Effect of Different Etching Time on Fabrication of an Optoelectronic Device Based on GaN/Psi

1  Introduction

III-nitride semiconductor materials, such as AlN, InN, and GaN, are widely used in the fabrication of modern optoelectronic devices because of their unique features for scientific and technological applications [1–3]. Gallium nitride (GaN) is one of the III-nitride semiconductor materials with different forms (powder, films, and nanoparticles) [4,5]. GaN has high electron mobility, high carrier saturation velocity, high thermal conductivity, a wide band gap of 3.4 eV, and a high optical absorption coefficient [6–9]. GaN-based optoelectronic devices include light-emitting diodes (LEDs), photodiodes, ultraviolet detectors, and high-power and high-frequency optoelectronic devices [10,11]. GaN devices can operate in fast and harsh environments and are thus ideal for photodiodes with small dimensions, good thermal and chemical stability, and low dark current and high breakdown field [12–14]. The gaN-based photodiode has interesting applications in civil and military, such as chemical sensing and heat, flame, and missile detection [15,16]. These features render optoelectronic devices operational in blue to near-ultraviolet spectral regions. Furthermore, GaN has a high temperature, power, and high frequency [17,18].

Several GaN thin-film deposition processes have been used, including chemical vapor deposition (CVD), such as molecular beam epitaxial, atmospheric pressure CVD, low-pressure CVD, plasma-enhanced CVD, metal-organic CVD, and hot wire CVD, and physical vapor deposition process, such as pulsed laser deposition (PLD) [19–23]. The PLD of III-nitride semiconductor materials has many distinct benefits, including simple method, versatility, rapid production of films with a strong forward-directed plume of materials that can be deposited with low pollution and specific stoichiometry [24–27]. Moreover, coating stoichiometry, crystallinity, topography, morphology, and roughness can be regulated by adjusting deposition parameters [28,29].

Porous silicon (Psi) is one of the most promising materials because of its excellent properties, showing great potential as a substrate for optoelectronic devices, such as sensors, LEDs, photodiodes, detectors, and computer chips [30–32]. One of the most important characteristics of the Psi layer is its large and reactive interior surface, which is considered the most critical property of Psi [33–36]. A Psi layer is different from bulk silicon because of its unique characteristics, such as strong visible photoluminescence (PL) at room temperature, direct bandgap, low cost, quantum confinement effect that promotes radiative transition, a high point of chemical reactivity and quick oxidation, which make Psi as a potential substrate for integrated optics and photonics devices [37,38].

The goal of this research is to study the effect of using different etching times in the preparation of Psi with the photoelectrochemical etching method. A solid-state laser (Nd: YAG laser) with a wavelength of 1064 nm was used in preparing a Gallium nitride on porous silicon (GaN on a Psi) “GaN/Psi” substrate with PLD. The deposition was carried out using laser energy of 1000 mJ and a vacuum pressure of 10−2 Torr. The effects of varying etching times on structural, spectroscopic, morphological, topographical, and electrical characteristics of GaN structure on a Psi substrate were explored in the present study.

2  Experimental Parts

2.1 Preparation of Psi Substrate

Silicon wafers (N-Type) (crystalline silicon, University Wafer, Inc., USA) of 500 μm thickness, 0.001–0.005 Ω/cm electrical resistivity “According to Saxena et al. [39], resistivity affects the porous silicon’s morphological structure. Where higher resistivity Si wafers produce aligned Si nanowire arrays-like structure after etching, whereas relatively lower resistivity Si wafers produce an interconnected porous (cheese-like) structure. But all the porous resistivity has visible photoluminescence due to the quantum confinement effect and that is all we need in our work” and (100) orientations were used in fabricating Psi substrate at room temperature with a photo-electrochemical etching method assisted by a diode laser of 660 nm wavelength and 100 mW power (China, Tongtool Company), DC power supply (0–30 V; China, Jiuyuan), digital multimeter (China, Victor Company, VC97), magnification beam expander of 10× (China, Carman Haas Company), electrolyte solution of 48% hydrofluoric acid (German, Thomas Baker Company) and 99.9% ethanol (German, Honeywell company). Si wafers were cut into 1 × 1 cm squares and cleaned with ultrasonic equipment in 99.9% ethanol for 5 min for the removal of surface contaminants and oxidation. The top-down approach of photoelectrochemical etching setup assisted by laser was used in preparing the Psi substrate. Additionally, the process was carried out using a Teflon cell with platinum electrodes with a purity of 95% (turkey) as cathode and silicon as the anode, as shown in Fig. 1.

Figure 1: Schematic diagram of photoelectrochemical etching process assisted by laser to prepare Psi substrates

Table 1 shows the parameters of the top-down photoelectrochemical etching process assisted by a laser for the preparation of Psi. The electrolyte concentration of HF: ethanol was fixed to 24% concentration with dilution equation [40–42], as shown in Eq. (1).

