Indium Phosphide (InP) Semiconductor Materials

Indium Phosphide (InP) is a binary compound semiconductor that consists of Indium (In) and Phosphorous (P). It is classified under the III-V semiconductor group of materials, and comes with a zincblende crystal structure. This semiconductor material is best known for its low noise figure and high operation frequency (W-Band and beyond).

Indium phosphide can be prepared from the reaction of white phosphorous and indium iodide at 400 °C. White phosphorous is a waxy and translucent solid that turns yellow upon exposure to light, and indium iodide is an orange-colored crystalline solid. The resulting compound, i.e. InP is brittle in nature, gray in color, and crystal semiconductor that must be ground from a high-purity melt.

 Indium Phosphide single crystal with polished side up

Properties of Indium Phosphide 

Indium Phosphide crystallizes in the zinc blende structure, which is a type of cubic crystal structure. It is known for its superior electrical, optical, thermal, and mechanical properties. One of its key features is its direct bandgap of 1.34eV, which allows it to efficiently emit and absorb light, particularly in the infrared spectrum. This makes InP a preferred material for fiber-optic communication systems in high-speed data transmission. It also has very high electron mobility of around 5400 cm2/V.s, which enables faster electron transport in high-frequency transistors and other high-speed electronic circuits.

In terms of optical properties, the Indium Phosphide crystal is transparent in the infrared region and has a high refractive index (~3.1) due to which it becomes effective in optoelectronic devices like lasers, photodetectors, and modulators. Thermally, it has a moderate thermal conductivity (~0.68 W/cm K), which is lower than materials like silicon but is sufficient for developing high performance electronic devices. The table below gives an overview of the properties of Indium Phosphide.

Production of InP crystals 

The production of bulk Indium phosphide (InP) crystals is primarily achieved through two methods: top-seeded crystal pulling with liquid encapsulation and vertical growth in contain with bottom seeding. In the top-seeded crystal pulling method, also called as the Liquid Encapsulated Czochralski (LEC) process, a high-purity indium and phosphorous mixture is melted in a crucible. A seed crystal is introduced at the top, and the crystal is slowly pulled upwards, allowing the molten material to solidify and grow onto the seed. This process is performed under a protective layer of borix oxide, which encapsulates the molten InP to prevent the evaporation of phosphorus. Although this method is cost-effective and is widely used in the industry, crystals made during this process have high dislocation densities. These dislocation densities arise from large thermal gradients and the strain experience by the growing crystal. They can impact the performance of the electronic devices which are made using these wafers. 

Indium Phosphide Wafer 

The vertical growth method with bottom seeding, is also called Vertical Gradient Freeze (VGF). It involves placing the seed crystal, which is filled with molten indium phosphide, at the bottom of the container. The temperature is gradually lowered from the bottom, allowing the crystal to grow upward as the material solidifies. The vertical orientation in this method allows for better thermal management, resulting in lower thermal gradients, and thus few dislocations in the crystal. The lower-stress environment produces crystals with higher quality and lower defect densities, making them more suitable for high-performance applications. Although high-quality crystals are obtained using the VGF process, but there are yield problems due to twinning and interface breakdown. 

Applications of InP Technology

The typical applications for InP devices include optical applications like lasers, photodetectors, avalanche photo diodes, optical modulators and amplifiers, waveguide-based devices, quantum photonic devices, and both optoelectronic and photonic integrated circuits, as well as new devices for optical communications, switching, networking, and signal processing. Many optoelectronic applications operating at speeds of 40 Gbits/s and above require compatibility with InP-based optical devices. Thus, InP-based high-electron mobility transistors offer a solution to low-noise operation at such speeds.  

The next-generation communication systems (6G), are expected to operate in 100 to 340 GHz frequency range. Power efficient systems which are operating at such high frequencies require RF front ends with transistor RF figure of merits (FOMs) such as transit frequency ft, and maximum oscillation frequency of 500 GHz. The Indium Phosphide heterojunction bipolar junction transistors (HBTs) and high-electron mobility transistors (HEMTs) are promising technologies for these extreamly high frequency applications.

InP HBTs provide higher output powers than SiGe HBTs due to their high carrier peak velocity and high bandgap of the collector. The HEMT technology offers excellent noise characteristics and the highest reported RF FOM (Figure of Merit), but low breakdown voltages. The InP HEMTs are mostly used in aerospace, and defense applications such as radiation sensing in space telescopes, test and measurement, weather radar, and readout circuitry for quantum computers. The excellent performance of the InP devices is highlighted in the figure below. The HBTs outperform SiGe BiCMOS counterparts, while the InP HEMT performance exceeds that of CMOS, silicon-on-insulator, and GaAs technologies.

Figure 2

Despite the excellent performance, InP technology has not been very widely adopted for commercial applications. For operating frequencies < 140 GHz, silicon based technologies deliver sufficient power output and efficiency with higher integration levels and are therefore ubiquitous. For frequencies above 140 GHz, power-efficient systems may require InP RF front ends if they can be mass manufactured. Currently, they are fabricated on small (100 -150 mm), expensive substrates, and use laboratory-scale processes, like electron-beam evaporation, e-beam lithography, metal liftoff, etc. As the demand for higher-frequency devices rises, economies of scale will kick in and lower the cost of InP devices and technology.