Open Access
Add to BibSonomy
Abstract
The effects of the bias voltage on photoluminescence (PL) intensity and the spectral responsivity are studied for an Au/Ni/undoped GaN/n+-GaN structure Schottky barrier photodetector. Near-band-gap PL of GaN quenches at low reverse bias but enhances at high reverse bias. Under high reverse bias, holes are accumulated in the region of the GaN adjacent to the Ni/Au. Only electrons below empty states at top of valence can be excited to the conduction band in this region, which reduces the absorption of near-band-gap luminescence. The decrease of the spectral responsivity for near the band gap under higher reverse bias also supports this assumption.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
GaN Schottky barrier diode (SBD) has important applications in photoelectric and electronic devices due to the wide direct bandgap and high breakdown voltage of GaN [1–9]. In a recent report, a bias-selective dual-operation-mode GaN Schottky barrier photodetector has been fabricated [10]. It shows different photo-responsivity behaviors under various biases. Hence, investigating the excitation and recombination behavior will help achieve wider applications and deeper understanding of the mechanism for GaN Schottky barrier devices.
Photoluminescence (PL) can be used to study the recombination of photo-generated carriers in the inorganic and organic semiconductor materials under applied electric field or bias voltage [11–13]. PL behaviors under various biases have been studied for InP [14] and GaAs [15] SBDs. However, there are only a few reports discussing the variation of PL with changing bias for GaN Schottky barrier photodetector. This letter reports the influence of the bias voltage on the PL intensity and spectral responsivity of the GaN Schottky barrier photodetector. Both the GaN Schottky barrier height and the band gap of GaN are measured by using the effect of the bias voltage on the PL intensity. This study may provide a further understanding of the materials and devices properties.
2. Method
The structure of GaN Schottky barrier detector investigated is grown by metal-organic chemical vapour deposition (MOCVD) system on a c-plane sapphire substrate. It consists of a 20 nm thick GaN nucleation layer, a 3 µm thickness of highly Si-doped (n∼2 × 1018 cm−3) GaN layer and a 0.4 µm unintentionally doped GaN layer. The detector mesa structure with an active area of 1.24 mm2 is patterned by standard photolithography techniques. The structure diagram of Schottky barrier detector is shown in Fig. 1. The device fabrication process is similar to that used in the Ref. [16]. Only undoped GaN layer instead of the p-GaN layer and a Ni(5 nm)/Au(5 nm) Schottky-contact layer instead of a Ni/Au p-contact layer are included. The I-V characterization of the Schottky detector gives the reverse saturation current Is = 3.08 × 10−16 A. The dark current density of the device under reverse bias 10V is about 4 × 10−8 A/cm−2.
The PL intensity at one photon energy vs bias voltage characteristics of the Schottky detector are measured by the setup as shown in Fig. 2. The setup consists of a 325 nm He-Cd laser, two focus lenses, a sample holder, a monochromator, a photomultiplier tube (PT), two KEITHLEY Model 2400 Source Meter, a Data Acquisition Module (DAM) and a computer. The size of focused laser beam on the surface of Schottky detector is less than the active area of the detector in order to keeping it in the active area. The monochromatic light beam coming from the monochromator is detected by the photomultiplier tube. One Source Meter as an Ampere meter measures the current of the photomultiplier tube. The other as voltage source supplies the bias voltage to the Schottky barrier photodetector and can scan the voltage from the negative (reverse bias) to positive (forward bias) voltage. When voltage source scans the bias, the Ampere meter measures the current of the photomultiplier tube. This function can be performed by LabTracer 2.0 Software running on the computer.
