Chapter 2 Basic Theory of Metal-Semiconductor-Metal (MSM)
2.4 Responsivity
The definition of photodetector’s responsivity is the ratio of incident light power to induced photocurrent. Take account of the cause of gain, MSM photodetector responsivity can be represented as
hv G
R=ηextqΓ
……….. (2.10)
Where R is the responsivity, ηextis external quantum effect, q is electron charge, h is plank’s constant, vis the frequency of incident light and Γ is internal gain. In general, external G quantum effect can be showed as
Where S means space width, W is finger width, R is surface reflectivity, α is absorption constant and d is the thickness of active layer. Another parameter ΓG can be represented as follows
e transmission time, S is space width and V is applied voltage.
e
ttr
The mobility difference of electron and hole induce this gain. Hole mobility is smaller then electron. When device is illuminated, electrons and holes produce in the conduction and valance band. Because of holes in valance band move much slower, when electrons in conduction band pass through semiconductor into metal, few holes in valance band still survived. In order to keep
the neutral of depletion region, the electrons in the cathode will inject into semiconductor and this is called gain. This result must be corrected by experiment. There is also the other phenomenon that the holes are trapped by acceptor states at the metal-semiconductor interface to lower the barrier height. The electrons would inject into the semiconductor from the cathode after lowering the barrier height so that the gain will be induced. [16-19]
Chapter 3
Fabrication Process and Measurement Techniques for the MSM photodetectors
In this chapter, fabrication processes and characterization techniques for the metal-semiconductor-metal (MSM) photodetectors are presented.
3.1 Design and layout of photo mask
We designed the MSM photodetector structures and the layout of mask patterns by using Tanner L-Edit. Three masks were used to fabricate various geometric structured GaN MSM photodetectors.
(1) metal finger mask: Finger mask delineates the MSM fingers. The pattern of some sampled fingers is shown in Fig. 3.1.
Fig.3.1 The metal finger mask.
(2) contact-opening mask: Contact-opening mask is used to open contact windows through the passivation layer to connect the finger metal with pad metal. The pattern of sampled contact openings corresponding to the fingers in Fig. 3.1 is shown in Fig.
3.2.
Fig.3.2 Contact-opening mask.
(3) padding mask: Padding mask delineates bonding pads. The pattern of sampled pads corresponding to the fingers in Fig. 3.1 is shown in Fig. 3.3.
Fig.3.3 Padding mask.
3.2 Fabrication processes
We fabricated MSM photodetectors on un-doped GaN. The processes were performed at laboratory of Vtera Technology Inc., Science-Base Industrial Park, Hsinchu, Taiwan.
To give clear illustration of fabrication procedures, some 3-D figures of processed sample are depicted accompanying with the following process descriptions.
3.2.1 Wafer initial cleaning (1) Immersing in ACE for 10 min.
(2) Immersing in IPA for 5 min.
(3) Dipping in D.I water.
(4) Blowing with N2.
3.2.2 Finger metal deposition 3.2.2.1Convential contact (1) Baking at 120℃.
(2) Coating photoresist AZ5214 with spin rate of 500 rpm for 5 s and then 3500 rpm for 25 s.
(3) Soft baking at 100℃.
(4) UV exposing with the metal finger mask for 4 s.
(5) Post exposure baking at 120℃.
(6) Exposing with a UV light.
(7) Developing with FHD-5.
(8) Rinsing with D.I water.
(9) Blowing with N2.
(10) After exposure inspection with optical microscope to observe lithographic results.
(11) Baking at 120℃
(12) Plasma etching the residual photoresist with O2 for 30 s.
(13) E-gun evaporation of MSM interdigited Schottky metals. Various kind of metal were chosen for this application
(14) Lifting off the unwanted metal.
(16) Observing the finger metals by using the microscope. (The temporally finished
sample structure is shown in Fig 3.4.)
Fig.3.4 Stereo graph of the sample after finger metal preparation.
3.2.2.1 Recessed contact [19]
(1) Baking at 120℃.
