Measurement temperature (K)
Chaper 4 Investigation of interfacial states for metal-semiconductor-metal photodetectors (MSM-PDs)
4.2.2 Persistent photoconductivity effects (PPC) for MSM-PDs with interfacial states
The persistent photoconductivity effects (PPC) in MSM-PDs with interfacial states by annealing treatment have been studied and observed. As light with photon
increases and becomes saturated after a period of time. The recombination phenomena between photogenerated carriers observed by continued illumination into the photocurrent saturated samples. The photogenerated electrons can occupy instable states and induce the photocurrent quenching after the saturation. After the incident light is turned off, the excess holes in the valence band recombine with free electrons which used to contribute to the dark current initially, resulting in the decreased dark current. It is necessary long recovery time that the electrons in the instable states tunnel out the potential barrier. As the photon energy (h ) less than energy bandgap (Eg), electrons are excited from deep levels or via interfacial states between metal and semiconductor to conduction band where they leak out to the instable states. Recently, many groups have reported the persistent photoconductivity effect (PPC) in n-GaN [97]-[110] and Mg doped p-GaN [111]-[113]. They found that the PPC effect is correlated to holes trap or vacancy, and discussed the trapping mechanism. In this work, we have studied the PPC effect in MSM-PDs with surface treatments.
In Fig.4-12, the dark current voltage characteristics with PPC phenomenon for MSM-PDs with annealing at 500 in H2 ambient (B2 sample) is observed by
3.35eV (Fig.13), and subsequent the dark current characteristics for MSM-PDs (B2 sample) was measured by without illumination. We found that the dark current and photocurrent could be increased due to by measuring method with continued step by step, hence the PPC phenomenon occurred in this case. The current-time characteristics with a constant reverse bias -2Volt and illuminated a photon energy of 3.35eV, 3.26eV and 3.1eV is utilized to confirm PPC effect as shown in Fig.4-13, Fig.4-14 and Fig.4-15, respectively. The initial dark current was measured by continued measurement to keep stable status, and the photocurrent was measured by incident light with a photon energy of 3.35eV, 3.26eV and 3.1eV provided. The photocurrent gradually increased and the dark current decreased with measured time is observed in the figure. These results may be attributed the presence of holes trap or acceptor-type trap states [100]-[105], [114]-[116]. When incident light is illuminated into the semiconductor, photoexcitation processes can occur due to emission of excited electrons in the metal over the barrier height (thermionic emission; TE) [117]- [119], or tunnel via surface energy level (field emission; FE), band to band excitation of electron hole pairs in the space charge region (h >Eg) and pass through deep level (h >Eg) as depicted in Fig.17 (a), (b), (c) and (d), respectively. The schematic photoexcitation process in the thermal equilibrium and higher reverse bias condition
probability for photocarriers emitted via deep level into conduction band is expected.
When the light was turned off, the excess holes in the valence band recombine with free electron from photoexcitation carriers, and recovered dark current to achieve stable status with a long time [120]. However, the recovered dark current can not attain to initial value by a long time after illuminated a photon energy of 3.35eV, 3.26eV and 3.1eV. Therefore, the excess holes occur in the valance band and more than photogenerated carriers can be induced due to these energy levels, resulting in greater internal gain and responsivity in MSM-PDs [89], [94]-[97], [103], [121], [122].
4.3 Summary
In summary, the higher responsivity and internal gain for metal semiconductor metal photodetectors (MSM-PDs) with different surface treatment have been demonstrated and fabricated. As light with photon energy is higher than bandgap energy (GaN~3.4eV) illuminated into MSM-PDs and applied reverse bias, the photogenerated current can be observed. The responsivity of MSM-PDs with or
gain can be observed. The responsivity of MSM-PDs with annealing at 400 (A1sample), 500 (A2 sample) and 600 (A3 sample) in N2 ambient by applying reverse bias -1Volt is 3.95A/W, 0.72A/W and 1.85A/W, respectively. The internal gain of MSM-PDs with annealing at 400 (A1 sample), 500 (A2 sample) and 600 (A3 sample) in N2 ambient at a photon energy of 3.35eV by applying reverse bias -1Volt is 195, 36 and 84, respectively. In general, photocurrent can’t be generated as photon energy (h ) is less than energy bandgap (Eg). However, the higher responsivity and internal gain characteristics for MSM-PDs with ICP etching process and annealing in N2 ambient at different temperatures are clearly observed. This result is attributed to interfacial states such as holes traps what capture or emit electrons or hole as applied higher reverse electrical field and illumination. Moreover, the internal gain for MSM-PDs with annealing in N2 ambient at 400 by applying higher -2Volt can achieve to 1734. On the other hand, the same results were in good agreement with the MSM-PDs with annealing at different temperatures in H2 ambient.
