Performance of polycrystalline GaN based metal-semiconductor-metal (MSM) photodetector with different contact

Journal Pre-proof Performance of polycrystalline GaN based metal-semiconductor-metal (MSM) photodetector with different contact N. Zainal, M.A. Ahmad, W. Maryam, M.E.A. Samsudin, S.N. Waheeda, M. Ikram Md. Taib PII:

S0749-6036(19)31731-8

DOI:

https://doi.org/10.1016/j.spmi.2019.106369

Reference:

YSPMI 106369

To appear in:

Superlattices and Microstructures

Received Date: 2 October 2019 Revised Date:

2 December 2019

Accepted Date: 10 December 2019

Please cite this article as: N. Zainal, M.A. Ahmad, W. Maryam, M.E.A. Samsudin, S.N. Waheeda, M.I.M. Taib, Performance of polycrystalline GaN based metal-semiconductor-metal (MSM) photodetector with different contact, Superlattices and Microstructures (2020), doi: https://doi.org/10.1016/ j.spmi.2019.106369. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Performance of polycrystalline GaN based metal-semiconductor-metal (MSM) photodetector with different contact N. Zainal1*, M. A. Ahmad1, W. Maryam2, M.E.A. Samsudin1, S.N. Waheeda1, M. Ikram Md. Taib1 1

Institute of Nano Optoelectronics Research and Technology (INOR), Universiti Sains Malaysia, 11800 USM, Penang, Malaysia. 2 School of Physics, Universiti Sains Malaysia, 11800 USM, Penang, Malaysia *

Corresponding author: [email protected]

Abstract This paper describes performance of polycrystalline GaN based metal-semiconductormetal (MSM) photodetector using different contact; Al, ITO, Ni and Pt. The performance of each photodetector was investigated in term of electrical resistivity (ρ), signal-to-noise ratio (SNR), responsivity (R), internal quantum efficiency (η) and temporal responsivity. From the results, Ni is suggested to be a good contact for the photodetector. This is due to diffusion of NixO, formed by residual oxides on the GaN layer, into the Ni contact reduced the resistivity, thereby increasing the electrical conductivity of the photodetector. The photodetector with Ni contact demonstrated significant increase in SNR behavior with increasing bias voltage, while its ρ value was measured to be 2.02 MΩ.cm2, and η was 3.13%, 2.36% and 1.52%, at λ = 342 nm, 385 nm and 416 nm, respectively. From the temporal responsivity measurement, the rise time = 1.75 sec, the recovery time = 1.87 sec and the sensitivity = 5840%.

Keywords: MSM photodetector, polycrystalline GaN; electrical contact, resistivity, signal-tonoise ratio, responsivity, internal quantum efficiency and temporal responsivity

Introduction While the earth's ozone layer protects us from UV-B and UV-C radiation, the UV-A radiation is not being filtered and therefore it penetrates into the atmosphere. The UV-A radiation with wavelength from 315 to 400 nm can be harmful to human’s skin by mutating the skin cells located deeper inside the skin structure [1-4]. Hence, this scenario justifies the importance and the urgency to have a photodetector as a device to detect the level of UV-A radiation towards preventing the risk to the human’s skin. In this regard, a photodetector based on single crystal GaN is not suitable for such application since it typically exhibits a limited detection range of below 360 nm [5-6]. In early reports, polycrystalline GaN based photodetector demonstrated a wider range of detection between 280 nm and 410 nm [7-10] which indicates polycrystalline GaN is a potential material for UV light photodetectors. At this point, polycrystalline GaN MSM photodetector is less demonstrated and less explored as compared to MSM photodetector based on single crystalline GaN. Our previous work [11] demonstrated a new technique of depositing polycrystalline GaN layer on sapphire substrate by electron beam (e-beam) evaporator with successive annealing in ammonia (NH3). In this present paper, we expand the scope of the work by fabricating the layer into functional metal-semiconductor-metal (MSM) photodetector using a different single contact; aluminum (Al), indium-tin-oxide (ITO), nickel (Ni) and platinum (Pt). While Al contact is less popular for GaN MSM photodetector, its potential in contact scheme for perovskite solar cell has been reported [12]. On the other hand, ITO is accepted for GaN MSM photodetector due to its high transparency and conductivity [13]. Ni and Pt contacts are commonly used since both have higher work function with respect to GaN, which then leads to low Schottky barrier at the metal-semiconductor interface. To date, technology of depositing metal or metal-oxide contacts is mostly carried-out through sputtering or evaporation techniques. As compared to e-beam evaporator, RF-

