1. Introduction

In recent years, solar-blind ultraviolet photodetectors (UV PDs) based on wide bandgap semiconductors have been broadly used in a variety of fields, such as environmental monitor, biological detection, astronomical exploration and ultraviolet communication [1–7]. Up to now, though various solar-blind UV PDs with different structures have been manufactured [8,9], there are still a large number of challenges in obtaining multi-spectral response and high-responsivity UV PD.

Due to their appropriate bandgap and intrinsic wavelength selectivity, wide bandgap semiconductor materials represented by GaN and Ga₂O₃ are outstanding candidates for the construction of UV PDs. Among kinds of GaN-based UV PDs, Schottky barrier diode (SBD) UV PD is one of the representative types owing to the simple fabrication process and prominent transient response [10]. However, GaN-based Schottky barrier diode UV PDs have the same problem as the other unipolar types PDs. The most striking issue is the high leakage current and low responsivity in the GaN-based Schottky barrier diode UV PDs. By contrast, Ga₂O₃, whose bandgap ranges from 4.7 eV to 4.9 eV, has also been fabricated for various unipolar type UV PDs, including metal-semiconductor-metal PDs and SBD PDs [11–15]. However, it is challenging to improve the p-type doping of Ga₂O₃. Therefore, the Ga₂O₃-based bipolar devices such as p-i-n UV PDs are still limited to a great extent. Furthermore, obtaining a high responsivity and UV-to-visible rejection ratio for Ga₂O₃-based UV PDs is also an urgent problem to be solved.

Multi-spectral UV PD has good detection ability and can be widely used in optical communication, medicine and other fields. However, the research of multi-spectral PD mainly focuses on infrared band. Only a few publications have been published on the ultraviolet band [16–18], and there are no comprehensive theoretical design models available.

In this work, we systematically proposed a Ga₂O₃/GaN ultraviolet photodetector with high responsivity, UV-to-visible rejection ratio and multi-spectral detection capability via comprehensive structure optimizations. The spectral response of the photodetector can be modulated effectively by optimizing the thickness ratio and doping concentration of light absorption layers. The simulated device after optimization showed a double absorption peak with a high responsivity of 892/950 A/W at 254/365 nm and a high UV-to-visible rejection ratio of 10³.

2. Device structure and physical models

The vertical device structure diagram obtained through Silvaco TCAD software is shown in Fig. 1(a). The work functions of the anode and cathode were set to 5 eV and 3.93 eV, respectively. We exposed the device to UV light ranged from 230 nm to 450 nm with the direction of incidence perpendicular to the upper surface of the device. In addition, the thickness and doping concentration of light absorption layers play important parts in the detection performance of photodetectors. During the simulation, in order to achieve a UV PD with high performance by structural optimization, the thickness of GaN/Ga₂O₃ was set to vary from 200/160 nm to 1600/300 nm, and the doping concentration of GaN also had a scope of variation which ranged from 5 × 10¹⁵ to 5 × 10¹⁶cm^-3. The optical index coefficient of light absorption layers can be found in the works of Peelaers [19] and Muth [20] et al.

 

Fig. 1. (a) Schematic of the high responsivity dual-band ultraviolet photodetector based on Ga₂O₃/GaN heterostructure. (b) Energy band diagram of the Ga₂O₃/GaN heterostructure in equilibrium.

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The framework used in this device simulation is Poisson equation and continuity equation. Besides, Gummel and Newton nonlinear combinatorial iterative method were used to make the simulation results converge. In the process of simulation, we mainly used four types of physical models, which are mobility model, carrier recombination model, carrier generation model (impact ionization model and band-to-band tunneling model) and carrier statistical model (Fermi-Dirac statistical model), respectively. In the mobility model, the low-field mobility model (conmob) and the parallel electric field-dependent model (fldmob) are selected as the impurity concentration dependent mobility models at 300 K. In addition, the expressions of the carrier recombination model represented by Shockley-Read-Hall (SRH) model, optical generation model and Auger model are listed as follows:

