[“Source” Observe the Autumn Series] Application and testing of the next generation of semiconductor gallium oxide device photodetectors

Gallium oxide (Ga2O3) detector is a photodetector based on ultra-wide bandgap semiconductor materials, mainly used for the detection of day-blind ultraviolet light. Its unique physical and chemical properties make it show broad prospects in multiple application fields. The performance of the detector will vary greatly due to different materials, structures, preparation processes and application scenarios. There are often constraints between performance indicators, such as dark current and output current, sensitivity and responsiveness, reliability and sensitivity, etc., which need to be weighed and compromised. The same is true for performance characterization. High responsiveness cannot be performed simultaneously with high-precision current characterization. Tektronix provides test instruments with a variety of performance and architectures to meet the needs of testing detectors at different extreme dimensions.

Characteristics, applications and detector classification of gallium oxide

Characteristics and Applications of Gallium Oxide

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.

Detector structure and type

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

• Metal-semiconductor-metal (MSM) type: This structure is simple, highly responsive, and suitable for large-scale integration, but the effective light absorption area is small.

• 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 and suitable for application scenarios that require wide coverage.

• Classification of Gallium Oxide Detectors

• 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 operate under a bias voltage of 0V. 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 achieve high-quality and stable p-type doping of Ga2O3, which limits the development of its pn homojunction devices, the dark current can be significantly reduced and the response can be improved by combining n-type Ga2O3 with other p-type or n-type materials to form a heterojunction device.

Gallium oxide detector performance indicators and test methods

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.

Intrinsic analysis mainly includes: transfer characteristic curve, output characteristic curve, characterization device control and carrier (electron-hole pair) migration characteristics under different working conditions, and electrical output characteristics. It can be easily completed through SMU source meter and 4200A-SCS parameter analyzer. The specific parameters and models of the required instruments need to be determined according to the voltage range and carrier range of the scan.

SMU SourceMeter Series

The 24 series and 26 series meet the needs of different test accuracy, especially for high-throughput carrier characterization of new and innovative preparations, structural designs, etc. of gallium oxide substrates and heterojunctions.

4200A-SCS Parameter Analyzer

A powerful tool for device testing, it has a built-in gallium oxide photoelectric test module, covering static and dynamic tests. It can also be equipped with a CVU module to increase CV test capabilities and quantitatively measure defects on device interfaces. In addition, it can control external pulse sources, oscilloscopes, etc. to complete dynamic response tests.

Figure: Transfer and output characteristics of a typical β-Ga2O3 detector [3]

Figure: Test block diagram

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

A performance comparison of various gallium-based oxide thin film day-blind ultraviolet detectors shows that 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.