The persistent photoconductivity effects (PPC) in MSM-PDs with interfacial states by annealing treatment have been studied and observed. As light was illuminated on the semiconductor surface, the photocurrent increases and becomes saturated after a period of time. The photogenerated electrons can occupy instable
light is turned off, the excess holes in the valence band recombine with free electrons which used to contribute to the dark current initially, resulting in the decreased dark current. It is necessary long recovery time that the electrons in the instable states tunnel out the potential barrier. As the photon energy (h ) less than energy bandgap (Eg), electrons are excited from deep levels or via interfacial states between metal and semiconductor to conduction band where they leak out to the instable states. We found that the dark current and photocurrent could be increased due to by measuring method with continued step by step, resulting in PPC phenomenon occurred. The initial dark current was measured by continued measurement to keep stable status, and the photocurrent was measured by illumination with a photon energy of 3.35eV, 3.26eV and 3.1eV. The photocurrent gradually increased and the dark current decreased with measured time is observed. These results may be attributed the presence of holes trap or acceptor-type trap states
-1.0 -0.8 -0.6 -0.4 -0.2 0.0
Fig. 4-1 Current voltage characteristics of MSM-PDs without surface treatment
Fig. 4-2 Current voltage characteristics of MSM-PDs with ICP etching treatment
360 380 400 420 440 460 1x10-5
1x10-4 1x10-3 1x10-2 1x10-1
Responsivity (A/W)
Wavelength (nm)
Spacing=20um, V=-1.5v R1
R2
Fig. 4-3 Responsivity of MSM-PDs with/without ICP etching treatment
320 340 360 380 400 420 440 460 0.01
0.1 1 10 100
Responsivity (A/W)
Wavelength (nm)
Spacing=20um, V=-1.5v A1 A2
A3
-2.0 -1.5 -1.0 -0.5 1
10 100 1000
Internal gain
Reverse bias (Volt)
Spacing=20um
Fig. 4-5 Internal gain of MSM-PDs with annealing at different temperatures in N2 ambient at a wavelength of 370nm
360 380 400 420 440 460
1x10-3 1x10-2 1x10-1 1x100 1x101
Responsivity (A/W)
Wavelength (nm)
Spacing=20um, V=-1.5v B1
B2 B3
Fig. 4-6 Responsivity of MSM-PDs with annealing at different temperatures in H2 ambient
-2.0 -1.5 -1.0 -0.5 0.1
1 10 100 1000
Internal gain
Reverse bias (Volt)
Spacing=20um
Fig. 4-7 Internal gain of MSM-PDs with annealing at different temperatures in H2 ambient at a wavelength of 370nm
5 10 15 20
1 10 100 1000
A1 sample
Responsivity (A/W)
Reverse bias -0.5V -1.0V -1.5V -2.0V
5 10 15 20
ambient (A1 sample) is depicted as a function of finger spacing at wavelength of 370nm..
-2.0 -1.5 -1.0 -0.5 0.0
5 10 15 20 1
10 100 1000 10000 100000
Internal gain
Finger spacing (um)
A1 A2 A3 B1 B2 B3
Fig. 4-11 Internal gain of MSM-PDs with annealing at different temperature is depicted as a function of finger spacing at wavelength of 370nm and reverse bias -1.5V.
Table 4-1 The detailed process conditions for measured PPC effects Dark-1st Illuminated h =3.35eV Iphoto-1st
Non-illumination Dark-2nd
Illuminated h =3.35eV Iphoto-2nd
Non-illumination Dark-3rd
Illuminated h =3.35eV Iphoto-3rd
Non-illumination Dark-4th
Illuminated h =3.35eV Iphoto-4th
Non-illumination Dark-5th
Illuminated h =3.35eV Iphoto-5th
Non-illumination Dark-6th
Illuminated h =3.35eV Iphoto-6th
Non-illumination Dark-7th
Illuminated h =3.35eV Iphoto-8th
Non-illumination Dark-8th
Illuminated h =3.35eV Iphoto-9th
-2.00 -1.95 -1.90 -1.85 -1.80
Fig.4-12 Dark current voltage characteristics for MSM-PDs with annealing at 50 in H2 ambient (B2 sample). After illuminated a photon energy of 3.35eV, the dark current subsequently was measured without illumination. The detailed measured process flow is described in Table 4-1.
