The Dark Horse of Semiconductor Industry
Group III-Nitride materials are the class of compound semiconductor consisting of elements namely Gallium, Indium and Aluminium with Nitrogen. The nature has blessed these semiconductor alloys system with persisting direct band gap. In today’s world these materials have proven the backbone of solid-state lighting application by achieving bright blue LEDs. The III-V blue LED technology has opened enormous new markets that were not accessible before. This technology has found its place in every person life living on the planet; from mobile display to television screen; from projectors to quantum computing hardware. The III-V semiconductor technology has revolutionised the modern electronic era, by saving 30% global energy. This technology yields Nobel Prize in Physics in the year of 2015 which was shared among three scientists—Isamu Akasaki, Hiroshi Amano, and Shuji Nakamura. It would be interesting to know the history of this semiconductor material and the hurdles that were faced by the researchers. The history will tell us about the hard endeavour was made by our research community to bring this technology for labs to fabs. Please allow me to take you towards 1930’s.
The first time GaN was synthesized was in 1932 at George Herbert Jones Laboratory by Johnson group by reacting gallium metal and ammonia at a very high temperature of 900-1000 ºC. Yet it took 37 years to obtain a single crystalline GaN layer. In 1969, Maruska and colleagues used a hydride vapour phase epitaxy (HVPE) process for the first time to deposit GaN on a c-sapphire substrate, in which Gallium was transported by HCl gas and nitrogen by ammonia.XRD analysis of the prepared layers revealed that they were oriented in the c-direction and had hexagonal wurtzite lattice structure. However, the layers were full of cracks and pits. In addition, the bandgap of GaN was found to be 3.29 eV at room temperature in this study. Moreover, the layers were unintentionallyn-type with an electron concentration of the order 1019cm-3. Later, various metals such as silicon, germanium, zinc, mercury, and magnesium were used as dopants to lower electron concentration and eventually, hole concentrations of the order of 1019 cm-3were achieved. However, reproducibility was a significant question.
In the year 1972, Pankove group reported the first gallium nitride-based blue light detector. It was deposited on a sapphire substrate with an unintentionally doped n-GaN base layer and aZn doped insulating GaN capping layer. Although the Zn concentration was not measured, it was in mild quantities. The fabricated diode was thermally stable and radiated 440 nm wavelength and worked well in reverse bias avalanche region. However, the efficiency of that in device was poor (power efficiency of 10-4) but it served the purpose by producing blue luminescence at room temperature.
The Maruska group demonstrated violet luminescence recently using a similar structure doped with Mg atoms to achieve high resistance in a top layer.The luminescence was considered due to the radiative recombination of electrons at deep accepters in the insulating region of GaN, which were created due to ionization by high electric field. The development of group III-nitride has been held back for several years due to poor-quality material prepared usingcurrently available growth techniques. Moreover, the irreproducibility of the result was a major concern.
The progress in group III-Nitride resumed in the early 1980s after good quality precursors, along with modern growth technologies such as metal-organic chemical vapour deposition (MOCVD) and Molecular Beam Epitaxy (MBE). The breakthrough was in the field was accomplished by Akasaki group, in the year of 1991, by achieving highest hole concentration up to the order of sixteen in GaN. Which led the development of first p-n junction UV light-emitting diode.
The availability of good quality precursors and modern characterization techniques in the 1990s enabled researchers to develop new ways of fabricating device structures based on group III-Nitrides. In the year of 1994, Nakamura group reported the first high bright blue LED, using In GaN/AlGaN double-heterostructures. Since then, the development of light-emitting device shifted towards the use of multiple quantum wells (MQW) as an active region in light emitting diodes (LEDs) and laser diodes (LDs). In 1996, Nakamura group demonstrated the very first violet In GaN MQW based laser diode.
HEMTs in Group III-Nitrides were not new back then, as GaAs/AlGaAs heterostructures had been demonstrated in the 1980s. GaN and related alloys, which have inherent polarization, could theoretically be used for high mobility devices. However, it was hard to find good quality materials until the 1990s. In the late ‘90s, a considerable amount of effort was made in the field of group III-nitride materials, which enhanced our understanding and resulted in the development of commercially available technologies. It was the year of 1991-92 when M. A. Khan’s group observed the first 2DEG in GaN/AlGaN heterostructure, the mobility of which was 834 cm2/Vs at room temperature. In addition, the M. A. Khan group demonstrated the first GaN/AlxGaN heterostructure grown by MOCVD. The electron mobility of this heterostructure was reported to be twelve time higher at room temperature as compared to bulk GaN of the same thickness. These achievements paved the way for the application of III-Nitride materials in power electronics. In the current market, GaN HEMT devices with ratings between 0.65 -1.2 kV are available
Although, we have come a long way and have enriched our understanding in the field of group III-Nitrides the full potential of these semiconductors has not been utilized yet. Still structural and electronic properties and growth mechanism need further exploration and investigation. The above said fundamental limitation in our understanding is delaying some of the potential applications of group III-Nitrides.