Inhibition of unintentional extra carriers by Mn valence change for high insulating devices

For intrinsic oxide semiconductors, oxygen vacancies served as the electron donors have long been, and inevitably still are, attributed as the primary cause of conductivity, making oxide semiconductors seem hard to act as high insulating materials. Meanwhile, the presence of oxygen vacancies often leads to a persistent photoconductivity phenomenon which is not conducive to the practical use in the fast photoelectric response devices. Herein, we propose a possible way to reduce the influence of oxygen vacancies by introducing a valence change doping in the monoclinic β-Ga2O3 epitaxial thin film. The unintentional extra electrons induced by oxygen vacancies can be strongly suppressed by the change valence of the doped Mn ions from +3 to +2. The resistance for the Mn-doped Ga2O3 increases two orders of magnitude in compared with the pure Ga2O3. As a result, photodetector based on Mn-doped Ga2O3 thin films takes on a lower dark current, a higher sensitivity, and a faster photoresponse time, exhibiting a promising candidate using in high performance solar-blind photodetector. The study presents that the intentional doping of Mn may provide a convenient and reliable method of obtaining high insulating thin film in oxide semiconductor for the application of specific device.

The intrinsic semiconductors always exhibit unintentional n-or p-type conductivity induced by native defects such as vacancies or interstitials 1,2 , leading to a difficulty in achieving high insulating material and limiting their use in the specific devices such as photodetector, dielectric layer, etc. In particular, for oxide semiconductors, the conductivity is often seen to exhibit a pronounced dependence on partial pressure of oxygen during growth 3,4 . It seems logical, that this conductivity is related to the presence of oxygen vacancies which served as the electron donors leading to a decrease in resistivity, as has been assumed for many years. In fact, with recent advances in growth techniques, particularly perhaps the advent of novel schemes of molecular beam epitaxy, it has become possible to grow thin films of oxide materials, as required for device applications, with rather high structural quality [5][6][7] . Notwithstanding this, however, oxygen vacancies have long been [6][7][8] , commonly and inevitably still are, attributed as the primary cause of conductivity in oxide semiconductors. Meanwhile, the presence of oxygen vacancies in oxides often leads to a persistent photoconductivity phenomenon since the carriers trapped by oxygen vacancies have longer lifetime and drift back to their original states slowly 9,10 . It is not conducive to the practical use in the fast photoelectric response devices. Take ZnO based photodetector as an example, the response time would extend to the order of minutes or hours due to the presence of oxygen vacancies 9,10 . In this work, we propose a possible way to suppress the unintentional extra carriers which are induced by oxygen vacancies and reduce the influence of oxygen vacancies by introducing a valence change Mn element doping, with a recent hot wide band gap semiconductor material -β-gallium oxide (Ga 2 O 3 ) using for solar-blind photodetector as an example.
β-Ga 2 O 3 , with ~4.9 eV direct band gap and tunable by alloying with Al 2 O 3 or In 2 O 3 , is particularly suitable for the deep ultraviolet (DUV) photodetector that is blind to wavelengths above 280 nm, which has a vast and ever growing number of military and civil surveillance applications such as missile tracking, fire detection, ozone holes monitoring, chemical/biological analysis, and so on [11][12][13] . The solar-blind photodetector could detect very weak signals accurately due to the "black background" with the absence of wavelength shorter than 280 nm in solar radiation and artificial lighting. During the past decade, solar-blind photodetectors based on wide band gap semiconductors such as AlGaN, ZnMgO, and diamond, β-Ga 2 O 3 have attracted intensive attentions 11,12,14,15 . But high quality epitaxial AlGaN film is difficult to be prepared due to high growth temperature, single wurtzite phase ZnMgO and diamond are not possible to be used to detect entire deep ultraviolet region due to their mismatched band gaps 11,12 . On the other hand, β-Ga 2 O 3 has great thermal and chemical stability, determining its possibility of working at high temperatures and being unaffected even by concentrated acids such as hydrofluoric acid 16,17 . Therefore, relatively, β-Ga 2 O 3 is considered as one of the most ideal candidates to fabricate solar-blind photodetector. In our previous work, high epitaxial β-Ga 2 O 3 thin films have been grown via laser molecular beam epitaxy (LMBE) and fabricated metal-semiconductor-metal (MSM) structure using for photodetector, showing an obvious DUV solar-blind photoelectric properties 12,18 . At the same time, we also note that oxygen vacancies widely exist inside the gallium oxide thin films and have a significant impact on the photoelectric performance 12,19,20 . Based on the above, β-Ga 2 O 3 epitaxial thin film using for solar-blind photodetector is an ideal candidate for studying the influence of oxygen vacancies and the suppression of unintentional extra carriers which are induced by oxygen vacancies.  To further evaluate the performance of the Ga 2 O 3 and (GaMn) 2 O 3 photodetectors comparatively, the time-dependent photoresponse to DUV illumination were investigated. Figure 5(a,b) show the time-dependent photoresponse of the Ga 2 O 3 and (GaMn) 2 O 3 photodetectors to 254 nm UV light illumination with varied optical  Fig. S3(a,b)]. However, there are also some obvious differences between two devices. For the Ga 2 O 3 photodetector, the current increases from approximately 465.6 nA of original dark current to a non-stable value of approximately 2490.6 nA of illumination current with an optical input power of 150 μW/cm 2 at 10 V [ Fig. 5(a)]. However, the recovery time is extremely long after the light is turned off. Such slow recovery should be attributed to the electron-hole trapping states, which would prevent charge-carrier recombination. And the original dark current cannot be obtained with a duration time of 20 s after turning off the 254 nm light. For the (GaMn) 2 O 3 photodetector, the dark current is approximately 3.1 nA at 10 V, which is low and favorable for practical detectors. Under 254 nm light with an optical input power of 150 μW/cm 2 illuminations, the current instantaneously increases to a stable value of approximately 238.5 nA. When the light turns off, the current decreases rapidly down to 3.5 nA, which is quite close to the initial dark value.

