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摘要:
为了提高Ga2O3基日盲紫外探测器的性能，本文使用分子束外延方法对β-Ga2O3薄膜进行Sn掺杂，并制备成MSM型日盲紫外探测器。结果表明，Sn掺杂可以改变薄膜晶体结构，使氧化镓薄膜由单晶向多晶相转变。同时，Sn掺杂紫外探测器的光电流和响应度相比于未掺杂器件产生了较大的提升，在254 nm、42 μW/cm2紫外光照下，Sn源温度900 ℃制备的薄膜探测器响应度为444.51 A/W，远高于未掺杂器件。此外，器件的-3 dB截止波长从252 nm调整到274 nm，表明Sn掺杂可以有效调控紫外响应的波长。Sn掺杂也会引入杂质能级，导致器件时间响应特性变差。
Abstract:In order to improve the performance of Ga2O3-based photodetectors (PDs), Sn-doped gallium oxide thin films were prepared on sapphire substrates by molecular beam epitaxy system. The influence of Sn doping on both Ga2O3 crystal structure and photoelectric properties of metal-semiconductor-metal (MSM) PDs were investigated. X-ray diffraction shows that gallium oxide films change from single crystal to polycrystalline phase when increasing the growth temperature of SnO2. When 254 nm and 42 μW/cm2 light was used, the responsivity of Sn-doped Ga2O3 photodetectors reached 444.51 A/W. Compared with the undoped β-Ga2O3 PDs, the photocurrent and responsivity of Sn-doped PDs were almost increased by two orders of magnitude, suggesting the improvement on PD performance. Spectral response shows that the cut-off wavelength of Sn-doped PDs changes from 252 nm to 274 nm by increasing Sn dose, which reveals an efficient way toward the development of the UV PDs focus on longer wavelengths. However, Sn doping also introduces impurity levels, resulting in poor time response of the MSM PDs.
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Key words:
- Sn doping /
- β-Ga2O3 /
- solar blind ultraviolet photodetector /
- responsivity
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Overview: Deep ultraviolet (DUV) photodetectors with solar-blind sensitivity (cutoff wavelength shorter than 280 nm) have received much attention because their photo response can be further restricted within the DUV region only even under sun or room illuminations. Solar-blind DUV photodetectors (PDs) are important devices that can be used in various commercial and military applications, such as flame detector, missile plume sensor, and ozone holes' monitor. Currently, Si-based photodiodes are the most commonly used ultraviolet photodetector in the commercial market because of their high compatibility with the highly mature silicon processes. However, expensive and cumbersome Wood's optical filters are required because Si is sensitive to infrared, visible, and near UV lights due to its small bandgap (1.1 eV~1.3 eV). Therefore, PDs based on wide-bandgap (Eg) semiconductors are regarded as more promising alternatives. In recent years, several wide bandgap materials consisting of AlGaN, Ga2O3, ZnMgO, BN, and diamond were proposed for solar-blind DUV photodetectors. Among these materials, gallium oxide, which has an Eg of 4.9 eV, is intrinsically suitable for solar-blind photodetection. In the past few years, gallium oxide-based metal-semiconductor-metal (MSM) PDs have been intensively explored.
It is found that the undoped Ga2O3 thin films are of n-type conductivity as defects such as oxygen vacancies will be introduced during the preparation process. However, pure β-Ga2O3 demonstrates poor conductivity at room temperature because of low electronic mobility, hindering its practical applications based on conductance response. In order to overcome this obstacle, intentionally controlled doping becomes a very important and feasible method. Tetravalent Sn is a superb doping candidate for Ga2O3 because it is not only an effective n-type dopant, but also has a close ionic radius with the octahedrally coordinated Ga3+. Masahiro Orita group used the method of pulsed laser deposition (PLD) to grow Sn-doped Ga2O3 film in 2002; Azuaki Akaiwa prepared a Sn-doped Ga2O3 film in 2012 using the spray-assisted mist chemical vapor deposition (mist CVD) method. In 2014, Du Xuejian and others from Shandong University of China used the metal-organic chemical vapor deposition (MOCVD) method to prepare a low-resistivity Sn-doped gallium oxide homoepitaxial film. However, there are few reports on the Sn-doped gallium oxide thin films preparation of molecular beam epitaxy (MBE), which is a new and widely used film preparation technology developed with the improvement of semiconductor crystal quality requirements. In this work, we explored the growth of Sn doped Ga2O3 film on sapphire substrates by MBE method. Solar-blind photodetectors with MSM structure based on Sn doped Ga2O3 thin films were fabricated and compared with the undoped PDs. Result shows that photocurrent and responsivity almost increased by two order of magnitude for Sn doped devices, suggesting devices performance of PD can be improved by doping Sn in Ga2O3 films.
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图 1 Ga2O3 MSM型日盲紫外探测器示意图
Figure 1. The schematic diagram of the Ga2O3 MSM solar-blind ultraviolet PD
图 2 β-Ga2O3以及不同温度Sn掺杂Ga2O3薄膜XRD θ~2θ扫描图谱
Figure 2. XRD spectra of β-Ga2O3 and Sn doped β-Ga2O3 films deposited at different SnO2 temperatures
图 3 Ga2O3 MSM型日盲紫外探测器的电流-电压特性曲线和器件响应度对比。(a)无紫外光照下的电流电压特性曲线；(b) 254 nm、42 μW/cm2紫外光照下的电流电压特性曲线；(c)线性坐标无紫外光照下的电流电压特性；(d)不同Sn掺杂Ga2O3对应的器件响应度
Figure 3. The current-voltage (I-V) of the Ga2O3 MSM PDs with different SnO2 temperatures. (a) Idark−V characteristics on asemilogarithmic scale; (b) I−V characteristics under 254 nm DUV illumination on a semilogarithmic scale; (c) Idark−V characteristics on a linear scale; (d) Responsivity of Sn-doped Ga2O3 MSM PDs with different SnO2 temperatures
图 4 Ga2O3 MSM型日盲紫外探测器的光谱响应特性
Figure 4. Spectral response of the Ga2O3 MSM PDs on a semilogarithmic scale
图 5 Ga2O3 MSM型日盲紫外探测器的时间响应特性。(a)探测器多周期时间响应；(b)单个周期探测器归一化时间响应；(c)未掺杂β-Ga2O3器件上升和衰减过程的实验数据和拟合曲线；(d) Sn源900 ℃ Ga2O3器件上升和衰减过程的实验数据和拟合曲线
Figure 5. Time-dependent photoresponse of the Ga2O3 MSM PDs with different SnO2 temperature. (a) Response for multicycles on a linear scale; (b) Normalized transient response on a linear scale; (c) Experimental and fitted curves of rise and decay processes for β-Ga2O3 PDs; (d) Sn doped β-Ga2O3 PDs with SnO2 temperature of 900 ℃
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