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We report on gallium (Ga) doped cubic MgZnO films, which have been grown by metal organic chemical vapor deposition. It was demonstrated that Ga doping improves the n-type conduction of the cubic MgZnO films. A two-orders of magnitude enhancement in lateral n-type conduction have been achieved for the cubic MgZnO films. The responsivity of the cubic MgZnO-based photodetector has been also enhanced. Depletion region electric field intensity enhanced model was adopted to explain the improvement of quantum efficiency in Ga doped MgZnO-based detectors.
© 2014 Optical Society of America
1. Introduction
Ultraviolet C (UVC, 100–280 nm) is a part of absent solar radiation at the Earth’s surface, due to absorption in the ozonosphere [1]. The gap of solar spectrum provides a “black background” for detection of weak UVC emitting sources such as missile plume. Therefore, photodetection, with a cutoff wavelength shorter than 280 nm, has been called solar-blind photodetection [2]. Solar-blind photodetectors have found many applications including flame sensing, non-line-of-sight optical communication, missile plume sensing, UV astronomy, chemical/biological analysis, etc [3–9]. Unfortunately, the commercially available photomultiplier tubes, covering solar-blind region, have to work at high operating voltages, which limits their applications.
Currently, some wide bandgap semiconductors based detectors, focused on solar-blind sensitive, have been researched, such as AlxGa1-xN [10], BGaN [11], diamond [12], cubic BN [13], monoclinic gallium oxide (β-Ga2O3) [14], silicon carbide [15] and MgxZn1-xO [16–18]. Among them, cubic MgxZn1-xO alloys have been receiving increasing intense attention due to its larger tunable bandgap (MgO 7.8 eV) and other unique optoelectronic natures [19–21]. Our group has obtained a responsivity of 396 mA/W under 10 V bias @ 246nm, which had been valued as one of the highest responsivity in MgZnO-based solar-blind photodetectors [22]. On account of the consuming scattering and absorption for UVC in the ambiance, the solar-blind signal will be much weakened in a limited spread distance [4]. Accordingly, the high quantum efficiency is one of the figures of merit for solar-blind photodetectors which practice demanded [23]. However, for the cubic MgZnO based detectors, the gain mechanism is still not controllable, because of the ultra-high resistivity of the as grown films [24–26]. This is an obstacle that hampers the realization of high-performance solar-blind photodetectors. In this report, by employing Triethylgallium as the n-type dopant source, gallium (Ga) doped cubic MgZnO films have been grown by metal organic chemical vapor deposition (MOCVD). It was demonstrated through electrical, optical, and structural studies that Ga doping improves the n-type conduction of the cubic MgZnO films. An enhancement of two-orders of magnitude in lateral n-type conduction has been achieved on the cubic MgZnO films. The responsivity of the planar structure metal-semiconductor-metal (MSM) photodetector has been also enhanced. Depletion region electric field intensity (EFI) enhanced model was adopted to explain the improvement of quantum efficiency (η) in Ga doped MgZnO-based detectors.
2. Experiments and results discussion
The ~600 nm thick undoped and Ga-doped cubic MgZnO films, with the same [Mg]/[Zn] mole flow ratio, were grown on (0001) sapphire substrate by MOCVD at 450 °C, respectively. The mole flow ratio for [Ga]/([Zn] + [Mg]) was fixed in 1:500. More details about the growth conditions of cubic MgZnO can be found in our works else [25–27]. The X-ray diffraction (XRD) pattern of Fig. 1(a) reveals that the films, both undoped and Ga doped, are crystallized in cubic structure with (111) preferred orientation. There are no other significantly strong signals related to other phases, suggesting that no phase segregation or emerging of wurtzite MgZnO or Ga related spinel occurs in each MgZnO film. Both undoped and doped films have high transmission of around 80% in the visible range, and the same steep transmission edge at around 260 nm, corresponding to a bandgap of 4.77 eV, as shown in Fig. 1(b). The composition of the films is calculated to be Mg0.44Zn0.56O from equation (Eg = 3.02 + 4.03x) [28].
Fig. 1 (a) XRD pattern of the MgZnO films. No phase segregation is observed in both films. (b) Transmission spectrum taken from both films illustrating a bandgap of 4.77 eV, which is well within the solar-blind region.
To confirm the incorporation of Ga into cubic MgZnO films, X-ray photoelectron spectroscopy (XPS) was carried out to investigate the elemental composition of the cubic MgZnO films. For the undoped films, no XPS signals from Ga could be detected, whereas for doped films a weak peak at 20.5 eV corresponding to Ga 3d core level was observed, as can be seen in Fig. 2, in addition to the strong Mg 2p and Zn 2p signals.