sputtering produces better film quality and step coverage. It also offers higher efficiency even at low pressure operation. A published work in [14] found that depositing ITO using RFsputtering enhanced light output of InGaN LED instead of using e-beam evaporator. This indicates that utilization RF-sputtering for depositing metal or metal-oxide contacts is important in fabrication technology of GaN devices, especially for photodetectors; e.g. [15]. In this work, Al, ITO, Ni and Pt contacts were deposited onto the polycrystalline GaN layer using RF-sputtering. Single contact is studied instead of multi-stack contact to avoid more formations of interface state density and new contact phases [16]. ‘Figure of merit’ of the photodetector with different contact is characterized by focusing on electrical resistivity (ρ), signal-to-noise ratio (SNR), responsivity (R), internal quantum efficiency (η) and temporal responsivity. The finding of this work would be the first step towards developing a viable polycrystalline GaN based MSM photodetector for wider UV range operation.

Experimental work

In this work, ~1 µm thick GaN layer was deposited on several pieces of m-plane sapphire substrate using e-beam evaporator, followed by 10 minutes annealing at 950 oC in ammonia (NH3) ambient. The purpose of the annealing is to transform the crystalline structure of the layer from amorphous to polycrystalline, while mitigating the nitrogen deficiency. The properties of the layer measured by SEM, AFM and XRD and the results are given in the figure 1 (a). The results have been published in [11]. In the next stage, the samples of the polycrystalline GaN layer were fabricated into MSM photodetector by depositing Al, ITO, Ni and Pt on the surface of the layer in separate step using a contact mask with an area of ~0.5 cm2 as shown in figure 1 (b). Each of the contacts was deposited at a

thickness of ~100 nm by RF-sputtering. In this experiment, the base pressure was around 3x10-5 to 5x10-5 mbar with argon flow rate at 12 sccm for Al, ITO and Ni, while 18 sccm for Pt. On the other hand, the applied power was adjusted depending on the contact; Al and ITO at 150 W, Ni at 180 W and Pt at 100 W. The contact was enhanced through nitrogen (N2) annealing at 750 oC for 10 minutes. The annealing was conducted using 3-zone furnace. Prior to the annealing, the furnace was purged-out using N2 flow in order to remove unwanted impurities from the environment.

(a) GaN (0002) Intensity (a. u.)

Surface roughness: 59.44 nm

GaN (10-11)

GaN (10-10)

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32

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(b) Inter-digitized finger contact

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Figure 1: (a) Characteristics of polycrystalline GaN layer as measured from SEM, AFM and XRD, and (b) schematic diagram of polycrystalline GaN based MSM photodetector with interdigitized finger contact.

Subsequently, a series of electrical analysis of the photodetectors was carried out to determine electrical resistivity (ρ), signal-to-noise ratio (SNR), responsivity (R) and internal quantum efficiency (η) through current-voltage (IV) measurement. In this experiment, a Xenon UV lamp (λ = 280 nm – 700 nm) was used to excite the samples. Energy dispersive xray (EDX) spectroscopy measurement was conducted to identify the presence of elemental composition in each contact. Further, temporal responsivity of the samples of the photodetector were measured under a pulsed UV illumination at λ = 385 nm. Towards the end of this work, the contact that is more suitable for the polycrystalline GaN based MSM photodetector than others will be proposed.

Results and discussions

The samples of photodetectors are labelled as in Table 1 with details of their respective resistivity, ρ. The resistivity was measured from gradient of IV graph of each sample. The correlation between the resistivity and the contact will be discussed. The contact resistivity should also be reported here. However, the samples in this work are limited and therefore are not available for transmission line method (TLM) measurement. Despite of that, the result shown in Table 1 could provide an insight on the quality of the electrical contact from each contact. In this work, the polycrystalline GaN based MSM photodetector with Al contact is defined as Sample PD1, ITO as Sample PD2, Ni as Sample PD3 and Pt as Sample PD4.

