Solar blind photodetector based on epitaxial zinc doped Ga2O3 thin film
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Phys. Status Solidi A, 1–7 (2017) / DOI 10.1002/pssa.201600688
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applications and materials science
Fikadu Alema*,1, Brian Hertog1, Oleg Ledyaev1, Dmitry Volovik1, Grant Thoma1, Ross Miller1, 1 2 2 2 2 Andrei Osinsky , Partha Mukhopadhyay , Sara Bakhshi , Haider Ali , and Winston V. Schoenfeld 1 2
Agnitron Technology Incorporated, Eden Prairie, MN 55346, USA CREOL, The College of Optics and Photonics, University of Central Florida, 4000 Central Florida Blvd., Orlando, FL 32816, USA
Received 12 September 2016, revised 12 December 2016, accepted 15 December 2016 Published online 10 January 2017 Keywords b-Ga2O3 thin film, MOCVD, responsivity, solar blind photodetector, zinc doped b-Ga2O3 thin film * Corresponding
author: e-mail fi
[email protected], Phone: þ1 952 937 7505, Fax: þ1-612 605 4327
We report on the fabrication and characterization of solar blind photodetectors (SBPs) based on undoped b-Ga2O3 and Zn doped (5 1020 cm3) b-Ga2O3 (ZnGaO) epitaxial films with cutoff wavelength of 260 nm. The epilayers were grown on c-sapphire by the metal organic chemical vapor deposition technique and their structural, electrical and optical properties were characterized using various methods. As grown films have a large number of defects, resulting in detectors with enhanced internal gain, hence, high spectral responsivity >103 A/W. Post growth annealing in oxygen improved the quality of the epilayers, leading to
detectors with reduced dark current (nA to pA) and increased out of band rejection ratio. At 20 V bias, a ZnGaO detector showed a peak responsivity of 210 A/W (at 232 nm) and an out of band rejection ratio (i.e., R232 nm/R320 nm) of 5 104. Alternatively, for a b-Ga2O3 detector these parameters were found to be five times and three times lower, respectively, suggesting that ZnGaO detectors have superior performance characteristics. These results provide a roadmap toward achieving high responsivity SBPs based on epitaxial ZnGaO films, laying a solid foundation for future applications.
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1 Introduction Solar blind photodetectors (SBPs) with cutoff wavelength 290 nm are of great interest for a variety of military and civil applications including flame sensing, missile interception, air and water purification, as well as space-to-space communications [1, 2]. Wide band gap semiconductors such as MgxZn1-xO and AlxGa1-xN have been widely studied and developed for SBP applications [1–6]. However, increasing the content of Mg (x > 0.37) [5] or Al (x > 0.45) [6], respectively, to achieve true solar blind photodetection is accompanied by phase segregation in MgZnO and film quality deterioration in AlGaN, making it difficult for their practical use. An alternative wide band gap semiconductor (Eg 4.9 eV) [7] which is naturally suitable for solar blind photodetection is monoclinic gallium oxide (b-Ga2O3) [8]. SBPs based on thin films and nanowires of b-Ga2O3 have been demonstrated [8–10] recently. The realization of these devices, however, relies on the feasibility of growing conductive doped b-Ga2O3 because pure, undoped, defect free, and stoichiometric b-Ga2O3 is an electrical insulator due to its large band gap [7, 11]. Successful growth of n-type
b-Ga2O3 thin films has been reported using Sn [12–15] and Si [16–18] dopants, but with mixed results in terms electrical conductivity. On the other hand, achievement of p-doping remains challenging largely due the to the lack of suitable dopants as is the case with other oxide semiconductors, and overcompensation by oxygen vacancies [19, 20]. Divalent ions including Mg2þ [21], Zn2þ [22, 23], and Cu2þ [24] substituting the site of trivalent Ga3þ in b-Ga2O3 have been investigated as potential p-type dopants, but without much success. In particular, we are aware of only a single group that has observed hole conductivity in (Zn) doped b-Ga2O3 (nanowires) [23]. Moreover, to the best of our knowledge solar blind photodetectors have not been