C1V1=C2V2 (1)

where:

C1 : Concentration of hydrofluoric acid.

V1 : Volume of hydrofluoric acid.

C2 : Concentration of ethanol.

V2 : Volume of ethanol.

2.2 Preparation of GaN Target

A highly impure GaN powder of 99.9% (Luoyang Advanced Material Company, China) was used in the pulsed laser deposition method and prepared by pressing GaN powder with a hydraulic press of 15 Kg cm2. The obtained GaN sample was circular, 5 gm, 2 cm in diameter, and 0.5 cm in thickness.

2.3 Preparation of GaN Films by PLD

A Q-Switching Nd: YAG laser (Guangzhou Dany Optical Technology Co., Ltd., China) with the energy of 1000 mJ, wavelengths of 1064 nm, and pulse duration of 7 ns was used in ablating the GaN target to deposit on the prepared Psi substrate. The process of PLD was carried out with a vacuum pressure of 10−2 mbar with practical parameters as presented in Fig. 2, and indicated in Table 2.

Figure 2: Schematic diagram of PLD method of GaN on Psi substrate using different wavelengths 1064, 532, and 355 nm

2.4 Preparing of Fingerprint Mask on GaN Films

A fingerprint mask-like aluminum (AL) mask has been deposited on the fabricated GaN films by a thermal evaporation system under 5 × 10−5 mbar for the maintenance of nanostructure performance. Fig. 3 shows the preparation processes of GaN-film-based optoelectronic devices.

Figure 3: Schematic illustration of the fabricated GaN/Psi optoelectronic device

2.5 Characterization by XRD, PL, FESEM, and AFM

Psi substrate and GaN thin film undergo several tests, such as X-ray diffraction (XRD) from Japan (XRD6000 Shimadzu Company) with copper radiation of 1.54060 Å was used in testing structural characteristics. Field emission scanning electron microscopy (FESEM) from German (ZEISS Company) was used in testing morphological properties with high resolution. Furthermore, atomic force microscopy (AFM) from the United States of America (TT-2 Workshop Company) was used in testing topographical properties. PL from the United States of America (Perkin Elmer Company) was used in testing spectroscopic properties.

2.6 Electrical Characterization

The current-voltage characteristics of Psi substrates and GaN thin film in dark and light for forward and reverse bias was investigated using a power supply (China, DAZHENG, 30 Volt, 5 A, PS-305D) and digital multimeters (UNI-T-UT33C) and (TEKR, CDM 250). Capacitance–voltage characteristic was explored using a programmable LCR meter (Taiwan, GW INSTEK, LCR-6100, and 10 Hz–100 kHz).

3  Results and Discussion

3.1 Structural Properties XRD

Fig. 4a shows the XRD patterns of the N-type Si <100> oriented wafer before the etching process, the Si <100> crystalline phase is the sole peak seen in the XRD pattern, indicating that no other crystalline phases were presented [43–45]. Fig. 4b shows the XRD patterns of the porous Si prepared at different etching times and the main substrates (N-Type Si). The main peaks were observed at etching times of 2.5, 5, 12.5, and 15 min, which mainly corresponded to (111) and (400) planes. Two main peaks were observed at etching times of 7.5 and 10 min, which mainly corresponded to the (200) and (400) planes. These peaks were indexed and consistent with the diffraction data of a silicon standard (JCPDS card 27-1402). A high and optimum crystallinity texture of the porous silicon structure appeared at an etching time of 10 min, according to Sampath et al. [46–48], and Fig. 4c presents the main peak (400), and it is clear that its splatted to two peaks one for Si and other for Psi [43].

Figure 4: XRD pattern (a) N-type Si, (b) prepared Psi substrates by photoelectrochemical etching method assisted by laser at different etching times, (c) the main peak of Si and porous Si (400)

Table 3 lists the structural features of the Psi substrate, as mentioned in Eq. (2). Crystallite size (D) was determined using Scherer’s formula [49–51], and the interplanar spacing (d) was obtained using the following formula [52–54]:

D=Kλ/βcoscosθ (2)

d=nλ2sinsinθ (3)

where:

D: Interplanar spacing in A∘

K: is a constant taken to be 0.9

λ : is the x-ray wavelength, wavelength of Cu K∝ = 1.54060 A∘

β : is fullwidth half maximum of XRD pattern

θ : is Bragg’s angle in degree

n: is diffraction order

Fig. 5 shows an XRD pattern of GaN on Psi film that ranges from 20° to 70° (in 2 θ axes) with no diffraction signal above 70°. Three crystal diffraction patterns were noticed after the deposition of GaN on Psi by the PLD method, corresponding to the particular hexagonal planes of GaN: (002), (110), and (103). These peaks were indexed and consistent with the diffraction data of the GaN standard (JCPDS card 01-074-0243). The high and most crystallographic texture appears in the (002) plane [55]. Notably, the crystal diffraction of Psi with an orientation of (400) was observed. Furthermore, Scherrer’s formula can be used in determining crystallite size (D) and interplanar spacing (d), as shown in Table 4.