Table 1: Electrical resistivity (ρ) for polycrystalline GaN based MSM photodetector with different contact. Sample

Contact

Electrical resistivity, ρ (MΩ.cm2)

PD1 PD2

Al ITO

6.10 4.25

PD3 PD4

Ni Pt

2.02 2.47

Figure 2 shows dark-current density (Id) and photo-current density (Ip) of each photodetector, including linear IV under dark illumination in order to observe Schottky behavior. It should be noted that the photodetectors with the contact of Al, ITO, and Ni show a similar dark-current density with a maximum of around 20 nA/cm2. In contrast, Sample PD4 with Pt contact exhibits dark-current density with one magnitude order higher than other samples. This is commonly observed for GaN based MSM photodetectors with Pt contact; e.g. [17]. On the other hand, the photo-current density from Sample PD1, Sample PD2 and Sample PD3 shows one magnitude order higher than that of their corresponding dark-current density. It was found that the photo-current density is maximum in the range of 200 - 900 nA/cm2. The result is better than a reported MBE grown GaN based photodetector [5]. In the case for Sample PD4, the photo-current density shows a small increase with respect to its dark-current density. Essentially, a combination of low photo-current density and high darkcurrent density would reduce the sensitivity of a photodetector. In this work, Pt is therefore not a preferable contact for polycrystalline GaN photodetector. The following measurements will lead to further discussion on this subject.

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Figure 2: Current-voltage (IV) characteristics of dark-current density and photo-current density for polycrystalline GaN based MSM photodetector with different contact. Inset figure is linear IV under dark-current measurement.

Referring to Table 1, the resistivity, ρ of all samples is relatively high but is lower than a reported UV photodetector in [5]. The resistivity is measured after the contact

5

annealing in N2 ambient at 750°C and its value is derived from the gradient of linear IV graph. From Table 1, the variation of resistivity depends on the contact used. Apparently, Al and ITO contacts give higher resistivity than Ni and Pt. While the resistivity is larger if the work function of contact is lower with respect to GaN, the incorporation of oxides in the Al and ITO contacts is potentially being another factor to increase the resistivity. To investigate this further, the oxygen atomic percentage (%) in each contact on the photodetector, before and after the contact annealing, was determined through EDX measurement and the result is shown in figure 3. Before the annealing, the oxygen level in the Al is much lower than Ni contact. Essentially, oxidation can easily be formed on the surface of Al and Ni when they are exposed to air. Given that Ni has a higher oxidation rate than Al [18], therefore the oxide film on Ni surface is thicker than that on Al surface. On the other hand, higher oxygen level in ITO is already expected since the ITO itself contains oxygen component, while the oxygen level in Pt is insignificant and close to the background.

Figure 3: (Colour online) EDX data to show oxygen (O) atomic percentage (%) in those contacts, before and after the contact annealing which were deposited onto polycrystalline GaN layer. Inset shows the PL spectra for Ni contact of Sample PD3.

After the contact annealing, an increase of oxygen is observed in Al, ITO and Ni contacts from the EDX data. On the other hand, the oxygen in Pt contact increases very slightly. Detailed investigation from our previous work [11] found that Ga2O3 in the polycrystalline GaN layer was not completely removed by the ammonia annealing. Therefore, upon the contact annealing as conducted in this present work, the oxygen component from the remaining Ga2O3 in the polycrystalline layer diffused towards the surface and reacted with the contacts to form more oxides than those before annealing. Based on figure 3, high resistivity of Sample PD1 (with Al contact) is owing to the increase of insulating Al2O3 inclusions resulting from the reaction of Al and the oxygen upon annealing. Such Al2O3 inclusions create physical barrier between metal-semiconductor interface. Likewise, the oxygen level in Sample PD2 (with ITO) raises. As been observed in many experiments; e.g. [13,19], the incorporation of oxygen into ITO reduces vacancies and this leads to higher resistivity. In contrast, Sample PD3 (with Ni contact) exhibits lowest resistivity despite the increase in the oxygen level after annealing. Moreover, its linear IV nearly Ohmic. Upon diffusion, the oxygen reacted with some of the Ni particles to form nickel oxide (NixO) inclusions. The evidence of the NixO inclusions is shown by a photoluminescence (PL) spectrum in the inset figure. Before annealing, only GaN related near band edge (NBE) emission peak is observed ~3.4 eV. On the other hand, a weak PL peak around 3.25 eV appears after annealing and this peak is very close to a reported PL peak of NixO [20]. The mechanism of how NixO inclusions in Ni contact can reduce resistivity is not well-understood at this point. However, a report by [12] found that contact between NixO with various metal including Ni will be forced to be Ohmic due to close Fermi level pinning to the NixO valence band maxima and therefore reduces the resistivity. As a matter of fact,