Results and Discussion
The sensitivity, the spectra responsivity (R λ ) and the external quantum efficiency (EQE) are the key parameters to evaluate the performance of a photodetector (the definition of these parameters please see Supplementary  Information) [25][26][27] . The larger values of sensitivity, R λ and EQE -the higher performance a photodetector has. These the parameters for our two photodetectors are listed in Table 1. The sensitivity increases with the increase of incident optical power, while it increases firstly and then decreases with increasing applied bias [ Fig. 5(e,f)]. Meanwhile, the R λ and EQE values increase with the increase of applied bias while decrease with increasing incident optical power. The sensitivity of the (GaMn) 2 O 3 photodetector is significant superior to that of the Ga 2 O 3 photodetector due to its tiny dark current. The maximum sensitivity of 67.1 was obtained at 10 V with an optical input power of 150 μW/cm 2 in the (GaMn) 2 O 3 photodetector. The band gap for 28.4 at.% Mn doping Ga 2 O 3 thin film can be estimated to about 4.75 eV, which has a bit red shift compared to undoped Ga 2 O 3 (4.92 eV) 23 . The light absorption to 254 nm UV light for undoped Ga 2 O 3 thin film is a bit stronger than Mn-doped one, which contributes to a higher sensitivity. However, the Ga 2 O 3 photodetector exhibits the bigger R λ and EQE values than the (GaMn) 2 O 3 photodetector. Herein, the maximum R λ value of 1.30 A/W was obtained at 20 V with an optical input power of 150 μW/cm 2 in the Ga 2 O 3 photodetector, which corresponds to an EQE ~634%.
Another important parameter for UV photodector is response time. A bi-exponential relaxation equation was used to analyze quantitatively the current rise and decay process of two devices (please see Supplementary  Information). As shown in Fig. 5(c,d) and [ Supplementary Fig. S3(c,d)], the photoresponse processes are well fitted. τ r and τ d are the time constants for the rise edge and decay edges respectively. We note that both the current rise and decay processes consist of two components with a fast-response component and a slow-response component for the Ga 2 O 3 photodetector, while there is only a fast-response component for the (GaMn) 2 O 3 photodetector. Generally, the fast-response component can be attributed to the rapid change of carrier concentration as soon as the light is turned on/off, while the slow-response component is caused by the carrier trapping/releasing owing to the existence of defects in β-Ga 2 O 3 thin films such as oxygen vacancies. For example, for the 254 nm illumination with an optical input power of 150 μW/cm 2 at 10 V, the decay process is rapid with a τ d of 0.28 s for the (GaMn) 2 O 3 photodetector, while the decay process of the Ga 2 O 3 photodetector is slow which consists of two components (τ d1 = 0.47 s, τ d2 = 6.87 s). The (GaMn) 2 O 3 photodetector presents a much faster response speed to light than that of the Ga 2 O 3 photodetector.