3. Results and discussion
The dependence of the luminescence intensity on the applied reverse voltage at various photon energies is shown in Fig. 3. The effects of bias voltage on PL intensity are different for it at 3.435 eV (P3.435eV), 3.416 eV (P3.416eV), and 2.296 eV (P2.296eV). P3.435eV and P3.416eV are induced by the band-band emission of GaN. P3.416eV is on the lower energy side of the band-band emission but P3.435eV is closer to the band-edge peak [17]. P2.296eV is related to the transitions between the conduction band or donors and deep acceptors (recombination centers) induced by defects in GaN [12,18,19]. Under forward bias condition, when the voltage is less than the value of the built-in potential Vbi, the voltage drop is mainly across the depletion layer to overcome the built-in electric field. Then the electric field in the depletion layer decreases and the separating effect of the photo-generated carriers in the depletion layer becomes subdued with increasing forward voltage. Therefore, the PL intensity at various photon energies enhances with the increasing forward voltage until the depletion region is fully collapsed as shown in part C in Fig. 3.
Fig. 3. The bias voltage dependent PL intensities at 3.435, 3.416, and 2.296 eV and the I-V characteristics of the Schottky detector. The insert shows the PL spectrum under zero bias.
In part D of Fig. 3, when the forward bias voltage is equal to or over Vbi, the depletion region is fully collapsed. With a higher forward bias voltage applied, the current and the electron density in the undoped GaN layer increase very rapidly. This electric field also causes the separation of the photo-generated electron-hole pairs in the undoped GaN layer [20]. The electrons move toward the transparent contact, and the holes move toward n+-GaN layer. During holes moving, they can be recombined by donors and recombination centers [21]. The PL intensity for photon energy near the band gap (P3.435eV) or slightly less than it (P3.416eV) decreases. Because the yellow luminescence (2.296 eV) is related to the transitions between the conduction band or donors and deep acceptors (recombination centers) induced by impurities or native point defects or around the edge dislocations, this effect makes P2.296eV increase with the rising forward bias [12,18,22–24].
P3.435eV and P3.416eV rise with the increasing forward bias voltage from zero to the value of the Schottky barrier height in units of eV (about 1 V). Then, their intensities decrease under higher forward bias voltage. This effect can be used to measure the GaN Schottky barrier heights. It can be explained as followed:
Metal(Au/Ni)/undoped GaN/n+-GaN contact structure in thermal equilibrium with no external voltage has a constant Fermi energy throughout the structure. The total built-in potential Vbi of the contact structure is equal to the difference between the work function of the metal and n+-GaN:
where Φm, ΦN is the work function of the metal (4.9 eV for Ni) [25] and n+-GaN (4.1 eV) [26] respectively. The Schottky barrier height ϕB of the metal/undoped GaN is equal to the difference between the work function of the metal and the electron affinity χs of GaN:The relationship between the work function ΦN of the n+-GaN and the electron affinity χs of GaN is
where δN is the energy difference between the Femi energy and bottom of the conduction band of n+-GaN. where Nc is the effective conduction band density of states in GaN, nN is the carrier density in n+-GaN layer, K is Boltzmann's constant, and T is absolute temperature. Therefore, the Schottky barrier height ϕB for the Metal/undoped GaN/n+-GaN contact structure isAt room temperature, nN = 2 × 1018 cm−3, kT = 0.0259 eV, Nc = 2.2 × 1018 cm−3, δN = 0.0025 eV. Because δN << (Φm − ΦN), ϕB ≅ Φm − ΦN = Vbi, that is to say, the value of the Schottky barrier height ϕB for the metal/undoped GaN/ n+-GaN structure is approximately equal to the value of built-in potential Vbi.
In part B of Fig. 3, under low reverse bias, the intensities of P3.435eV, P3.416eV, and P2.296eV decrease with increasing reverse bias voltage. The intensity of electric field in the depletion layer and the width of the depletion layer increase under higher reverse bias voltage. Hence, photo-generated electron-hole pairs can be separated more efficiently, which causes the PL drop. However, as shown in part A of Fig. 3, under high reverse bias, P3.435eV rises with increasing reverse bias voltage while P3.416eV and P2.296eV keep dropping. The behavior of P3.435eV is very abnormal and interesting. Therefore, it will be discussed in detail in the following.