the introduction of NixO in contact design has reduced the resistance in various optoelectronics devices; e.g. [21-24]. Unlike Sample PD1 and PD2, the existence of the NixO inclusions in the Ni contact (Sample PD3) helps to reduce the resistivity. Interestingly, Sample PD4 shows the opposite behavior, whereby the oxygen level is almost equal before and after annealing. This suggests that the formation of oxides in Pt is almost negligible in this case. Figure 4 illustrates the process of electron–hole pairs are generated and collected by electrodes as the MSM photodetector is illuminated by UV light at emission energy higher than the bandgap of the GaN. In principle, increase in the Schottky barrier height (SBH), qΦB at the metal-semiconductor interface will increase in the resistivity.

qΦB

e-

e- e- eEf

UV light

Contact

h+ h+

e-h pair generation

h+

h+

Figure 4: (Colour online) Schematic energy band diagram of polycrystalline GaN based MSM photodetector under UV light.

In this work, SBH for each sample is estimated using this equation: Φ =



∗  ln  

 (1)

where A is the contact area that is 0.5 cm-2, A* is the Richardson constant = 24 A cm-2 K-2, q is the electron charge, T is the temperature and Is is the saturation current determined by

linearly extrapolating dark-current to zero voltage for calculations. The calculated SBH are listed in Table 2. Due to low current density obtained in this work, the SBH for all samples is almost similar. However, the SBH values are comparable to those reported in literatures, except for the case of the Al contact because of the formation of the oxide layer in this study. Apart from Ni and Pt, metal-oxide ITO exhibits a lower SBH with respect to Al contact. While ITO is commonly used as transparent conductive layer to spread current uniformly to LEDs by contacting the metal-oxide to p-type GaN layer, this study shows the potential of ITO contact to n-type GaN layer. With further optimization for deposition and annealing steps, ITO contact may give higher light absorption for more current generation in MSMphotodetector.

Table 2: Estimated Schottky barrier height (SBH) for GaN based MSM photodetector with different contacts. Contact

Estimated SBH, qΦB (eV)

Reported SBH, qΦB (eV)

Al ITO Ni Pt

0.95 0.92 0.90 0.86

0.6-0.8 [25] 0.91 [26] 0.92 [27] 0.89-1.13 [28]

Figure 5 shows signal-to-noise ratio (SNR) as a function of voltage for all samples. The SNR value is derived from the ratio of photo-current density (Ip) to dark-current density (Id) from figure 2. Clearly, Sample PD3 shows an increasing trend of SNR after ~0.3 V before the plateau between 1.5 V and 4.0 V. The highest SNR for the sample is recorded at a ratio of 100. This result is further implying that the advantageous of NixO inclusions in the Ni contact for polycrystalline GaN based MSM photodetector. On the other hand, Sample PD1 and Sample PD2 show almost constant SNR ratio of ~17, and ~35 above the bias voltage of 0.5 V. The number of the photo-generated carriers that contributes to the photo-current is limited due to high resistivity in the Al and ITO contacts, limiting the SNR for both samples.

On the other hand, Sample PD4 exhibits the least SNR ratio near to 1 due to high dark-

Signal-to-noise ratio

current characteristic as shown in figure 2 (d).

Figure 5: (Colour online) Signal-to-noise ratio (SNR) of polycrystalline GaN based MSM photodetector with different contact.