(2) Coating photoresist AZ5214 with spin rate of 500 rpm for 5 s and then 3500 rpm for 25 s.
(3) Soft baking at 100℃.
(4) UV exposing with the metal finger mask for 4 s.
(5) Post exposure baking at 120℃ for 2 min.
(6) Exposing with a UV light.
(7) Developing with FHD-5.
(8) Rinsing with D.I water.
(9) Blowing with N2.
(10) After exposure inspection with optical microscope to observe lithographic results.
(11) Baking at 120℃.
(12) Plasma etching the residual photoresist with O2 for 30 s.
(13) Wet etching was performed to recess the sample to the depth of 100 angstrom.
(13) E-gun evaporation of MSM interdigited Schottky metals. Various kind of metal were chosen for this application
(14) Lifting off the unwanted metal.
(16) Observing the finger metals by using the microscope. (The section of the temporally finished sample structure is shown in Fig 3.5.)
Metal
GaN
Metal
GaN
Fig 3.5 The schematic structure of the sample after recessed-electrode preparation.
3.2.3 Passivation and contact opening
(1) Wafer cleaning with H2O:NH4F:CH3COOH = 120:1:1 for 2 mins.
(2) Deposit oxidation (SiO2, or Si3N4) with thickness of 2500 Å by plasma enhanced chemical vapor deposition (PECVD).
(3) Photolithographic processes with contact-opening mask. The detailed procedures are the same to processes from (1) to (10) for finger metal deposition.
(4) Etching oxidation (SiO2, or Si3N4) for opening by inductor coupled plasma (ICP) etching technique.
(5) Photoresist removal with ultrasonic ACE and IPA.
(6) Residual chemical removal and drying sample.
(7) Observing the contact opening patterns by using the microscope. (The temporally finished sample structure is shown in Fig 3.6.)
Fig.3.6 Stereo graph of the sample after contact opening.
3.2.4 Bonding pad metallization (1) Wafer cleaning.
(2) Photolithographic processes with contact-opening mask. The detailed procedures are the same to processes from (1) to (10) for finger metal deposition.
(3) Slightly plasma etching the residual photoresist with O2 for 45 s.
(4) Metallization of Ti-Al with E-gun evaporation technique.
(5) Lift off the unwanted metal to form bonding pads connecting to finger metal.
(6) Cleaning and drying sample.
(7) Observing the pad patterns by using the microscope. (The finished sample structure is shown in Fig 3.7.)
Fig.3.7 Stereo graph of the sample after the final process.
3.3 I-V measurement
Fig.3.8 illustrates the setup used for I-V measurement to characterize the performance of MSM photodetector, in term of dark currents and photo currents versus device bias voltage. Keitheley source-measurement-unit 236 is used to measure the I-V characteristics of MSM detectors with/without He-Cd laser beam illumination. The measurement results were recorded for further analysis.
3.4 Measurement of spectrum response
The optoelectronic properties of MSM photodetector, such as photo current versus bias voltage under the monochromatic light illumination, were characterized. Fig. 3.9 shows the set up for the spectrum response measurement of MSM photodetectors.
Experimental setup:
1. Hg-Xe lamp: A light source of continuing wavelength.
2. HeCd laser: The wavelength is 325 nm and the power is 12.6 mW.
3. Monochromator: ARC SPECTRO-275 Monochromator with 27.5 cm focal length was used to extract nearly single wavelength light from the broad band Hg-Xe light
source to impinge on the MSM photodetectors.
4. lock-in amplifier: STANDFORD RESEARCH SYSTEM SR850.
5. Digital voltage meter: HP 3478A which be used to measurement DC information.
8. pre-amplifier: STANDFORD RESEARCH SYSTEM SR850. It is used to amplify the small signal current to a large voltage.
Fig 3.9 shows the spectrum response measurement setups of MSM. The light emitted from a Hg-Xe lamp is modulated by chopper to a special frequency. Through the monochromator, the light is separated to near single wavelength light which illuminates on the samples. The low current signal produced by devices is amplified to voltage signal and then was measured by a lock-in amplifier synchronized with the modulated frequency.