Figure 5: XRD pattern of the deposited GaN on Psi substrate

3.2 Spectroscopic Properties

Fig. 6 shows the photoluminescence (PL) spectrum obtained from the prepared Psi substrates with the photoelectrochemical etching method at different etching times. The prepared Psi substrates exhibited yellow-orange visible PL with peak wavelengths in a range of 607–638 nm because of the surface states and the quantum confinement that developed on the Psi after the photoelectrochemical etching process as shown in Fig. 7 [56].

Figure 6: Room temperature photoluminescence of prepared Psi substrates by photoelectrochemical etching method assisted by laser at different etching time

Figure 7: Photoluminescence peak wavelength vs. etching time

The PL of Psi variant with the variation of etching time because PL intensity is affected the total volume of crystallites on the surface of Psi [57]. The energy gap was then calculated using Eq. (4) [58–60], where the Psi energy gap is definitely having higher energy gaps compared to Silicon (1.11 eV) and it increases from (1.94–2.1) eV as the etching time increases as shown in Fig. 8.

Egap=hcλ (4)

where:

Figure 8: Prepared porous silicon energy gap vs. etching time

Egap is energy gap of the prepared Psi

h is Planck’s constant

c is the speed of light

λ is the peak wavelength of the photoluminescence of prepared Psi at different etching time

Notably, at the etching time of 10 min, the peak PL was observed at 589 nm, which fell in the yellow visible band [61].

According to Li et al. [62], the photoluminescence spectral of the prepared porous silicon substrate at the etching time of 10 mA/cm2 has been shifted after the deposition of GaN material on it due to the changes in the surface chemistry achieved.

Fig. 9 shows the PL spectrum obtained from GaN/Psi deposited at 1064 nm wavelength by the PLD method. Two PL peaks were observed after GaN/Psi was deposited, which fall in the ultraviolet (UV) band of 374 nm corresponding to GaN and in the infrared (IR) band of 730 nm corresponding to Psi.

Figure 9: Room temperature photoluminescence of the deposited GaN on Psi substrate by PLD method

3.3 Surface Topography AFM

Fig. 10 shows the 3D AFM images of Psi substrates with the photoelectrochemical etching method assisted by laser at different etching times for surface topography analysis. At the etching time of 2.5 min, pores began to form slightly on the surface with an oval shape of low height and did not cover all of the surfaces. As the etching time increased to 5 and 7 min, the pores on the surfaces had irregular distributions over the surface, oval shapes, and low heights. As the etching time reached 10 min, the pores formed homogeneously over the entire surface with a steeper oval shape.

Figure 10: 3D AFM images of prepared Psi substrates by photoelectrochemical etching method assisted by laser at different etching times

Furthermore, at etching times of 12.5 and 15 min, nonhomogenous pores started to dissociate. Root mean square height and surface roughness are provided in Table 5. Notably, the particle size distribution of the prepared Psi substrates was investigated at the nanometer range and at different etching times. Table 4 shows the AFM parameters prepared by Psi substrates at different etching times.

Fig. 11 shows a 3D AFM image of GaN deposited on the Psi layer with the PLD method for surface topography analysis. The GaN film had sharp oval particles with root means square (RMS) height and surface roughness of 17.49 and 4.83 nm, as shown in Table 6.

Figure 11: 3D AFM image and particle size distribution of GaN films deposited at 1064 nm laser wavelength

3.4 Surface Morphology FESEM

Fig. 12 shows FESEM images of the prepared Psi substrates at different etching times for surface morphology analysis. At etching times of 2.5 and 5 min, similar FESEM images were observed. The pores started to form on the surface. As the etching time increased to 10 min, dark pores with small diameters were observed, accompanied by uniformly distributed and homogeneous pores. Moreover, the cross-section of the FESEM image at 10 min displayed the thickness of the Psi layer to 36.02 nm.

Figure 12: FESEM and EDS images of prepared Psi substrates by photoelectrochemical etching method assisted by laser at different etching time

Furthermore, at etching times of 12.5 and 15 min, the pore diameters increased, and the pore walls were widened, leading to decreased number of pores on the surfaces of the prepared Psi substrates. The average diameters of the prepared Psi substrates at different etching times (2.5–15 min) were obtained from FESEM software images: 48.21, 66.23, 57.54, 49.02, 61.89, and 60.91 nm.