The PL spectra is recorded under different reverse bias voltages, shown in Fig. 4(a). Under high reverse bias voltage, near-band-gap PL (3.425∼3.473 eV) enhances with increasing reverse bias voltage, which is the same as the behavior of P3.435eV in part A of Fig. 3. The photon absorption of the detector can influence the intensity of PL. For example, shown in Fig. 4(c), the band-edge PL can be absorbed in the whole undoped GaN layer under low reverse bias. This will decrease the intensity of near-band-gap PL.
Fig. 4. (a) PL of the detector under several reverse bias voltages of −3, −9, and −15 V respectively. (b) The difference of PL intensity under various reverse bias voltages. (c) The schematic diagram of the band structure of the Schottky barrier under low reverse bias. (d) The schematic diagram of the band structure of the Schottky barrier under high reverse bias.
The photo-generated electron-hole pairs excited by 325 nm Laser beam rapidly relax to the bottom of the conduction band for electrons and to the top of the valence band for holes by emitting nonequilibrium phonons through electron-phonon interaction [27,28]. Shown in Fig. 4(d), under high reverse bias, photo-generated holes are accumulated in the region of undoped GaN adjacent to the Ni/Au transparent contact (Ni/Au film) due to the band bending. In other word, there is a hole accumulation region. In this region, the states at top of the valence band are occupied by holes. Therefore, only electrons below these states can be excited to the conduction band. Hence, the absorption edge energy in the hole accumulation region rises under higher reverse bias voltage. The photon energy in a very narrow range near the band gap of the GaN is almost transparent in this region. Near-band-gap PL of GaN will not be absorbed in the hole accumulation region. With increasing reverse bias, the density of the holes in the hole accumulation region raises and the width of this region increases. Therefore, near-band-gap PL enhances under higher reverse bias. On the other hand, more holes in the hole accumulation region under higher revise bias will contribute to the radiative recombination band-band emission too. Hence, PL in the narrow range near the band gap enhances.
Under high reverse bias, the hole accumulation region will reduce the absorption of near-band-gap PL. Moreover, the high density of holes in this region will also enhance the radiative recombination band-band emission. As a result, near-band-gap PL is enhanced with the increasing reverse bias. Hence, the difference of PL intensity under various reverse bias voltages can show this variation of PL induced by the increasing reverse bias more clearly. Moreover, the band-edge PL should be most enhanced because these effects only contribute to the intensity of PL for photon energy in a very narrow range near the band edge. The energy position of the maximum peak of the curve −15V-(−3V) in the Fig. 4(b) is 3.439eV which agrees well with the extrapolation of B. Monemar's [29] data about the band gap of the GaN at 298 K. With changing voltage difference between high and low reverse bias, the peak position of the curve in the Fig. 4(b) does not shift obviously. This supports the previous discussion that the band-edge PL should be most enhanced due to the increase of the absorption edge and the enhancement of the band-edge emission in the hole accumulation region. The difference of PL intensity between high reverse and low reverse bias applied to GaN Schottky barrier detector can be used to measure the GaN band gap.
To further verify the analysis above, the spectral responsivity of the detector under various reverse biases is measured as shown in Fig. 5. Under low reverse bias (0∼−4 V), the spectral responsivity of the detector rises with the increasing reverse voltage for all photon energies in Fig. 5 due to the stronger separation of photoexcited electron-hole pairs [30,31]. The radiative and no-radiative recombination decreases with increasing reverse bias due to the separation of photoexcited electron-hole pairs. The decreasing recombination of photo-generated carriers contributes to the rise of the spectral responsivity. Under high bias voltage (−4∼−12 V), because all the light for photon energy greater than the GaN band gap can be absorbed the responsivity for it rarely changes. On the other hand, the enhancement of the responsivity for photon energy smaller than the GaN band gap is a result of Franz-Keldysh effect [32,33]. Carriers is transported to energy levels that are lower than the band-gap energy with the assistance of the electric field. The absorption of photons in this energy range is increased. However, the responsivity for near band gap is reduced under higher reverse bias voltage. The hole accumulation region leads to a slight rise in the absorption edge energy of the hole accumulation region, just like the Burstein–Moss effect [34,35]. The width of the hole accumulation region also becomes larger with the increasing reverse bias. Therefore, the absorption for near-band-gap light declines with the rising reverse bias. This also supports our previous discussion that the near-band-gap luminescence is almost transparent in the hole accumulation region under high reverse bias.