fabricated from p-type b-Ga2O3. In this work, we report on the epitaxial growth of high quality Zn doped b-Ga2O3 (ZnGaO) and undoped b-Ga2O3 films on c-plane sapphire by metal organic chemical vapor deposition (MOCVD), as well as the fabrication and characterization of photodetectors with metal-semiconductor-metal (MSM) structure. Defects in the films were annealed out in an oxygen atmosphere. At a bias of 20 V, a ß 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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ZnGaO detector showed an absolute responsivity of 210 A/W at 232 nm and an out of band rejection ratio of 5 104. Under the same conditions, these parameters were found to be five times and three times, respectively, lower for the b-Ga2O3 detectors, indicating the superior performance characteristics of the ZnGaO detectors. 2 Experimental 2.1 Growth of undoped and Zn doped Ga2O3 epitaxial films Epitaxial undoped Ga2O3 (0.24 mm) and Zn doped Ga2O3 (ZnGaO, 0.36 mm) thin films were grown by a vertical rotating (1000 rpm) disc MOCVD system customized by Agnitron Technology for optimal growth of Ga2O3 epitaxial films. The films were grown on c-sapphire (200 ) substrates using triethylgallium (TEGa), diethylzinc (DEZn), and oxygen as precursors for Ga, Zn, and O2, respectively. Argon (Ar) was used as a carrier gas. Point of use gas purifiers were used for both Ar and O2 to reduce the impurity level 80%) in the visible and deep UV regions. It has been shown both theoretically and experimentally that the b-Ga2O3 is a direct
Figure 3 Room temperature UV-VIS transmission for b-Ga2O3 (black) and ZnGaO (blue) for as grown epitaxial films. The inset shows the plot of (ahn)2 versus hn used to estimate the band gap. ß 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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band gap semiconductor [7, 30]. Therefore, the optical band gap (Eg) of the films can be evaluated using the plot of (ahn)2 versus hn (Fig. 3, inset), where a is the absorption coefficient and hn is the photon energy. Both films have an estimated band gap of 4.9 eV, consistent with reported values in the literature [7, 31]. The Zn doping had no noticeable influence on the band gap of the b-Ga2O3, in contrast to what was reported by Wei et al. [22]. Even though the XRD, SIMS, and XPS results show the incorporation of Zn into the film, the resulting material is not electrically conductive. Room temperature Hall measurements revealed that both the Zn doped and undoped films were resistive. Post growth annealing of the films in an oxygen atmosphere did not activate the Zn species despite significant structural improvement. The possible reasons for this behavior could be an overcompensation of Zn by oxygen vacancies or Zn may form a deep level acceptor in Ga2O3 [19]. The current voltage (I–V) curves with and without UV illumination (260 nm, power density 1.7 mW cm2, detector area 2.5 103 cm2) for MSM devices based on as grown b-Ga2O3 and ZnGaO films are shown in Fig. 4a. The increase of current through both devices with UV illumination clearly indicates that the films have photoconductive properties [8, 10] with the photocurrent (Iph) to dark current (Idk) ratio being over three orders of magnitude. Up to 30 V bias was applied to the devices; the maximum Idk measured for b-Ga2O3 and ZnGaO was, respectively, 10 and 23 nA. The former is about 8 times lower than that reported elsewhere for undoped b-Ga2O3 [8]. In SBPs, such a low dark current is crucial for achieving a high signal to noise (S/N) ratio as it reduces shot noise in the devices [4, 32]. Although all the devices have low dark current, the Zn doped devices have dark currents twice as large as the undoped b-Ga2O3 devices. This could be due to a high concentration of defects including oxygen vacancies and substitutional defects in the ZnGaO film that result from doping. The formation energy of oxygen vacancies in Zn doped and undoped b-Ga2O3 has been estimated by first
Figure 4 Dark and UV-illuminated I–V curves for MSM devices fabricated from as grown (a) and annealed (b) b-Ga2O3 (black) and ZnGaO (blue) epitaxial films. ß 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