H e C d L a se r
- MSM
d e vice
Ke ith e ily 2 3 6 so u rce me a su re me n t u n it
-C o mp u te r
GPIB in te rfa ce
Fig 3.8 Setup of dark and photo current measurement for MSM photodiodes.
Fig 3.9 Setup of spectral responsivity for MSM photodiodes.
Chapter 4
Results and Discussion
In this chapter, the fabrication process results and optoelectronic characteristics of metal-semiconductor-metal photodetectors (MSM-PDs) are presented. Dark current and photocurrent of MSM-PDs of various geometric structures are measured and analyzed. The factors determine the dark and photo currents of MSM-PDs are discussed. Spectrum responsivity of MSM-PDs was measured to confirm the UV detection capabilities of these GaN MSM-PDs.
4.1 Process results of GaN MSM photo detector
We have successfully fabricated MSM-PDs on GaN. Fig. 4.1 (a)-(c) show the photographs of samples after processes (a) finger metal deposition, (b) contact opening, and (c) bonding pad metallization, respectively, by optical microscopy.
(a)
(b)
(c)
Fig 4.1 Photographs of samples after processes (a) finger metal deposition, (b) contact opening, and (c) bonding pad metallization.
In our design, there are 20 different geometry sets of MSM-PDs. For the interdigit metal finger structure, we made various MSM-PD chips with a finger width of 3, 4, 5, or 8µm and with a finger spacing of 2, 3, 4, 5, or 8µm, and all with the same chip area of 250 × 250µm2. We also fabricated the MSM-PDs with the recessed electrodes. For distinguishing the device geometric structures, sample #sw is adopted to denote the MSM-PD detector with sµm spacing and wµm finger width. i.e., sample 38has 3µm finger spacing and 8µm finger width.
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.
4.3 Photo current of GaN MSM photo detector
Fig 4.7 illustrates the photo and dark current of (a) planar and (b) recessed electrode MSM PDs with finger spacing width 2µm and finger width 3µm versus applied bias under illumination (He-Cd laser :0.0066mW). In the planar MSM PDs, the photo current is 4.0071E-4 A when applied voltage is 20V and the ratio of photo to dark current is 71(Idark= 5.6343E-6A). In the recessed electrode MSM PDs, the photo current is 0.01104 A when applied voltage is 20V and the ratio of photo to dark current is 326 (Idark= 3.3787E-5A). From Fig.4.7, it was found that photocurrent to dark current contrast ratios were 326 and 70 for the MSM-PDs with and without recessed electrodes under a 20V applied bias, respectively.
0 5 10 15 20
(b) Recessed-electrode MSM PD
Fig 4.7 The photo and dark current of (a) planar and (b) recessed-electrode MSM PDs with finger spacing width 2µm and finger width 3µm versus applied bias under illumination (He-Cd laser :0.0066mW).
0 5 10 15 20 1E-5
1E-4 1E-3 0.01
I photo(A)
V(volt)
Recessed contact planar contact 23 SiO2 passivation layer
He-Cd laser (325nm):0.0066mW
Fig. 4.8 The I-V characteristics of the planar and recessed-electrode GaN MSM PDs under illumination (He-Cd laser :0.0066mW).
Fig.4.8 shows the I-V characteristics of the planar and recessed-electrode GaN MSM PDs under illumination (He-Cd laser :0.0066mW). We found that photocurrent observed from the MSM-PD with recessed electrodes was lager than that of planar MSM-PD by about one order of magnitude. From Fig 4.7 and 4.8, we believe these observations should be related to the enhanced electric field or the uniform electric field distributed through the gap space in the recessed electrode structure.
0 5 10 15 20 1E-6
1E-5 1E-4 1E-3
He-Cd (325nm):0.0066mW SiO2 passivation layer planar contact
43 44 45 48 I photo(A)
V(volt)
Fig 4.9 Photo current of the planar MSM PDs with the finger space width s = 4µm and different finger width w = 3, 4, 5, 8µm versus applied bias.