3. Results and discussions

In order to elaborate the transport principle of carriers in the Ga₂O₃/GaN heterostructure, the energy band diagram is shown in the Fig. 1(b). At equilibrium, Ga₂O₃ and GaN have a conduction band offset, which can motivate further separation of photogenerated carriers [21]. Meanwhile, a low built-in potential barrier is generated at the heterostructure interface due to the conduction band offset between Ga₂O₃ and GaN. With this potential barrier, the flow of photogenerated electrons from Ga₂O₃ layer into GaN layer is thereby greatly accelerated. Instead, photogenerated holes in the valence band cannot be transported efficiently due to high valence band offset and low mobility. When a positive bias is applied to the GaN side, the degree of band bending in heterostructure interface becomes larger. This promotes it easier to transfer electrons from Ga₂O₃ to GaN and increases the barrier of photogenerated holes in GaN. The modulation of Ga₂O₃/GaN heterostructure band offset and electric field results in the transport of photogenerated electrons and the blocking of photo-excited holes, thereby enhancing the photoconductive gain of the device. Therefore, electrons will be continuously transported under the effect of electric field before the photogenerated carrier recombination, thus enhancing the photoconductive effect.

Responsivity (${R\; = \; }\frac{{{{I}_{{light}}}\; { \textrm{-} \; }{{I}_{{dark}}}}}{{{{P}_{{op}}}}}$) is an important parameter to characterize the performance of photodetectors, where ${{I}_{{light}}}$ and ${{I}_{{dark}}}$ stand for the current density under illumination and dark circumstance, respectively. In addition, ${{P}_{{op}}}$ means the power density of incidence light whose value is a constant in this work. Spectral response curves of the Ga₂O₃/GaN dual-band UV PD with different thicknesses of GaN are shown in Fig. 2(a). Meanwhile, the thickness of Ga₂O₃ and n-doping concentration of GaN is fixed at 200 nm and 1 × 10¹⁶cm^-3, respectively. As seen in Fig. 2(a), when the thickness of GaN ranges from 200 nm to 1500 nm, there are always two absorption peaks with varying intensity at wavelengths of approximately 254 nm and 365 nm corresponding to cut-off wavelengths of the two absorption layers. In addition, steep cut-off edges appear near the two absorption peaks, which is due to a larger response speed resulting from the plummet of absorption coefficient of the material. GaN exhibits an increasing trend in responsivity at the wavelengths of 230-254 nm with a maximum responsivity of up to 1931 A/W when its thickness varies from 200 nm to 1500 nm. The responsivity of wavelengths between 255 nm and 365 nm first increases and then falls with the increasing of GaN thickness, and the maximum responsivity in this waveband can be able to reach 618 A/W at the thickness of 600 nm. In the case of applying a positive bias on one side of GaN, the changing of GaN thickness will affect the electric field distribution of the heterostructure. Besides, the change trend of responsivity with electric field intensity will be explained in detail later. The two absorption peaks in these bands show a similar phenomenon in that they redshift as GaN thickness increases. The redshift of absorption peaks is attributed to the lack of optical dead space which is a region where photogenerated carriers are collected before recombination [22]. Due to the deep energy level in the GaN absorption layer, the device still shows an optical response when the incident wavelength is greater than 365 nm, which makes the electrons jump from the deep energy level to the conduction band and then form the photocurrent to generate the spectral response.

 

Fig. 2. Spectral response curves of the Ga₂O₃/GaN dual-band UV PDs under different (a) GaN thickness and (b) GaN doping.

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The responsivity and rejection ratio of absorption peaks (254 and 365 nm) under various GaN thicknesses are displayed in Fig. 3(a) in order to better understand how the thickness of the GaN absorption layer affects the detection performance. From the perspective of responsivity, the responsivity of absorption peak at 254 nm decreases from 1157 to 123 A/W and that of absorption peak at 365 nm gradually increases from 50 to 497 A/W (at the thickness of 600 nm) before decreasing to 128 A/W as the thickness of GaN layer increases from 200 nm to 1500 nm, which is also consistent with the findings in Fig. 2(a). The rejection ratio, which is defined in this work as the ratio of responsivity at the wavelength of absorption peaks (254 nm and 365 nm) to 280 nm and 390 nm, is another crucial statistic to characterize the detection performance. Though R₂₅₄/R₃₉₀ and R₃₆₅/R₃₉₀ decreased gradually with increasing GaN thickness, they were still at high levels (1119 and 917, respectively) at the maximum intensity of 365 nm absorption peaks. To describe the relative intensity of absorption peaks, we additionally introduce R₂₅₄/R₂₈₀ and R₃₆₅/R₂₈₀. The rejection ratio of R₂₅₄/R₂₈₀ and R₃₆₅/R₂₈₀ at the thickness of 600 nm are 1.28 and 1.04, indicating high responsivity at the absorption peaks. Therefore, the value of 600 nm was determined as a fixed requirement in the subsequent optimization once the thickness of GaN had been optimized.