Subsequently, Xenon UV illumination coupled with a monochromator was used as the excitation source to determine the spectral responsivity (S) and the internal quantum efficiency (η) of all the samples. Figure 6 indicates that all samples, especially Sample PD3 are capable of detecting incident wavelength from UV to violet as the peak responsivity is

detectable at the wavelength of ~342 nm, ~385 nm, and ~416 nm. Correspondingly, this result confirms that our polycrystalline GaN based MSM photodetectors can extend the cutoff wavelength further as compared to some reported MSM photodetectors based on single crystal GaN; e.g. [5]. It should be noted that the extension of the peak responsivity appears at 416 nm (2.98 eV) despite the photon energy of the UV light is lower than the bandgap of GaN. This behavior is unclear and therefore needs further investigation. Nonetheless, there is possibility the peak responsivity at 416 nm it is related to defective properties of the polycrystalline GaN.

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Wavelength (nm) Figure 6: (Colour online) Responsivity of polycrystalline GaN based MSM photodetector with different contact. Inset figure shows the photoluminescence spectra of the polycrystalline GaN layer without any contact.

On top of that, the responsivity shows a clear cut-off at ~362 nm and ~405 nm as a result from high probability of photo-generated carriers to recombine and to emit photon.

This corresponds to the photoluminescence spectrum (see inset figure), of which the PL of near band edge (NBE) at 362 nm and blue line (BL) at 405 nm are clearly observed. Table 3 shows the internal quantum efficiency (η) of all samples. The efficiency is derived from the peak responsivity (R) at ~342 nm, ~385 nm and ~416 nm. In this work, the efficiency is calculated using this equation: =

  ×  ()

(2)

R is the peak responsivity and λ is the corresponding wavelength of the peak responsivity. As compared to others, Sample PD3 exhibits the highest internal quantum efficiency of 3.13 %, 2.36 % and 1.52 % at the wavelength of 342 nm, 385 nm and 416 nm, respectively. The internal quantum efficiency for longer wavelength excitation decreases due to the reduction of probability for an excited photon with lower energy to generate electron-hole pairs in the polycrystalline GaN. Furthermore, it is clear that high resistivity of Sample PD1 and Sample PD2 causes the deterioration in the internal quantum efficiency of these samples. In the case of Sample PD4, its poor internal quantum efficiency is attributed to the high dark-current density as observed in figure 2(d). Moreover, Pt contact is non-transparent and therefore it limits the photo-generated current.

Table 3: Peak responsivity (R) and internal quantum efficiency (η) of polycrystalline GaN based MSM photodetector. Sample/ Wavelength PD1 PD2 PD3 PD4 Ref. [8] Ref. [29]

Peak responsivity x 10-3 (A/W) 342 nm 385 nm 416 nm 2.69 2.00 1.39 2.13 1.56 1.08 8.58 7.31 5.10 0.81 0.60 0.46 -

Internal quantum efficiency (%) 342 nm 385 nm 416 nm 0.98 0.65 0.42 0.78 0.50 0.32 3.13 2.36 1.52 0.30 0.19 0.14 0.4 at λ = 360 nm 1.36 at λ = 400 nm

It is worth noting that the internal quantum efficiency of Sample PD3 is higher than the reported GaN based MSM photodetectors [8,29]. This may be attributed to the improved crystalline quality of the polycrystalline GaN as demonstrated in this work alongside the use of Ni for the contact. Next, temporal responsivity measurement was conducted to evaluate rise time (tr), recovery time (tf) and sensitivity (S) of all samples. Typically, an excitation source with a wavelength that corresponds to the highest internal quantum efficiency will be selected. In this work, the highest internal quantum efficiency is expected to be at λ = 342 nm. However, the equivalent LED light source with sufficient brightness is not available when this work was conducted. Alternatively, a LED with a wavelength of 385 nm was chosen as the excitation source since it also enables clear evidence of responsivity for all samples. The oscillation profile between photo-current density and dark-current density for all samples is shown in figure 7. Upon a closer inspection, the Ip (rising just after the excitation source started to pulse) can be divided into two parts. The rapid increase of current (Ip1) during the start of each light pulse originates from the rapid collection of electron-hole pairs generated near the inter-digitized fingers. Subsequently, the gradual increase of current (Ip2) comes from the delayed collection of electron-hole pairs generated away from the inter-digitized fingers. This delay is attributed to the reduction of the built-in potential gradient at the semiconductor-contact interface [30]. With reduced gradient, possibility to have rapid drift by the photo-generated electron-hole pairs away from the inter-digitized fingers can be lessen.