For the different finger width, we also measure their photo current. As the thickness of our metal is 100 Å that is like transparent metal, most of the depletion region under the finger metal can also absorb the photon to generate the photo current. The total area of finger region plays the more important role than the space region because of the transparent metal. In Fig.4.9, the photo current of the conventional MSM PDs with the finger space width s = 4µm and different finger width w = 3, 4, 5, 8µm versus applied bias are shown. The wide finger width the lower photo response can be seen in Fig.4.9.
0 5 10 15 20
Fig 4.10 Photo current of the recessed-electrode MSM PDs with the finger space width s = 4µm and different finger width w = 3, 4, 5, 8µm versus applied bias.
Fig. 4.10 shows the photo current of the recessed-electrode MSM PDs with the finger space width s = 4µm and different finger width w = 3, 4, 5, 8µm versus applied bias. The wide finger width the lower photo response can be also seen in Fig.4.10. In other words, more photo current can be induced when raises the density of finger metal.
4.4 Photo response of GaN MSM photo detector
The optical spectrum measurement setups of MSM PDs are shown in Fig 3.9. Fig. 4.11 illustrates the normalized responsivity of the planar MSM PD with Pt metals under a 2V applied bias. An abrupt exponential cut-off wavelength of 365nm demonstrates the structure impressive wavelength selective below and above the gap.
340 350 360 370 380 390 0.1
1
Normalized responsivity(a.u.)
Wavelength(nm) 28
Pt planar contact SiO2 passivation layer
Under 2V bias 365 nm
Fig. 4.11 The normalized responsivity of the planar MSM PD with Pt metals under a 2V applied bias.
The response in the UV is almost three orders of magnitude greater than visible region (500 nm) of the spectrum, which compares favorably well with the other reported results, indicating a good spectral selectivity and high quantum efficiency up to cut-off wavelength.
The spectral selectivity and below band gap response (>365 nm) are important for UV application, which we believe may be associated with deep-level defect states, its magnitude is an indication of the density of defects in the GaN. Because the GaN has a wide band gap, crystal defects or impurities in the material may easily create energy levels within the gap.
These levels form recombination centers that harm the performance of UV photodetectors, in
particular their visible blindness. Better control of the material growth technology (reduction of impurities and defect densities) is needed to improve the visible blindness for our GaN-based UV detectors.
Fig. 4.12 and Fig 4.13 show the responsivity of the planar and recessed-electrode MSM PDs with the finger space width s = 2µm and different finger width w = 3, 4, 5, 8µm under a 2V applied bias. The wide finger width the lower responsivity can be seen in Fig.4.12 and Fig.4.13. This trend is matched with Fig. 4.9 and Fig. 4.10. The responcivity of MSM PDs increases when raises the density of the finger metal. The response range of these detectors is from 310 to 370nm.
220 240 260 280 300 320 340 360 380 400 420 440 -0.0002
Fig 4.12 The responsivity of the planar MSM PDs with the finger space width s = 2µm and different finger width w = 3, 4, 5, 8µm under 2V bias.
220 240 260 280 300 320 340 360 380 400 420 440
Fig 4.13 The responsivity of recessed-electrode MSM PDs with the finger space width s = 2µm and different finger width w = 3, 4, 5, 8µm under 2V bias.
To get the relative responsivity of MSM PDs, we divided the original spectral response of MSM PD to that of a PMT under the same measurement condition and multiplied the responsivity of the PMT device. The monochromatic light is calibrated with PMT and an optical power meter. Fig. 4.14 shows spectral responses of the recessed-electrode and the planar MSM PDs with a 2V applied bias. With a 2V applied bias and an incident light wavelength of 360 nm, it was found that measured responsivities were 0.019 and 0.0094A/W for the MSM-PDs with and without recessed electrodes, respectively.
.
320 330 340 350 360 370 380
1E-4 1E-3 0.01 0.1
SiO2 passivation layer Bias 2V
Wavelength(nm)
Responsivity( A/W)
recessed contact planar contact
recessed contact planar contact