4. Conclusions
In conclusion, the bias voltage applied to the GaN Schottky barrier detector plays a very important role on the PL emission intensity at various photon energies. The near-band-gap PL of the Schottky detector shows an enhancement with increasing reverse bias in our test. The high density of holes in the hole accumulation region may reduce the absorption of near-band-gap luminescence and enhance band-band emission, which will increase the intensity of near-band-gap PL under high reverse bias. This effect can be used to measure the bad gap of the GaN at room temperature.
Funding
National Key R & D Program of China (Grant No. 2017YFB0405001); National Natural Science Foundation of China (Grant Nos. 62034008, 62074142, 62074140, 61974162, 61904172,61874175); Youth Innovation Promotion Association of the Chinese Academy of Sciences (Grant No. 2019115).
Acknowledgments
This work was supported by the National Key R&D Program of China (Grant No. 2017YFB0405001), the National Natural Science Foundation of China (Grant Nos. 62034008, 62074142, 62074140, 61974162, 61904172, 61874175), and the Youth Innovation Promotion Association of Chinese Academy of Sciences (Grant No. 2019115).
Disclosures
The authors declare that there are no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
References
1. J. Lee, J. Yoo, H. Kang, and J. Lee, “840 V/6 A-AlGaN/GaN Schottky barrier diode with bonding pad over active structure prepared on sapphire substrate,” IEEE Electron Device Lett. 33(8), 1171–1173 (2012). [CrossRef]
2. Y. Saitoh, K. Sumiyoshi, M. Okada, T. Horii, T. Miyazaki, H. Shiomi, M. Ueno, K. Katayama, M. Kiyama, and T. Nakamura, “Extremely low on-resistance and high breakdown voltage observed in vertical gan Schottky barrier diodes with high-mobility drift layers on low-dislocation-density GaN substrates,” Appl. Phys. Express 3(8), 081001 (2010). [CrossRef]
3. R. Yin, Y. Li, C. P. Wen, Y. Fu, Y. Hao, M. Wang, and B. Shen, “High voltage vertical GaN-on-GaN Schottky barrier diode with high energy fluorine ion implantation based on space charge induced field modulation (SCIFM) effect,” in 2020 32nd International Symposium on Power Semiconductor Devices and ICs (ISPSD), 2020), 298–301.
4. M. Kumar, C. Y. Lee, H. Sekiguchi, H. Okada, and A. Wakahara, “Demonstration of a large-area AlGaN/GaN Schottky barrier photodetector on Si with high detection limit,” Semicond. Sci. Technol. 28(9), 094005 (2013). [CrossRef]
5. W. Y. Weng, T. J. Hsueh, S. J. Chang, G. J. Huang, and H. T. Hsueh, “A β-Ga2O3/GaN Schottky-barrier photodetector,” IEEE Photonics Technol. Lett. 23(7), 444–446 (2011). [CrossRef]
6. A. M. Noor Elahi, M. R. M. Atalla, C. Mo, W. Zhang, S. Liu, Z. Zhang, Z. Jiang, J. Liu, X. Sun, M. Chang, X. Zhang, and J. Hsu, “The effect of the undoped GaN/buffer-layer interface on the operation of Schottky diodes and MESFET devices,” Microelectron. Eng. 214, 38–43 (2019). [CrossRef]
7. S.-H. Shin, B.-K. Jung, J.-H. Lee, M.-B. Lee, J.-H. Lee, Y.-H. Lee, and S.-H. Hahm, “Superior characteristics of RuO2/GaN Schottky-type UV photodetector,” phys. stat. sol. (a) 188(1), 341–344 (2001). [CrossRef]