 

Fig. 3. Responsivity and rejection ratio of the Ga₂O₃/GaN dual-band UV PDs under different (a) GaN thickness and (b) GaN doping. The red solid sphere and triangle represent the responsivity of the 254 nm and 365 nm absorption peaks, respectively. The blue solid (hollow) sphere and triangle represent the ratio of responsivity at 254 nm and 365 nm to 390 nm (280 nm), respectively.

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Device detection performance can be significantly impacted by doping of absorption layers. Therefore, we listed a series of GaN doping levels to modulate the responsivity and rejection ratio of Ga₂O₃/GaN dual-band UV PDs after optimizing the thickness of GaN absorption layer, as shown in the Fig. 2(b). There are always two absorption peaks in the spectral response when the doping of GaN absorption layer changes from 5 × 10¹⁵ to 5 × 10¹⁶cm^-3. At the wavelength of 230-254 nm, the responsivity of the Ga₂O₃/GaN dual-band UV PD increases with the doping concentration of GaN absorption layer and the maximum responsivity can be able to reach 1213 A/W with the doping concentration of 5 × 10¹⁶cm^-3. This is attributed to a markedly reduced voltage drop in the GaN region caused by the increasing doping concentration of GaN, which raises the voltage drop in the Ga₂O₃ region, thus enhancing the responsivity. On the contrary, the responsivity of the device steadily falls in the range of 255 nm to 365 nm as the doping of GaN absorption layer increases, while the maximum responsivity can still reach 1416 A/W when the doping concentration of GaN absorption layer is 5 × 10¹⁵cm^-3. Additionally, we found that the two absorption peaks have a blueshift with the increasing doping concentration of the GaN absorption layer, which can be attributed to the Moss-Burstein effect [23,24]. Namely, when the carrier concentration of n-type doping is too high and some electrons have occupied the conduction band, requiring more energy to make the electrons jump from the valence band to the conduction band.

The relationship between the responsivity and rejection ratio at the absorption peaks of 254 nm and 365 nm and the doping concentration of the GaN absorption layer is shown in Fig. 3(b), so as to further investigate the influence of the doping concentration of the GaN absorption layer on the device detection performance. According to Fig. 3(b), the rejection ratio of the device (R₂₅₄/R₃₉₀ and R₃₆₅/R₃₉₀) is generally in an upward trend when the doping concentration of GaN absorption layer increases, and the maximum values can be able to reach 2.76 × 10⁵ and 1.37 × 10³, respectively. Unfortunately, there is a rapid decline in the responsivity at the wavelength of 254/365 nm from 1139/1416 to 696/238 A/W when the doping concentration of GaN absorption layer increases from 5 × 10¹⁵ to 2 × 10¹⁶cm^-3. Therefore, there must be a trade-off between responsivity and rejection ratio. When the doping concentration of the GaN absorption layer is 1 × 10¹⁶cm^-3, the R₂₅₄/R₃₉₀ and R₃₆₅/R₃₉₀ of the device are 1119 and 917, respectively. At this time, the responsivity still remains at a high level (827/678 A/W correspond to 254/365 nm). Finally, we get to the conclusion that the ideal doping concentration for the GaN absorption layer is 1 × 10¹⁶cm^-3.

As shown in Fig. 4, the thickness and doping concentration of the GaN absorption layer can greatly affect the electric field distribution of the devices. Due to the energy band offset of the two absorption layers and the piezoelectric polarization effect at the interface, there are strong electric field spikes at the heterojunction of the two absorption layers from Fig. 4(a) and Fig. 4(c). In addition, Fermi level is found above the conduction band by observing the energy band. In the GaN absorption layer, some electrons transfer to the direction of cathode because the conduction band shifts downward with the increase of depth, so there is an electric field valley at the right side of the heterojunction. As can be seen from Fig. 4(b), taking the depth of 0.2 µm (0.35 µm) as an example, the electric field intensity of the device presents a trend of initially increasing and then monotonically decreasing with the increase of GaN thickness, which makes the responsivity of corresponding waveband of the device also present a same trend of first increasing and then monotonically decreasing with the increase of GaN thickness (see Fig. 2(a)). The same conclusion can be drawn in Fig. 4(d) that the electric field intensity at 0.2 µm (standing for Ga₂O₃ layer) /0.7 µm (standing for GaN layer) shows a monotonically increasing/decreasing trend with the increase of GaN doping, respectively. As a result, the responsivity of corresponding waveband of the device also exhibits the same changing trend (see Fig. 2(b)).