To understand the impact of the Mn dopants on the conducting properties, the crystal structure was built [ Fig. 6(a)] 28 , and the electronic structure was calculated with density functional theory (DFT). Figure 6(b,c) shows the electronic structure of undoped and Mn-doped β-Ga 2 O 3 respectively. The Fermi level was set at zero-point of energy. For the calculation of band structure of undoped β-Ga 2 O 3 , a 1 × 2 × 1 supercell doubling the monoclinic unit along the b direction was modeled. The calculated width of the band gap is 2.068 eV (uncorrected), which is less than half the experimental value of 4.9 eV. This is because DFT theory is based on the ground state theoretically, resulting in that the exchange-correlation potential between the excited electronic has been underestimated. Before the calculation of band structure of Mn-doped β-Ga 2 O 3 , the site occupancy of the Mn dopants was investigated by the total energy density functional theory calculation. With the Mn doping concentration of 28.4 at.%, two Mn ions replacing with two Ga ions in a conventional unit cell with a dopant concentration of 25 at.% was modeled. Comparing the total energy, those two Mn ions substituting two adjacent octahedral sites is found energetically stable, and the electronic band structure is given in Fig. 6(c). Relative to undoped β-Ga 2 O 3 , there are six new bands within the band gap occupying energies. For the neutral cell, which corresponds to 3+ valence for the Mn, two thirds of these new states are occupied, and the other unoccupied; that is, the Fermi level is located in these defect states. Mn 3+ is amphoteric since it can accept and donate an electron if the Fermi level crosses the Mn 2+ /Mn 3+ acceptor or the Mn 3+ /Mn 4+ donor level 29,30 . In our previous report, two valence states of Mn ions (Mn 2+ /Mn 3+ ) were observed in our (GaMn) 2 O 3 thin films by using XPS 23 . Therefore, the Fermi level should be located within kT of the Mn 2+ /Mn 3+ transition level, which is depicted schematically in Fig. 6(c). In this model, if the Fermi level is higher, additional electrons would enter the Mn d-shell, and thus decreasing the valence to 2+ [29][30][31] . In other words, if Mn doping is the dominant defect in β-Ga 2 O 3 , the Fermi level will be pinned close to the Mn 2+ /Mn 3+ transfer level. In our undoped β-Ga 2 O 3 thin film, oxygen vacancies always existed as the donor-type defects 23   converted to negatively charged acceptors (Mn 2+ ). The change of Mn valence suppresses the unintentional extra carriers, and the Mn ion acts as the "carrier killer". Therefore, the (GaMn) 2 O 3 thin film has a higher resistivity compared to the undoped Ga 2 O 3 , leading to a lower dark current in the application of photodetector. At the same time, the oxygen vacancies are usually acted as the carrier trapping/releasing centers in the photodetector. Under the illumination of 254 nm light, some of the photogenerated carriers are captured by the trapping states of oxygen vacancies. When the illumination is turned off, these carriers captured by the oxygen vacancies would be released and recombined. Generally, traps in a wide band gap semiconductor are extremely deep. The time constant of the transient decay is governed by the depth of these traps and can be very long, and this process is responsible for the slow-response component. For the as-grown undoped Ga 2 O 3 thin films, the II peak of O 1s XPS which is usually associated with the oxygen vacancy is obvious 23 . While for the (GaMn) 2 O 3 thin film, the II peak of O 1s is highly suppressed, indicating that the oxygen vacancy concentration decreased markedly [ Supplementary Fig. S4]. The presence of many oxygen vacancies prevents carriers' recombination and causes the slow recovery time for pure β-Ga 2 O 3 thin film. And the reduction of oxygen vacancies concentration by Mn doping contributes to a faster response speed to light for (GaMn) 2 O 3 photodetector.
In     lattice. Meanwhile, the prototype photodetector devices with a MSM structure have been fabricated and investigated based on the undoped and Mn-doped Ga 2 O 3 epitaxial thin films. In compare with the undoped Ga 2 O 3 , the Mn-doped photodetector takes on a lower dark current, a higher sensitivity and a faster photoresponse time, which are attributed to the higher resistivity and a lower trapping states concentration of oxygen vacancies. For the pure Ga 2 O 3 , there are a few oxygen vacancies as donor-type defects leading to the origin of intrinsic n-type conductivity. While for Mn-doping, the extra electrons generated by oxygen vacancies would enter the Mn d-shell with the change Mn valence from + 3 to + 2; that is, the Fermi level would be pinned mid-gap at the Mn 2+ /3+ transition level, which is predicted by DFT. The study presents that doping with Mn may provide a convenient and reliable method of obtaining high insulating β-Ga 2 O 3 thin film which is a promising candidate for use in high performance solar-blind photodetector.

Methods
The thin films were prepared on 10 × 10 mm α-Al 2 O 3 (0001) substrates by the LMBE technique at a repetition frequency of 1 Hz and with a fluence of ~5 J/cm 2 . The thin film deposition was grown in a vacuum environment of 1 × 10 −6 Pa and at a substrate temperature of 800 °C. For the (GaMn) 2