8. O. Katz, V. Garber, B. Meyler, G. Bahir, and J. Salzman, “Vertical versus lateral GaN Schottky ultraviolet detectors and their gain mechanism,” phys. stat. sol. (a) 188(1), 345–349 (2001). [CrossRef]
9. Y. Lian, Y. Lin, J. Yang, C. Cheng, and S. S. H. Hsu, “AlGaN/GaN Schottky barrier diodes on silicon substrates with selective Si diffusion for low onset voltage and high reverse blocking,” IEEE Electron Device Lett. 34(8), 981–983 (2013). [CrossRef]
10. F. Xie, H. Lu, D. Chen, F. Ren, R. Zhang, and Y. Zheng, “Bias-selective dual-operation-mode ultraviolet Schottky-barrier photodetectors fabricated on high-resistivity homoepitaxial GaN,” IEEE Photonics Technol. Lett. 24(24), 2203–2205 (2012). [CrossRef]
11. J. H. Kim, J. H. Yu, T. S. Kim, T. S. Jeong, C. J. Youn, and K. J. Hong, “Electric-field effect of near-band-edge photoluminescence in bulk ZnO,” J. Mater. Sci. 45(15), 4036–4039 (2010). [CrossRef]
12. M. Matys and B. Adamowicz, “Mechanism of yellow luminescence in GaN at room temperature,” J. Appl. Phys. 121(6), 065104 (2017). [CrossRef]
13. T. Wernicke, L. Schade, C. Netzel, J. Rass, V. Hoffmann, S. Ploch, A. Knauer, M. Weyers, U. Schwarz, and M. Kneissl, “Indium incorporation and emission wavelength of polar, nonpolar and semipolar InGaN quantum wells,” Semicond. Sci. Technol. 27(2), 024014 (2012). [CrossRef]
14. K. Ando, A. Yamamoto, and M. Yamaguchi, “Bias-dependent photoluminescence intensities in n-InP Schottky diodes,” J. Appl. Phys. 51(12), 6432–6434 (1980). [CrossRef]
15. A. Ahaitouf and A. Bath, “Electrical and gated photoluminescence intensity studies on Schottky and oxidized Schottky structures,” Thin Solid Films 342(1-2), 136–141 (1999). [CrossRef]
16. W. B. Liu, D. G. Zhao, X. Sun, S. Zhang, D. S. Jiang, H. Wang, S. M. Zhang, Z. S. Liu, J. J. Zhu, Y. T. Wang, L. H. Duan, and H. Yang, “Stable multiplication gain in GaN p–i–n avalanche photodiodes with large device area,” J. Phys. D: Appl. Phys. 42(1), 015108 (2009). [CrossRef]
17. K. B. Nam, J. Li, J. Y. Lin, and H. X. Jiang, “Optical properties of AlN and GaN in elevated temperatures,” Appl. Phys. Lett. 85(16), 3489–3491 (2004). [CrossRef]
18. D. C. Look, G. C. Farlow, P. J. Drevinsky, D. F. Bliss, and J. R. Sizelove, “On the nitrogen vacancy in GaN,” Appl. Phys. Lett. 83(17), 3525–3527 (2003). [CrossRef]
19. D. G. Zhao, D. S. Jiang, H. Yang, J. J. Zhu, Z. S. Liu, S. M. Zhang, J. W. Liang, X. Li, X. Y. Li, and H. M. Gong, “Role of edge dislocations in enhancing the yellow luminescence of n-type GaN,” Appl. Phys. Lett. 88(24), 241917 (2006). [CrossRef]
20. S. De, A. Layek, S. Bhattacharya, D. K. Das, A. Kadir, A. Bhattacharya, S. Dhar, and A. Chowdhury, “Quantum-confined stark effect in localized luminescent centers within InGaN/GaN quantum-well based light emitting diodes,” Appl. Phys. Lett. 101(12), 121919 (2012). [CrossRef]
21. E. Gaubas, S. Juršėnas, S. Miasojedovas, J. Vaitkus, and A. Žukauskas, “Carrier and defect dynamics in photoexcited semi-insulating epitaxial GaN layers,” J. Appl. Phys. 96(8), 4326–4333 (2004). [CrossRef]