principle calculations [33]. It was found that the formation energy of oxygen vacancies is lower in the presence of Zn dopants, suggesting that the incorporation of Zn into the b-Ga2O3 lattice increases the concentration of oxygen vacancies, leading to a higher dark current. Figure 4b shows the I–V characteristics (dark and UVilluminated) for the MSM devices fabricated from post growth annealed b-Ga2O3 and ZnGaO epitaxial films. Annealing in oxygen reduced the Idk of the devices by three orders of magnitude. In particular, at a 30 V bias, Idk of 3 and 20 pA were measured for b-Ga2O3 and ZnGaO devices, respectively, indicating a substantial improvement in the S/N ratio of the devices. Similarly, the Iph of both devices was reduced by two orders of magnitude, but the ratio of Iph/Idk improved to 4 orders of magnitude. Annealing the films in an oxygen atmosphere very likely annihilates oxygen vacancies, leaving the samples with cation vacancies which may form deep level accepters [19]. In addition, the observed increase in the coherent scattering domain size (Fig. 1c) indicates a reduction in lattice defects which along with oxygen vacancies reduces the electrical conductivity [17]. The spectral response for the b-Ga2O3 and ZnGaO detectors at a 20 V bias are shown in Fig. 5. For the MSM detectors based on the as grown b-Ga2O3 and ZnGaO epitaxial films (Fig. 5a), absolute peak responsivity of 17,000 A/W (at 242 nm) and 3600 A/W (at 256 nm), respectively, were measured. Such high responsivities are likely due to defects in the as grown films which results in trapping states that enhance internal gain [4, 25]. One of the possible mechansims could be the increase in excess carrier lifetime as a result of hole trapping by an oxygen vacanacy [34]. After annealing, the responsivity for the devices dropped significantly (Fig. 5b) to a peak responsivity of 46 A/W (at 236 nm) for the b-Ga2O3 photodetectors and of 210 A/W (at 232 nm) for the ZnGaO photodetectors. The lowering of the responsivity with
Figure 5 Spectral response of MSM devices based on as grown (a) and annealed (b) b-Ga2O3 (black) and ZnGaO (red) solar blind photodetectors at a 20 V bias. (c) Normalized responsivity (in logarithmic scale) to compare the out of band rejection ratio of the devices. After annealing, the solar blind selectivity of the ZnGaO increased by 50 times. www.pss-a.com
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annealing is in agreement with the observed reduction in the photocurrent (Fig. 4b). The cutoff wavelength for the devices was 260 nm except for the devices based on the as grown ZnGaO film which was 285 nm. Shown in Fig. 5c are normalized responsivity spectra comparing the degree of selectivity of the devices to solar blind radiation. As seen from the figure, the devices based on the as grown films have an out of band rejection ratio (R[peak l]/R[320 nm]) [4] of 103 which increases to 2 104 and 5 104 for annealed b-Ga2O3 and ZnGaO devices, respectively. This high rejection ratio demonstrates that the devices can operate in the solar blind spectral window without interference from UV-A (320–400 nm) and visible radiation. Comparing the devices based on annealed epitaxial films, the out of band rejection ratio for the ZnGaO detector is 3 times higher than that for the undoped b-Ga2O3, indicating stronger selectivity to solar blind radiation. As shown in Fig. 5a and b, the cutoff wavelength for the devices is longer than 253 nm (¼4.9 eV, bandgap estimated in Fig. 3), showing the presence of a sub-band response which results from carrier transitions involving defect levels or band tails due to defects or stacking faults introduced during the growth process of the films [4, 25, 32]. Nevertheless, the degree of the influence of sub-band response on our devices is different for the two materials. For the as grown b-Ga2O3 photodetectors, the appearance of a peak responsivity at 242 nm indicates that the photocurrent comes mainly from band to band transitions of electron-hole pairs. Conversely, the red shifts in the cutoff wavelength (285 nm) and peak responsivity