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Figure 7: Temporal responsivity of polycrystalline GaN based MSM photodetector with different contact. The inset figure shows the temporal responsivity for Sample PD4 in greater details

The rise time (tr) is the period for the photo-current to rise from 10% to 90% of the maximum value, while the recovery time (tf) is the period for the photo-current to fall from 90% to 10% of the maximum value. From figure 7, the parameters, including sensitivity (S) were calculated and the result is shown in Table 4. The table also shows the result from

selected reports for a photodetector with Ag/Al and Al contact and for photodetector with Ti/Al/Pt/Au contact. Due to the asymmetrical current profile in figure 7, tr and tf of all samples in this work is much longer than other reports [7-8,31-33]. This is always expected in a MSM photodetector with Ohmic behavior. It is important to note that the dark-current is consistent for each period. From the aspect of tr, Sample PD3 with Ni contact exhibits the fastest response, while the slowest response is observed for Sample PD2. This is due to high resistivity in the ITO contact layer as discussed earlier. In parallel, Sample PD3 shows tf of two times larger than tf of Sample PD4. However, the limited sensitivity of Sample PD4 due to poor Pt contact is not suitable for polycrystalline GaN based MSM photodetector. Moreover, tr and tf as shown in Sample PD3 are significantly improved as compared to reported ones [34-35]. Such behavior is attributed to low series resistivity and high SNR resulting from the formation of NixO inclusions in the Ni contact. From the aspect of sensitivity (S), Sample PD3 demonstrates the highest sensitivity than other samples. In fact, the sensitivity of Sample PD3 is 25 times higher than [34]. On the other hand, Sample PD2 shows a lower sensitivity; two times lower than Sample PD3. Overall, it can be observed that Sample PD3 shows better result in terms of tr, tf and S. Therefore, Ni is suggested to be a good contact for polycrystalline GaN based MSM photodetector.

Table 4: Rise time (tr), recovery time (tf) and sensitivity (S) of polycrystalline GaN based MSM photodetector with different contact. Sample

Contact

Rise time, tr (s)

PD1 PD2 PD3 PD4 Ref. [34] Ref. [35]

Al ITO Ni Pt Ag/Al and Al Ti/Al/Pt/Au

3.97 16.99 1.75 6.30 7.1 9

Recovery time, tf (s) 3.23 2.98 1.87 0.75 10.4 21

Sensitivity, S (%) 1600 3319 5840 115 233 -

Conclusion This work successfully demonstrated a series of functional metal-semiconductormetal semiconductor (MSM) photodetector based on polycrystalline GaN with different contact of Al, ITO, Ni and Pt. The photodetector with Ni contact showed better result, especially at λ = 385 nm due to the presence of NixO inclusions in the Ni contact. This particular photodetector demonstrated an internal quantum efficienty of 3.13%, 2.36 % and 1.52% at the wavelength of 342 nm, 385 nm and 416 nm, respectively, with an improvement of rise time, recovery time and sensitivity of 1.75 s, 1.87 sec and 5840%. Interestingly, all photodetectors exhibited a clear responsivity at the detection wavelength of 342 nm, 385 nm and 416 nm. The cut-off wavelength was extended due to the use of the polycrystalline GaN for MSM photodetector, as opposed to reported single crystal GaN MSM photodetector.

Acknowledgements This project has been funded by Fundamental Research Grant Scheme (203/CINOR/6711562) under Ministry of Education Malaysia and Research University Grant (1001/CINOR/8014033) under Universiti Sains Malaysia.

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30th September 2019 Editorial in Chief Superlattices and Microstructures Dear Sir/Madam, Highlights for the manuscript As per title, the highlights of our manuscript are as follows: 1. Ni is a good electrical contact to polycrystalline GaN based MSM photodetector. 2. NixO inclusions in Ni leads to improving the performance of the photodetector 3. All photodetectors have extended the cut-off wavelength. Please contact the following for future communication: Assoc. Prof. Norzaini Zainal Institute of Nano Optoelectronics Research and Technology (INOR) Universiti Sains Malaysia, 11800, Penang. MALAYSIA (e-mail address: [email protected]) Thanking you in advance.

Sincerely, Norzaini Zainal, PhD