Chapter 4 Results and Discussion
4.2 Dark current of GaN MSM photodetectors
Theoretically, the dark current of the MSM potodetectors consists of surface leakage current and the internal leakage current (reverse saturation current). Fig.4.2 shows the two main mechanisms for leakage currents of the MSM PDs. We use the simple concept to analyze our experimental results and to induce the relation between dark current and different geometric structured MSM-PDs.
Fig 4.2 illustrates two ways for leakage currents
According to the formerly research results of our group, we have found that Pt is a suitable metal to be made the electrode in the GaN MSM-PDs.[20] In this study, Pt is selected for the metal material in the MSM structure. Fig.4.3 shows the dark I-V characteristics of the conventional and recessed-electrode GaN MSM-PDs with or without oxidation. With oxidation, it was found that the dark currents were 57µA and 0.4µA for the MSM-PDs with and without recessed electrodes under 20V bias. Without oxidation, it was found that dark currents were 96µA and 28µA for the MSM-PDs with and without recessed electrodes under 20V bias. The large leakage current observed in MSM-PD with recessed electrodes is
attributed to the wet etching induced surface damages. From these I-V curves, it should be noted that the dark current of MSM-PDs with recessed electrodes reaches saturation faster than that of MSM-PDs without recessed electrodes. At the same time, we also found the smaller dark current of MSM PDs with oxidation. The dark current of MSM PDs with passivation layer is lower because of the lower surface density.
0 5 10 15 20
1E-11 1E-10 1E-9 1E-8 1E-7 1E-6 1E-5 1E-4
I(A)
without oxidation (recessed) oxidation(recessed)
without oxidation (planar) oxidation (planar)
88
V(volt)
Fig.4.3 The dark I-V characteristics of the planar and recessed-electrode GaN MSM-PDs with or without oxidation. (For example, PD with 3 µm spacing and 8 µm wide metal finger is denoted as 38.)
0.0 2.5 5.0 7.5 10.0 12.5 15.0 0.0000000
0.0000001 0.0000002 0.0000003
I(A)
V(volt) 43
44 45 48
SiO2 passivation layer planar contact
Fig.4.4 Dark current versus bias voltage for various the planar Pt GaN MSM-PDs with finger width =3, 4, 5, 8µm and the same finger space = 4µm.
For the different geometry figures, we have measured the dark current versus bias voltage.
From Fig.4.2 and Fig.4.4, we discuss further the dark current of the planar MSM-PDs.
(1) The internal leakage current, Iin, flows through the depletion regions and semiconductors from the reverse-bias contacts to the forward-bias contacts. The leakage increases with the finger density and the finger width of MSM PDs,
W S Iin W
∝ + ………. (4.1) For the finger width 3µm, the ratio from Equation (4.1) is
7
3, and 8µm, the ratio from
Equation (4.1) is 12
8 , with the finger spacing width 4µm, the dark current of the former
should be lower than the latter. Fig 4.4 shows the oppositive trend. The internal leakage current is not the main control in the conventional MSM PDs.
(2) Surface leakage current flows on the surface of GaN. It increases with the finger density of MSM PDs and decreases with the finger spacing width.
S
For the finger width 4µm, the ratio from Equation (4.2) is 32
1 , and 8µm, the ratio from
Equation (4.2) is 48
1 , with the finger space width 4µm, the dark current of the former
should be near 1.5 times higher than the latter. In Fig 4.4, these two MSM PDs shows the same tend. Under 15V bias, the dark current of the device with finger spacing width 4µm and the finger width 4µm is near 0.22µA and the dark current of the device with finger spacing width 4µm and the finger width 8µm is near 0.13µA. The surface leakage current controls the dark current of planar GaN MSM PDs mainly. On the other hand, the dark current of the recessed electrode MSM-PDs is also shown in Fig.4.5.
0.0 2.5 5.0 7.5 10.0 12.5 15.0
Fig.4.5 Dark current versus bias voltage for various the recessed Pt/GaN MSM-PDs with finger width =3, 4, 5, 8µm and the same finger space = 4µm.
For the finger width 3µm, the ratio from Equation (4.1) is 7
3, and 8µm, the ratio from
Equation (4.1) is 12
8 , with the finger spacing width 4µm, the dark current of the former should
be lower than the latter. Fig 4.5 shows the same trend. The internal leakage current is the main control in the recessed MSM PDs. Under 15V bias, the dark current of the device with finger spacing width 4µm and the finger width 4µm is near 3.7µA and the dark current of the device with finger spacing width 4µm and the finger width 8µm is near 4.7µA. The internal leakage current controls the dark current of recessed GaN MSM PDs mainly.
0.0 2.5 5.0 7.5 10.0 12.5 15.0
(b) The recessed-electrode MSM PDs
Fig.4.6 The I-V characteristics of (a) the planar and (b) recessed-electrode the MSM PDs with finger space width =2, 3, 4, 5, 8µm and the same finger width = 8µm.
For the finger width 4µm, the ratio from Equation (4.2) is 32
1 , and 8µm, the ratio from
Equation (4.2) is 48
1 , with the finger space width 4µm, the dark current of the former should
be near 1.5 times higher than the latter. In Fig 4.5, these two MSM PDs shows the oppositive tend.
Fig.4.6 shows the I-V characteristics of (a) the planar and (b) recessed electrode the MSM PDs with finger space width =2, 3, 4, 5, 8µm and the same finger width = 8µm. From Fig.4.6 (a), we can find that the surface leakage current dominates the dark current in the planar MSM PDs. Furthermore, Fig.4.6 (b) shows the internal leakage current is the important part of the dark current in the recessed electrode MSM PDs.