position (256 nm) for the as grown ZnGaO photodetector suggests that the majority of the photocurrent is due to the transition of carriers between defect states in the forbidden gap and valance or conduction bands. In addition to the defects present in the b-Ga2O3 film, the ZnGaO film could have an additional defect state or acceptor level created as a result of Zn ions substituting for Ga ions ðZnGa Þ0 . Interestingly, both devices fabricated from the annealed films have sharp cutoff wavelength 260 nm suggesting a much smaller role for defects in the generation of holeelectron pairs. This demonstrates that annealing in an oxygen atmosphere mitigates the effect of the ðZnGa Þ0 defect, dramatically improving the cutoff wavelength, dark current, and out of band rejection ratio of the devices. The response speed of the photodetectors both on the as grown and annealed films were evaluated by measuring the temporal response of the detectors. Table 1 presents the the rise and fall times for the detectors. Generally, the devices were found to be slow which consistant with the observed high responsivity values. However, a significant improvement was observed for the annealed devices. To understand the origin of the sub-band response in the photodetectors, we studied the films by CL spectroscopy. Figure 6 shows the CL spectra of the as grown and annealed b-Ga2O3 and ZnGaO epitaxial films. As seen from the figure, the films show no near band edge emission, www.pss-a.com
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Table 1 Rise and fall times of the MSM photodetectors. as grown devices
annealed
rise time (s) fall time (s) rise time (s) fall time (s)
b-Ga2O3 4.0 ZnGaO 8.4
14.2 20.6
4.5 3.2
0.8 1.4
suggesting the presence of a high concentration of compensating deep levels [15]. The as grown b-Ga2O3 film in Fig. 6a shows a broad blue emission band centered around 430 nm (2.88 eV). After annealing, the CL spectrum maintained the same u7 shape with a small shift in the peak position of the band to 410 nm (3.02 eV). The broad blue emission band is characteristics of b-Ga2O3 and has been previously reported in the CL [15, 35] as well as photoluminescence [36] studies of the material. It is a unique signature of undoped b-Ga2O3 which is associated with the recombination processes between intrinsic donoracceptor pair (DAP) centers. In b-Ga2O3, the donor and acceptor levels are introduced 000 and gallium V by oxygen V•• O Ga vacancies, respectively [35, 36]. However, instead of a single heavily charged gallium 0 vacancy V000 Ga , a gallium-oxygen vacancy pair ðVO ; VGa Þ is believed to be the most likely type of defect that forms an accepter level [36]. These defects form trapped excitons, resulting in blue emission from the following process: X VO; VGa þ VXO ! ðVO ; VGa Þ þ V•O þ hn [36, 37]. The
Figure 6 CL spectra of undoped b-Ga2O3 (a) and ZnGaO (b) epitaxial thin films. ß 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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blue shift in the CL peak after annealing can be attributed to the decrease in the concentration of oxygen vacancies. In Fig. 6b, the CL spectra for ZnGaO epitaxial films are shown. For the as grown film, a strong UV emission band centered at 370 nm (3.35 eV) and a broad blue emission band 460 nm are observed. The red shifted blue emission is attributed to DAP recombination involving electrons in oxygen vacancies and holes on the ðZnGa Þ0 defect. This phenomenon has been observed for Zn doped b-Ga2O3 thin films grown by PLD [28]. Moreover, a similar UV emission (3.34 eV) was observed elsewhere for a single crystal b-Ga2O3 at 150 K and attributed to recombination of selftrapped excitons [36]. In our case, the UV emission in uncooled ZnGaO may result from interactions between ðZnGa Þ0 and ðVO ; VGa Þ defects, which appears to have a “quenching” effect on the activities of ðVO ; VGa Þ, reducing the intrinsic blue emission while at the same time promoting the UV emission. Further investigations such as time and temperature resolved CL measurements are required to more fully understand the nature of the interaction. After