As illustrated in Fig. 3(a), Au Schottky contacts, with the planar MSM structure, were fabricated on the undoped and Ga-doped cubic MgZnO films, using vacuum evaporation, photolithography and wet etching procedure. The fingers were 500 µm long and 2 µm wide with a spacing of 5 µm. It should be noted that the Au electrodes are translucent in both the photodetectors, which will benefit to absorption of light. To explore the electrical properties, the current-voltage (I-V) curves of each photodetectors were measured in the dark. Figure 3(b) shows that the conductivity of Ga doped films is enhanced with two-orders of magnitude in lateral, which demonstrates that Ga-doping may help to make the conduction of cubic MgZnO controllable. To further contrast the optoelectronic properties of the undoped and doped samples, the photoresponse of the photodetectors was measured. As shown in Fig. 3(c), both the two photodetectors show a high sensitivity in solar-blind region. The Ga-doped sample shows an enhanced responsivity about 50 times at 265 nm under 10 V bias, compared to the undoped sample. Note that the rejection ratio of solar-blind UV to visible light, both samples, is about two orders of magnitude, demonstrating the good performance of the Ga doped cubic MgZnO. Figure 3(d) shows the responsivity as the function of bias voltage under 265 nm light illumination. In the investigated range, the Ga-doped sample always shows η is 50 times higher than the undoped sample.
Fig. 3 (a) Schematic diagram of the MSM photodetectors. (b) I-V characteristics of the two photodetectors in dark. (c) Spectral response of the detectors at 10 V bias revealing that the devices are both blind to solar light. (d) The dependence of the maximum responsivity of the photodetectors on the external bias.
3. Theoretical modeling and simulation
The model of depletion region EFI enhanced is used to analysize the experimental results. In general, for a Schottky contact [29], if E00 << kBT, the thermionic emission is dominated in the junction electronic transport process without tunneling, where kB is the Boltzmann constant, T is absolute temperature, E00 is the characteristic energy related to the tunneling probability. E00 can be expressed by following formula:
where q is the elementary charge, ħ is the reduced Planck constant, N is the carrier density, m* is the effective mass and εs is the relative dielectric permittivity. In this work, m* = 0.3m0 and εs = 12 for cubic MgZnO [30,31], and the carrier concentration N is about 1016 cm−3 and 1014 cm−3 for the Ga-doped and undoped films, respectively, which were estimated by the resistance of the devices. So, E00 is about 2.4 meV and 0.24 meV for the doped and undoped films, which are both much smaller than the thermal energy kBT at room temperature (26 meV). Therefore, the I-V curve of this Schottky contact is determined by thermionic emission model. Considering the MSM structure, consisting of two Schottky contacts connected back to back on a coplanar surface, the I-V of the detectors can be described as following:where V is the applied bias, A is the contact area, n is the ideality factor, qϕB is Schottky Barrier Height (SBH) and A n* is the effective Richardson constant, which is 36 Acm−2K−2 for cubic MgZnO [32]. Then, according to the I-V curves [Fig. 3(b)], the SBH can be calculated. Here, for the doped sample, qϕB1 is 0.908 eV, which is almost equal to that of the undoped sample, qϕB2 (0.907 eV). It suggested that the built-in potential ϕbi1 is equal to ϕbi2, approximately.Meanwhile, for the Schottky contact, the depletion width (WD) can be described as WD = [(2εs/qND)(ϕbi-V-kBT/q)]1/2, where ND is the donor concentration. In this work, the ND1 (Ga doped sample) is larger than ND2 (undoped sample) by at least two-orders of magnitude, so WD2/WD1 ∝ (ND1/ND2)1/2 ≥ 10. In view of the above analysis, the Ga-doped MSM photodetector has the narrowing depletion region with the same built-in potential, which leads to a higher EFI in the effective layer. The enhanced EFI could separate the photogeneration carriers, effectively, as illustrate in Fig. 4.
To make the investigations visualized, simulations of the EFI distribution on MSM photodetectors were performed by using the COMSOL software. The proposed MSM structures and the simulation results are shown in Fig. 5. The EFI distribution, in vertical and horizontal, is shown in Figs. 5(b) and 5(c). In vertical, Ga-doped detector has a higher and abrupt EFI even reaching 600 nm. Furthermore, in horizontal, both the two detectors have the same effective EFI width. It makes sure that photogeneration carriers will be more effectively separated in the Ga-doped MSM photodetector. That is the reason why the η has been improved in the Ga doped MgZnO-based detectors.
Fig. 5 Simulations of the EFI distribution on MSM photodetectors (a) whole vision, (b) in vertical, and (c) in horizontal.
4. Conclusion
In conclusion, by employing Ga-doping, the lateral conduction of cubic MgZnO films has been increased by two-orders of magnitude compared to the undoped case. The responsivity of the MSM photodetector has also been significantly improved. The EFI enhanced model was adopted to explain the improved η in the Ga-doped MgZnO-based detectors. From all the above results, it is indicated that Ga-doping (or other effective doping), which makes the electrical property of cubic MgZnO films controllable, is a valid method and prerequisite to realize the improvement of the MgZnO based photodetectors performance.
Acknowledgments
This work is supported by the National Basic Research Program of China (973 Program) under No.s 2011CB302002 and 2011CB302006, the National Natural Science Foundation of China under No.s 11134009.
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