Moreover, the EDX spectrum of the prepared Psi substrates at different etching times was investigated to show the high peak of silicon.

Fig. 13 shows the FESEM image of GaN deposited on the Psi layer by the PLD method to investigate the surface morphology. The magnification of surface morphology of the GaN films was at 80,000× and 160,000×. The FESEM image showed that the average particle diameter was 51.88 nm. The GaN particle completed covered the Psi layer without voids or cracks, forming a uniform and homogeneous spherical particle size of 51.88 nm and showing cauliflower-like morphology.

Figure 13: FESEM images of GaN film deposited at the wavelength of 1064 nm, (A) Magnified 160,000×, (B) Magnified 80,000×, and (C) FESEM cross section that shows the thickness of GaN deposited Psi substrate

Also, through this examination, the thickness of the GaN nanostructures measured, which were deposited using the pulsed laser deposition technique, where it was found that the thickness of these films is approximately 117.4 nm, as presented in Fig. 13c.

3.5 Electrical Properties

The I–V characteristic of the prepared Psi substrate at an etching time of 10 min (Fig. 14A) was examined in the dark and under illumination by halogen light (100 mW/cm2 intensity) at room temperature. The current that passed through the Psi substrate increased with applied voltage because of an increase in the resistivity of the Psi layer [63]. Charge transfer created a depletion region in the prepared Psi near the electrical dipole, thereby causing a rectifying behavior [64].

Figure 14: Electrical characteristic of prepared Psi substrate under light and dark. (A) I–V characteristic, (B) C–V characteristic, (C) 1/C2 vs. voltage characteristic

As shown in Fig. 14B, C–V, the characteristic of prepared Psi substrate at an etching time of 10 min was examined at room temperature under an applied voltage of 0–3 V. The capacitance of the prepared Psi substrate decreased with increasing applied voltage. This effect was referred to as the increase in depletion region at increasing built-in potential [65,66].

As shown in Fig. 14C, 1/C2 vs. voltage characteristic of the prepared Psi substrate at an etching time of 10 min was investigated. A linear relation was observed between C2 and applied voltage. The potential built-in was determined by extrapolating the linear portion of the presented curve to a 1/C2 value of 0 points. The potential built-in was 0.34 eV.

As shown in Fig. 15, the I–V characteristic of GaN/Psi heterojunction in the dark and under illumination were examined under the following conditions: halogen light intensity of 100 mW/cm2, room temperature, and forward and reverse bias. The forward current increased with bias voltage because of the decrease in the depletion layer width. Moreover, the GaN/Psi film showed rectification characteristics, and the current transport mechanism in both layers was recombination tunneling [67,68].

Figure 15: I–V characteristic of GaN on Psi layer deposited by PLD method

As shown in Fig. 16, the C–V characteristic of the GaN/Psi film deposited at 1064 nm wavelength was examined under an applied voltage of 0–2 V. Capacitance decreased with increasing applied voltage because of the increase in the width of the depletion region and subsequent decrease in capacitance at the junction [69].

Figure 16: C–V characteristic of GaN films deposited at different laser wavelengths

Fig. 17 shows the 1/C2 and voltage characteristics of GaN films deposited at 1064 nm laser wavelength. The fabricated GaN on the Psi film was an abrupt junction, and the value of the built-in potential decreased from 0.34 to 0.31 eV after GaN was deposited at a laser wavelength of 1064 nm [70–74].

Figure 17: 1/C2–V characteristic of GaN films deposited at 1064 nm laser wavelength

4  Conclusion

High-performance GaN/Psi film was fabricated with a pulsed laser deposition method using 1064 nm Nd:YAG laser. A suitable Psi substrate was prepared with the photoelectrochemical etching method assisted by laser at different etching times. The prepared Psi substrate at 10 min of etching time had a high and crystallographic texture with PL at the yellow visible band and pores formed homogeneously over the entire surface. The GaN/Psi film displayed the high and hexagonal crystallographic texture in the (002) plane. The spectroscopic properties results revealed that the PL emission of the deposited nano-GaN films was in the ultraviolet band (374 nm) related to the GaN material and in the near-infrared band (730 nm) related to the Psi substrate. The topographical and morphological results of the GaN films confirmed that the deposited film had spherical grains with an average diameter of 51.8 nm and surface roughness of 4.8 nm. The GaN/Psi surface showed a cauliflower-like morphology. The built-in voltage decreased from 3.4 to 2.7 eV after deposition. The GaN/Psi film is a potential material ultraviolet optoelectronic device because of its high photosensitivity in the ultraviolet region.

Acknowledgement: The authors would like to thank the Laser and Optoelectronics Engineering Department/university of Technology for the logistic support of this work.

Funding Statement: The authors received no specific funding for this study.

Conflicts of Interest: The authors declare that they have no conflicts of interest to report regarding the present study.