i–n PDs prepared on PSS (Crys-
talwise Co). The epi-structure is grown by the metal-
organic chemical vapor deposition method. To obtain
crack-free lms, a 2.25 mm undoped GaN buer layer
is grown on PSS. The p–i–n structure consists of a 1.75
mm n-type GaN layer (n~3×10
18
cm
-3
), a 400 nm undoped
GaN photon absorption layer, and a 60 nm p-type GaN
contact layer (p~7×10
17
cm
-3
). The full-width at half-
maximum (FWHM) of the rocking curve of the (102)
reection is obviously narrower for the device structure
grown on PSS (298 arcsec) than that grown on stan-
dard sapphire substrate (SSS) (396 arcsec), as shown in
Fig. 2, indicating a decrease in the screw or edge-disloca-
tion density in the lms
[12]
. The standard photolithogra-
phy and chlorine-based plasma dry etching processes are
used to dene and pattern the device mesa. The lift-o
technique is employed to dene the n-contact electrode
region. A Ti/Al/Ni/Au (10/60/10/50 nm) metal stack
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