22. F. J. Sánchez, F. Calle, D. Basak, J. M. G. Tijero, M. A. Sánchez-García, E. Monroy, E. Calleja, E. Muñoz, B. Beaumont, P. Gibart, J. J. Serrano, and J. M. Blanco, “Yellow luminescence in Mg-doped GaN,” MRS Internet J. Nitride Semicond. Res. 2, e28 (1997). [CrossRef]
23. M. A. Reshchikov, D. O. Demchenko, A. Usikov, H. Helava, and Y. Makarov, “Carbon defects as sources of the green and yellow luminescence bands in undoped GaN,” Phys. Rev. B 90(23), 235203 (2014). [CrossRef]
24. Y. Linkai, Q. Haoran, H. Jialin, Z. Mei, Z. Degang, J. Desheng, Y. Jing, L. Wei, and L. Feng, “Influence of dislocation density and carbon impurities in i-GaN layer on the performance of Schottky barrier ultraviolet photodetectors,” Mater. Res. Express 5(4), 046207 (2018). [CrossRef]
25. M. Grundmann, “Diodes,” in The Physics of Semiconductors: An Introduction Including Nanophysics and Applications, 2nd ed., M. Grundmann, ed. (Springer Berlin Heidelberg, 2010), pp. 519–598.
26. Y. Goldberg, M. Levinshtein, and S. Rumyantsev, “Properties of advanced semiconductor materials: GaN, AIN, InN, BN, SiC, SiGe,” SciTech Book News 25, 93–146 (2001).
27. D. V. Khveshchenko and M. Reizer, “Piezoelectric electron-phonon interaction in impure semiconductors: Two-dimensional electrons versus composite fermions,” Phys. Rev. B 56(24), 15822–15826 (1997). [CrossRef]
28. G. Qin and M. Hu, “Nontrivial contribution of Fröhlich electron-phonon interaction to lattice thermal conductivity of wurtzite GaN,” Appl. Phys. Lett. 109(24), 242103 (2016). [CrossRef]
29. B. Monemar, “Fundamental energy gap of GaN from photoluminescence excitation spectra,” Phys. Rev. B 10(2), 676–681 (1974). [CrossRef]
30. A. David and M. J. Grundmann, “Influence of polarization fields on carrier lifetime and recombination rates in InGaN-based light-emitting diodes,” Appl. Phys. Lett. 97(3), 033501 (2010). [CrossRef]
31. J. H. Ryou, P. D. Yoder, J. Liu, Z. Lochner, H. Kim, S. Choi, H. J. Kim, and R. D. Dupuis, “Control of quantum-confined Stark effect in InGaN-based quantum wells,” IEEE J. Sel. Top. Quantum Electron. 15(4), 1080–1091 (2009). [CrossRef]
32. T. Maeda, M. Okada, M. Ueno, Y. Yamamoto, M. Horita, and J. Suda, “Franz–Keldysh effect in n-type GaN Schottky barrier diode under high reverse bias voltage,” Appl. Phys. Express 9(9), 091002 (2016). [CrossRef]
33. C. Wetzel, T. Takeuchi, H. Amano, and I. Akasaki, “Piezoelectric Franz–Keldysh effect in strained GaInN/GaN heterostructures,” J. Appl. Phys. 85(7), 3786–3791 (1999). [CrossRef]
34. M. Grundmann, “Optical Properties,” in The Physics of Semiconductors: An Introduction Including Nanophysics and Applications, 2nd ed., M. Grundmann, ed. (Springer Berlin Heidelberg, 2010), pp. 265–307.
35. Z. M. Gibbs, A. LaLonde, and G. J. Snyder, “Optical band gap and the Burstein–Moss effect in iodine doped PbTe using diffuse reflectance infrared Fourier transform spectroscopy,” New J. Phys. 15(7), 075020 (2013). [CrossRef]