 

Fig. 4. (a) Electric field intensity distribution curves of the Ga₂O₃/GaN dual-band UV PDs under different GaN thickness; (b) Electric field intensity versus the thickness of GaN at 0.2 µm (in Ga₂O₃ absorption layer) and 0.35 µm (in GaN absorption layer); (c) Electric field intensity distribution curves of the Ga₂O₃/GaN dual-band UV PDs under different GaN doping; (d) Electric field intensity versus the doping concentration of GaN at 0.2 µm (in Ga₂O₃ absorption layer) and 0.7 µm (in GaN absorption layer).

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Following the optimization of the GaN absorption layer, it is necessary for improving the detection performance to optimize the thickness of intrinsic Ga₂O₃ absorption layer. As is shown in Fig. 5(a), there is the relationship between Ga₂O₃ thickness and responsivity of Ga₂O₃/GaN dual-band UV PDs. When the wavelengths of incident light range from 230 nm to 254 nm, the responsivity of Ga₂O₃/GaN dual-band UV PD responds progressively as the thickness of Ga₂O₃ absorption layer. This increasing of responsivity at the waveband of 230-254 nm can be attributed to the increase of layer thickness so that more electrons transition to the conduction band to form photogenerated carriers. On the contrary, the responsivity in the 255-365 nm range decreases with the increasing Ga₂O₃ thickness, which is attributed to the significantly decreasing voltage drop in both GaN and Ga₂O₃ region. The effect of thickness of Ga₂O₃ absorption layer on the responsivity and rejection ratio at the wavelength of 254 and 365 nm has been described in the Fig. 5(b). As the thickness of Ga₂O₃ layer increases from 160 nm to 300 nm, the responsivity at 254/365 nm absorption peak reduces dramatically from 982/1258 to 792/108 A/W. This can be explained by Fig. 5(c) and Fig. 5(d) which show that different Ga₂O₃ thicknesses affect the distribution of the electric field intensity of the devices. Namely, the responsivities at 254 nm and 365 nm exhibit the similar variation trend because the electric field intensity in the Ga₂O₃ and GaN absorption layers decreases with the increasing of Ga₂O₃ thickness (take the depth of 0.2/0.7 µm as an example). In the view of rejection ratio, R₂₅₄/R₃₉₀ and R₃₆₅/R₃₉₀ still keep a high level (10³-10⁴) and have a tendency to increase continuously with thickness. As shown in the red line in Fig. 5(a), it is expected to achieve as larger responsivity as possible with a high rejection ratio, which can be obtained at the Ga₂O₃ thickness of 180 nm. At this time, both absorption peaks have relatively high intensity (both R₂₅₄/R₂₈₀ and R₃₆₅/R₂₈₀ > 1) and responsivities of the two absorption peaks are 892 and 950 A/W, respectively.

 

Fig. 5. (a) Spectral response curves of the Ga₂O₃/GaN dual-band UV PDs under different Ga₂O₃ thickness and (b) responsivity and rejection ratio of the Ga₂O₃/GaN dual-band UV PDs under different Ga₂O₃ thickness. The red solid sphere and triangle represent the responsivity of the 254 nm and 365 nm absorption peaks, respectively. The blue solid (hollow) sphere and triangle represent the ratio of responsivity at 254 nm and 365 nm to 390 nm (280 nm), respectively; (c) Electric field intensity distribution curves of the Ga₂O₃/GaN dual-band UV PDs under different Ga₂O₃ thickness; (d) Electric field intensity versus the thickness of Ga₂O₃ at 0.2 µm (in Ga₂O₃ absorption layer) and 0.7 µm (in GaN absorption layer).

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4. Conclusion

In summary, we obtained a Ga₂O₃/GaN dual-band ultraviolet photodetector with an ultrahigh responsivity and UV-to-visible rejection ratio by means of optimization strategy. An optimal device design scheme is presented by optimizing the thickness ratio and doping concentration of the heterostructure. It is shown by using numerous simulation calculations that the responsivities of Ga₂O₃/GaN dual-band ultraviolet photodetector can reach 892 and 950 A/W at the wavelength of 254 and 365 nm, respectively. Furthermore, UV-to-visible rejection ratio of the optimal scheme can be greater than three orders of magnitude. The high performance optimization scheme in this work will make important theoretical preparation for the reasonable design of multi-spectral UV photodetectors in the future.