-2.00 -1.95 -1.90 -1.85 -1.80
-300n
-160n After illuminated =3.35eV
Photocurrent (Amp)
0 200 400 600 800 1000 1200 1400 1600 1800 -1.8µ
-1.6µ -1.4µ -1.2µ -1.0µ
-800.0n Bias=-2v
illuminated =3.35eV Dark
Current (Amp)
Time (s)
Fig.4-14 The current-time characteristics with a constant reverse bias -2Volt and illuminated a photon energy of 3.35eV.
0 200 400 600 800 1000 1200 1400 1600 1800 -1.3µ
-1.2µ -1.1µ -1.0µ
-900.0n Dark Bias=-2v
illuminated =3.26eV
Current (Amp)
Time (s)
Fig.4-15 Current-time characteristics with a constant reverse bias -2Volt and illuminated a photon energy of 3.26eV.
0 200 400 600 800 1000 1200 1400 1600 1800 -1.3µ
-1.2µ -1.1µ -1.0µ -900.0n -800.0n
Bias=-2v
illuminated =3.1eV Dark
Current (Amp)
Time (s)
Fig.4-16 Current-time characteristics with a constant reverse bias -2Volt and illuminated a photon energy of 3.1eV.
(a) (b)
(c)
(d)
B
Va
Fig.4-18 Schematic photoexcitation process (a) thermionic emission, (b) field emission, (c) band to band and (d) acceptor level or donor level to conduction band in the higher reverse bias.
(d) (a) (b)
(c)
B
Va
Ch
apter 5 Application of interfacial states for p-i-n-photodiodesFigure 5-1 shows the current-voltage (I-V) characteristics of AlGaN-based photodiode devices with different annealing gases under the illumination (0.13 W) with at a wavelength of 330nm, and the photocurrents are also near a constant at various reverse biases. For the N2-treated and H2-treated sample, a lower dark current could be observed as shown in inset of Fig.5-1. For the Non-treated PDs, a dark and under illumination (330nm, 0.13 W) current are increased with the reverse bias increased. Some leakage paths in parallel with the diode may be formed in the Non-treated sample. The ICP induced defects which turn the reverse bias leakage current of AlGaN-based photodiode devices may create an increase of surface states density and sidewall damage [39].
Comparing to the N2-treated and H2-treated PDs, the photocurrents are rapidly increased with more than reverse bias voltage 6 Volt for H2-treated PDs. Therefore, the external quantum efficiency of the AlGaN-based photodiode devices for the H2-treated PDs may increase more than two times of magnitude comparing to that for the N2-treated PDs. These results are attributed to hydrogen interacts with ICP induced defects or formation of dangling bonds on the sidewall or ICP damage areas, after the GaN is annealed with hydrogen ambient. Hydrogen can create in a number of interfacial states such as bound at donors/acceptors or trapping and recombination centers due to H2 diffusion and reaction chemically into sidewall or ICP damage areas [81]-[85]. When the bias increases, electrons can tunnel by trap levels, and may be
illumination, the I-V curve of the N2-treated PDs is almost the same. However, comparing to the Non-treated PDs and H2-treated PDs, some currents under the illumination could be observed. Since the incident photon energy is less than the band gap, these currents could be characterized as the defect-assisted photocurrent or band bending effects. The photocurrent of the H2-treated PDs at a wavelength of 360nm (0.23 W) is larger than that of the Non-treated PDs. These results are same as above mentioned, and are attributed to trapping and recombination centers due to H2
diffusion and reaction chemically into the sidewall or ICP damage areas.
Figure 5-3 shows the I-V characteristics illuminated with a wavelength of 400nm (0.4 W). In the dark and under the illumination, the I-V curve of the Non-treated PDs and of N2-treated PDs is almost the same. Yet, the photocurrent of the H2-treated PDs is increased in a small amount. This result indicates H2-induced defects may response at a wavelength of 400nm (3.1eV).