annealing, the ðZnGa Þ0 related blue component disappeared while the UV emission band shifted to 360 nm (3.44 eV) and narrowed in width. This substantiates the observed improvement in crystal quality (Fig. 1 [blue]) and detector properties that result from annealing in an oxygen atmosphere. The annihilation of oxygen vacancies in the ZnGaO film may leave the sample dominated by “inactive” cation vacancies [19], thus, reducing DAP processes which leads to the disappearance of Zn related blue emission. Undoped b-Ga2O3 exhibits n-type conductivity that has long been attributed to oxygen vacancies [36]. However, density functional theory (DFT) calculations show that oxygen vacancies are deep donors and cannot dominate the n-type conductivity in b-Ga2O3 [38]. Instead, the conductivity can be attributed to silicon and hydrogen impurities [38, 39]. In our case, we do not expect these background impurities, but the presence of a high concentration of oxygen vacancies is apparent from the blue CL emission observed for our films [28, 35–37]. However, the films were highly resistive, suggesting that the oxygen vacancies in the films are deep donors with ionization energy (Ed) >1 eV [38].The activation energy for ðVO ; VGa Þ acceptor (Ea) in b-Ga2O3 has been estimated to be 0.3 eV [36]. Assuming this holds true here, the CL peak energy (ECL) and Eg of the films can be used to estimate the donor (Ed) and acceptor (EA) activation energies for 0 the oxygen vacancy (V•• O ) and the ðZnGa Þ defect, respec tively. Accordingly, the donor level (Ed) of V•• O : Ed ¼ Eg Ea ECL , where ECL ¼ 2.88 eV (430 nm) for undoped b-Ga2O3 is calculated to be 1.72 eV, which is comparable with previous reports [28, 38]. Similarly, the accepter level (EA) for the ðZnGa Þ0 defect ¼ Eg Ed ECL was estimated to be 0.48 eV and is consistent with the value obtained by DFT calculation [40]. In Fig. 7, the energy levels of donors and acceptors for both b-Ga2O3 and ZnGaO materials are shown, schematically, based on the above ß 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Figure 7 Schematic diagram showing energy levels of donor V •• O , and accepter ðVO ; VGa Þ0 as well as ðZnGa Þ0 in the b-Ga2O3 (left) and ZnGaO (right) epitaxial films.
results. Clearly, the Zn dopant forms a deep accepter level, explaining why the ZnGaO film remains resistive and making it difficult to observe p-type conductivity. From the above analyses, the photocurrent for the MSM detector on the as grown ZnGaO film is dominated by the transition of carriers between ðZnGa Þ0 accepter and conduction or valance bands. After annealing, the ðZnGa Þ0 defect is “frozen” as observed from the CL emission, indicating a reduced contribution of defect states to the generation of photocurrent carriers. This also follows from the observed 24 nm (0.50 eV) blue shift in the responsivity peak of the annealed ZnGaO detector (Fig. 5b) which is comparable to EA (¼0.48 eV) of the ðZnGa Þ0 defect. For the b-Ga2O3 detector, however, the contribution of defect states is limited both before and after annealing. 4 Conclusions In summary, we have demonstrated solar blind photodetectors (SBPs) based on b-Ga2O3 and ZnGaO epitaxial films grown on sapphire. The as grown films have a large number of defects, resulting in detectors with high responsivity >103 A/W. Annealing the films in an oxygen atmosphere reduces the dark current of the devices from the nA to the pA range. Improved device performance was observed for ZnGaO based detectors with a responsivity of 210 A/W (at 232 nm) and an out of band rejection ratio of 5 104. Results in this paper provide a roadmap for achieving high responsivity SBPs based on epitaxial ZnGaO films, laying a solid foundation for their future applications. Moreover, we expect, that by using b-Ga2O3 substrates, the film/substrate mismatch induced defects will be significantly reduced, thereby leading to improved performance of ZnGaO detectors. Acknowledgement This work was supported by the US Department of Defense Army Research Office (ARO) contract No. W911NF-14-C-00157; monitored by W. Clark and M. Gerhold.
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