Infrared Photoluminescence In Semiconductor Industry

INTRODUCTION

PL (Photoluminescence spectroscopy) is a contactless, nondestructive technique for studying the optical properties of materials. It relates to exciting a material’s surface with light and then measuring the emitted light can provide information about the band structures, defects, and impurities.

PL spectroscopy has been widely used in various fields of physics, chemistry, and biology fields. For physics, this method is well-known for playing an essential role in the semiconductor industry. It is a powerful method for investigating the quantum confinement effects in nanocrystals, typically applied to semiconductor photocatalysts, LED, and optoelectronics. For example, direct Z-scheme heterojunction between two semiconductors yields valuable information by PL spectroscopy.

The probe-assisted PL spectroscopy has been widely applied to thin films and bulks. Referred documents about semiconductor substances described from mono elements to compounds, clarified mostly group IV (silicon) in the element periodic for the former and II-VI (CdSe, CdTe, etc.), III-V, and IV-IV compositions for the latter.

Their emission covers the complete spectral range from UV to IR, with excellent quantum yields of 60 – 90%. All covered wavelength ranges can provide two-dimensional maps of semiconductor properties. The short wavelengths of the PL method give the best spatial resolution, while long-wavelength infrared light can deeply penetrate materials’ structures. Mid- to far-infrared radiation of PL is useful in measuring samples with penetration of depth micrometers instead of only tens to hundreds of nanometers in the short range. The infrared range covers very broad range, spanning the near infrared (approximately 1 – 5 um), the middle infrared (5 – 50 um), and the far infrared (50 – 1000 um).

PRINCIPLE

When a material is excited with light, electrons are promoted to higher energy levels. These electrons then relax back to their ground state, emitting light in the process. The energy of the emitted light is related to the energy difference between the excited state and the ground state (Described in Figure 1).

To excite electrons in the valance band (VB) to the conduction band (CB) needs higher energy than the forbidden energy of materials. Utilizes a laser beam to capture the light emitted from a substance as it transitions from the excited state to the ground state upon laser irradiation. By analyzing the luminescence spectrum, material imperfections, and impurities can be detected. A perfect example is Silicon with a bandgap as about 1.1 eV, which is transparent to infrared light with wavelengths ranging from 1 to 10 μm.

Figure 1. The schematic describes the electrons’ movements of the photoluminescence principle.

APPLICATION

Overall, infrared photoluminescence (IR-PL) has contributed to the semiconductor industry in 6 key applications as below:

1. Semiconductor materials characterization: doping concentration and crystalline quality of semiconductor materials are provided, which is crucial for device design and optimization.
2. Thin Film Analysis: the thickness, composition, and quality of the thin films are provided, which is essential for device performance and reliability in LED, TCO, etc.
3. Quantum Dot and Nanostructure Characterization: the size, composition, and electronic structure of these nanostructures are provided, which are critical for applications in quantum computing, optoelectronics, and photovoltaics.
4. Defect and Impurity Identification: determine the type and concentration of defects, which is valuable for process optimization and quality control.
5. Interface and Heterostructure Characterization: the quality of the interfaces between different semiconductor layers is provided, which is crucial for device performance and reliability in transistors, lasers, and photodetectors.
6. In-situ and Non-destructive Monitoring: IR-PL can be used for in-situ monitoring of semiconductor fabrication processes, allowing for real-time feedback and optimization.

To clarify the usage of IR-PL, in this application note we refer to the experiment result from the article titled “Photoluminescence Excitation Spectroscopy of Diffused Layers on Crystalline Silicon Wafers”.

The samples under study are float-zone boron-doped c-Si wafers oriented in the <1 0 0> direction. The doping profiles of the diffused layers were analyzed using the electrochemical capacitance-voltage (ECV) technique. Sample 1 was found to be shallower but with a higher concentration compared to sample 2. The excitation wavelengths used ranged between 510 and 810 nm.

The spectral analysis in Figure 1 reveals two distinct peaks. The first peak, at approximately 1130 nm, corresponds to the band-to-band emission from the underlying c-Si substrate and was utilized for normalizing the spectra across different excitation wavelengths. The second peak, at around 1165 nm for sample 1 and 1157 nm for sample 2, is attributed to the band-to-band emission from the heavily doped layer near the surface, referred to as the HDBB peak. This HDBB peak exhibits a shift to longer wavelengths compared to the c-Si peak, which is attributed to the band-gap narrowing effects in heavily doped silicon.

Figure 1. The evolution of normalized PL spectra of samples 1 and 2, respectively, with increasing excitation wavelengths from 510 to 810 nm.

The HDBB peak of sample 1 exhibits a lower energy level (i.e., longer wavelength) and a broader shape when compared to that of sample 2. This suggests that the diffused layer of sample 1 is more heavily doped than that of sample 2, shown in Figure 2.

Figure 2. Comparison of normalized PL spectra between sample 1 and sample 2, excited with the 510- and 810-nm wavelengths

RECOMMENDED EQUIPMENT

For this application note, Intins recommends that customers use the NIRQuest+2.5 spectrometer. This device plays the most important role in a PL measurement system. The measurement precision and outstanding characteristics span a wavelength range of 900-2450 nm. Designed for high-sensitivity performance, the optical bench is ideal for NIR applications. The NIRQuest features an exceptional InGaAs array detector, delivering high performance with data acquisition speeds as fast as 5 milliseconds. The detector is internally cooled and temperature-regulated for optimal signal-to-noise ratio and sensitivity. Data is transferred in 64-byte packets (USB 1.1) or 512-byte packets (USB 2.0). Additionally, this product line includes an integrated trigger mode.

To activate the system Ocean Optics also supports high-power lasers with various single wavelengths of 532nm, 638nm, 785nm, and 1064nm. In addition, a VIS-NIR fiber optics cable is also provided to complete a system for measuring photoluminescence. The VIS-NIR fiber ranges from 400nm to 2100 nm) and solarization-resistant (180-800 nm and 200-1100 nm) options are also available.