Figure 5-4 shows the spectral response of surface treated samples with different annealing gases under reverse bias voltage 8 Volt. The cutoff wavelength is around 330nm as the absorption of Al0.13Ga0.87N (3.7 eV) for these PDs. For the N2-treated PDs, there is less surface defects as discussed above, and higher rejection ratio which is around three to four orders of magnitude in the spectral responsivity with annealing at 600 in N2 ambient could be achieved,. However, under higher reverse bias, the defect-assisted photocurrent may be enhanced, and it may increase the responsivity in the rejection band. The spectral responsivity at a wavelength of 330nm with annealing at 600 in H2 ambient is around 0.1 A/W. It is larger than that with in N2 ambient, which is around 0.05 A/W. For the H2-treated PDs, some extra defect levels around 3.4 eV (360nm) can be observed at reverse bias voltage 8 Volt. For the N2-treated PDs,
In order to characterize the surface treated effects, the AlGaN-based photodiode devices (PDs) with KOH treatement can be also achieved. Figure 5-5 shows the current-voltage (I-V) characteristics for AlGaN-based photodiodes (PDs) without (“as grown PDs” sample) and with the KOH treatment (“KOH-treated PDs” sample), and the photocurrent of both PDs is also near a constant at various biases voltage. For the KOH-treated PDs, a lower dark current could be observed. For the as grown PDs, a dark current is increased with the reverse bias increased. Under illumination (330nm, 0.7 W), for the as grown PDs, the current is increased with bias. Some leakage paths in parallel with the diode may be formed in the as grown sample [23]. Comparing to a stable current for the KOH-treated PDs, these defects may be characterized as the surface defects. To clarify the defects effect, with the illumination of photon energy less than the absorption edge, the results of sample illuminated would be studied.
Figure 5-6 shows the I-V characteristics illuminated with a wavelength of 400nm (2.8 W). The I-V curve of the KOH-treated PDs taken in the dark and under the illumination is almost the same. Yet, for the as grown PDs, some currents under the illumination could be observed. Since the incident photon energy is less than the band gap, these currents could be characterized as the defect-assisted photocurrent. Thus, lower surface defects could be expected for the KOH-treated PDs.
Figure 5-7 shows the spectral response of as grown and KOH-treated PDs under 0 Volt and reverse bias 10 Volt. The cutoff wavelength is around 335nm as the absorption of Al0.13Ga0.87N (3.7 eV) for both PDs. The responsivity is 0.023 A/W and
responsivity in the rejection band. For the as grown PDse, some extra defect levels around 3.4 eV (360nm) can be observed at reverse bias voltage 10 Volt. For the KOH-treated PDs, there is no such level in this case as shown in Fig.5-7.
Figure 5-8 (a) shows the scanning electron microscopy (SEM) images of the AlGaN-based photodiodes for the as grown PDs after it has been etched to n+-GaN region. Due to the worse protection of the photo resist during etching process or the worse property of AlGaN and p-GaN, the sidewall with whisker-like features [123]
for the as grown PDs can be observed on top of n+-GaN layer after ICP etched process.
These whisker-like features may result from mix and edge dislocations [124], and of diameter are around 50-100nm. Thus, some leakage currents through these whisker-like features can be expected. After the KOH treatment, these whisker-like features can be removed [125] as shown in Fig.5-8 (b). Under the smoother surface on the edge, more uniform electric field can be achieved and less leakage current on the edge can be expected.
-10 -8 -6 -4 -2 0 2
Fig. 5-1 Current-voltage (I-V) characteristics of AlGaN-based photodiode devices (PDs) with annealing at different gases under the illumination (0.13 W) is at a wavelength of 330nm.
The insert of figure shows a lower dark current.
-10 -8 -6 -4 -2 0 2
-10 -8 -6 -4 -2 0 2
Fig. 5-3 Current-voltage (I-V) characteristics of different annealed treated samples illuminated with a wavelength of 400nm (0.4 W).
320 340 360 380 400 420 440
10-5
Fig. 5-4 Spectral response of surface treated samples with different annealing ambients under reverse bias voltage -8 Volt.
-10 -5 0
-4x10-8-2x10-8 0 2x10-8
4x10-8 as grown PDs, dark as grown PDs, illuminated KOH PDs, dark
KOH PDs, illuminated
C ur re nt ( A )
Bias (V)
Fig. 5-5 Current-voltage characteristics for as grown and KOH-treated PDs under the illumination (0.7 W) with wavelength 330nm
-10 -5 0
-5x10-9 -4x10-9 -3x10-9 -2x10-9 -1x10-9 0
C ur re nt (A )
Bias (V)
as grown PDs, dark as grown PDs, illuminated KOH PDs, dark
KOH PDs, illuminated
280 300 320 340 360 380 400 420 440 460 1x10-6
1x10-5 1x10-4 1x10-3 1x10-2 1x10-1
Responsivity (A/W)
Wavelength (nm)
as grown PDs, 0V as grown PDs, -10V KOH PDs, 0V KOH PDs, -10V
Fig. 5-7 Spectral responsivity for as grown and KOH-treated PDs at different biases
Fig. 5-8(a) Scanning electron microscopy (SEM) images of the AlGaN-based photodiodes for the as grown sample after it has been etched to n+-GaN region.