"Source" to detect the slightest detail: Application and testing of the next generation of semiconductor gallium oxide device photodetectors

The testing of gallium oxide detectors can be divided into material testing, device intrinsic analysis (static) and optoelectronics (dynamic). Material testing usually uses TEM, XRD and other methods to analyze the microscopic crystals, film structure, surface morphology and other characteristics of the prepared materials. This article aims to briefly introduce the electrical and optoelectronic properties.

Transfer characteristic curve and output characteristic curve characterize the control of the device and the migration characteristics of carriers (electron-hole pairs) under different working conditions, as well as the electrical output characteristics. This can be easily accomplished using the SMU source meter and 4200A-SCS parameter analyzer. The specific parameters and models of the required instruments need to be determined based on the voltage range and carrier range to be scanned.

Especially for optoelectronic devices, it is necessary to characterize the responsiveness characteristics from input light to output current. In addition, when there is no light, there is a dark current generated by the random drift of electron-hole pairs in optoelectronic devices. Especially for MSM and heterojunction devices, the performance of dark current directly determines the defects in the preparation of different materials.

Photoresponsivity R = (JPhoto - JDark)/P

Where R is the photoresponsivity, JPhoto is the photocurrent density, JDark is the dark current density, and P is the incident light power.

It can be seen that the dark current test still has very high requirements for the test instrument. The current is in the pA level, and a high-precision source meter is required in conjunction with a low-leakage probe station to achieve this level. It is recommended to use the Keithley 2600 series source meter, or add a PA module to the 4200A-SCS to achieve accuracy in the pA level and resolution in the aA level. In the test system, the choice of light source depends on the application scenario. The excitation light source can be changed to achieve wavelength dependence testing. Different light source types also determine the cost of the test system. Usually, UV LED can be selected, which contains a rich UV spectrum and is low in cost, but it is impossible to achieve a certain wavelength; lasers and monochromators have strong wavelength selection, but the cost is high.

The dynamic parameter test of gallium oxide mainly characterizes the photoelectric response speed, response stability (light-dark cycle), etc. The response time refers to the time from the detector receiving the light signal to the output of the electrical signal. A shorter response time means that the detector can detect the change of the light signal faster, which is crucial for applications that require real-time monitoring or high-speed communication. The response stability test can evaluate the performance consistency of the detector under different working conditions (such as temperature changes, long-term operation, etc.). Ensuring that the detector can maintain a stable response time and sensitivity in various environments is crucial for practical applications.

Figure: Typical response speed and stability of gallium oxide ^[3]

According to the literature ^[1], which compares the performance of various types of gallium-based oxide thin film day-blind ultraviolet detectors, the response rise time ranges from nanoseconds to seconds, spanning 9 orders of magnitude, and the dark current ranges from picoamperes to nanoamperes.

Usually, when testing current changes in the ms range, SMU can be used. By using the autoscale of SMU, if time tests in the us range are required, DMM6500 can be used, connected to the test system, and high-speed current sampling can be performed. At the same time, it is compatible with smaller current test ranges and test accuracy. If the current changes in the ns range, an oscilloscope is required to complete it, but the current test capability of an oscilloscope is usually in the mA range, and an external TIA with a fixed stable gain needs to be used for amplification.

Gallium oxide has an ultra-wide band gap of 4.4 to 5.3 eV, which can effectively cover the solar-blind band (200-280 nm) of ultraviolet light. This feature makes gallium oxide an ideal solar-blind ultraviolet detection material, because the light in this band is strongly absorbed by the ozone layer in the atmosphere, and the ground background interference is small, which can provide higher detection accuracy. At the same time, the most stable isomer of gallium oxide, β- _Ga2O3 , has a band gap of 4.8 _eV and a theoretical breakdown electric field of about 8 MV/cm. As a result, Baliga's Figure of Merit is as high as 3444, far exceeding gallium nitride (GaN) and silicon carbide (SiC), which means that it has great potential for use in power devices, making it a candidate material for the next generation of semiconductor power electronics. Due to different application scenarios, the ideas for the monomer, doping, and device preparation and testing of gallium oxide are not used. This article focuses on the application of gallium oxide in photodetectors and the testing methods.

The structures of gallium oxide detectors are mainly divided into the following types:

■ Metal-semiconductor-metal (MSM) type: This type of structure is simple, has high responsiveness, and is suitable for large-scale integration, but has a small effective light absorption area.

■ Schottky junction: The Schottky barrier formed between metal and gallium oxide has a faster response speed and lower dark current.

■ Thin-film transistor (TFT) type: Able to amplify gain while suppressing dark current, suitable for applications with high response speed.

■ Array type: Mainly used for large-area imaging, suitable for application scenarios that require wide coverage.

■ MSM device: It is composed of photosensitive material and two back-to-back Schottky contact (metal semiconductor contact with rectification voltage and current characteristics) electrodes. When an external bias is applied to the device, one Schottky junction is forward biased and the other is reverse biased, so the dark current is small. At the same time, the device also has the advantages of simple structure, easy preparation, and small junction capacitance.

■ Schottky junction device: It is composed of a photosensitive material and two electrodes that form a Schottky contact and an ohmic contact (a metal semiconductor contact with non-rectifying voltage and current characteristics). The light response of the Schottky junction device comes from the photovoltaic effect and can work under a 0V bias. At the same time, because the space charge region is close to the electrode and closer to the surface, the device usually has a higher external quantum efficiency and a faster response speed.

■ Heterojunction: Since _{it is still very difficult to perform} high-quality and stable p-type doping _{on Ga2O3} , which limits the development of its pn homojunction devices, by combining n-type Ga2O3 _with other p-type or n-type materials to form a heterojunction device, the dark current can be significantly reduced and the response can be improved.

[1] DOI: 10. 37188/CJL. 20230146 Research Progress of Gallium-based Oxide Thin Film Solar-blind Ultraviolet Detectors

[2] Liu, Y.; Huang, R.; Lin, T.;Dang, J.; Huang, H.;Shi, J.; Chen, S.Preparation of Sn-Doped Ga2O3 ThinFilms and MSM Ultraviolet DetectorsUsing Magnetron Co -Sputtering. Materials 2024, 17, 3227.

[3] https://doi.org/10.1016/j.mtphys.2024.101380 Enhancement of photodetection performance of Ga2O3 / Si heterojunction solar-blind photodetector using high resistance homogeneous interlayer

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