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Inspec hand in hand with semiconductor development

Semiconductor technology has developed over the last century from a curiosity to its modern-day applications. The Inspec database classification codes have evolved hand in hand with the subject.

Starting in the 20th century, there were several reports of light emission from materials under application of electric fields, a phenomenon which was termed Electroluminescence.

A7860F: Electroluminescence (condensed matter)
B4220: Luminescent materials

The materials' properties were poorly controlled and the emission mechanisms misunderstood. The first instance in 1923 of blue electroluminescence was based on light emission from particles of silicon carbide (SiC) which had been manufactured as sandpaper grit and contained accidental p-n junctions.

B2520M: Other semiconductor materials
B2530B: Semiconductor junctions

In the following decades, blue SiC light emitting diodes (LED's) were never substantially improved.

B4260D: Light-emitting diodes

In the end, the best SiC LED's, emitting blue light at 470nm, had an efficiency of around 0.03%. In 1954 industrial growth of III-V compound semiconductors commenced. For example in the mid-1950s, large crystal boules of gallium arsenide (GaAs) were extracted from the melt, and sliced and polished wafers were used as substrates for the growth of p-n junction diode structures.

A8110F: Crystal growth from melt
A8160C: Surface treatment and degradation in semiconductor technology
B0510: Crystal growth
B2520D: II-VI and III-V semiconductors
B2550E: Surface treatment (semiconductor technology)

Infra-Red LED's based on GaAs were first reported in 1962. In order to get visible light emission, GaAs was alloyed with gallium phosphide (GaP), and red LED's were demonstrated. By 1968, iso-electrical doping of GaP with nitrogen had been investigated, and much brighter yellow-green (550nm) LED's, had reached efficiencies of 0.3%.

A6170T: Doping and implantation of impurities
B2550B: Semiconductor doping

In order to create a full-colour image, there must be red, green and blue pixels in a display. Red LED's existed, made out of GaAsP, and also green, made out of GaP:N. All that was needed was a bright blue LED.

B7260B: Display materials

In 1971 Pankove (RCA Princeton Laboratory) demonstrated a metal-insulator-semiconductor LED based on gallium nitride (GaN). The device consisted of an insulating Zn doped layer which was contacted with two surface probes, and blue light peaking at 475nm was emitted.

B2530F: Metal-insulator-semiconductor structures

Pankove then made a device consisting of an undoped n-type region, an insulating Zn doped layer and an In surface contact, this proved to be the first actual GaN LED, and it emitted green light. In 1972, Maruska began growing Mg doped GaN films using the hydrogen vapour phase growth (HVPE) technique and produced a bright violet LED emitting at 430nm.

A6855: Thin film growth, structure, and epitaxy
A8115J: Ion plating and other vapour deposition
B0520X: Other thin film deposition techniques

The two major problems that needed to be solved in order to fabricate GaN based LED's were to produce sufficient high-quality crystalline layers and to achieve p-type doping.

The difficulty of growing high-quality GaN crystalline films lies in finding a suitable substrate material. Yoshida et al grew GaN/AlN heterostructures by MBE in 1983.

A8115G: Deposition by sputtering
B0520D: Vacuum deposition

Akasaki demonstrated in 1986 that metal organic chemical vapour deposition (MOVPE) growth of high-quality GaN layers was possible on sapphire.

A8115H: Chemical vapour deposition
B0520F: Chemical vapour deposition

In the conventional MOCVD technique, semiconductors are grown by flowing reacting gases over a substrate. Nakamura pioneered a method whereby the gases flow in two directions instead of one. It was discovered that at high substrate temperatures approx 1000C the gas flow of the reactants was not favourable due to convection, a second gas jet was introduced consisting of nitrogen and hydrogen gas, perpendicular to the substrate surface, this growth technique acts by pushing the reactants towards the growth surface, leading to improved crystal growth.

A significant advance was made by Akasaki et al when they discovered that p-type conduction GaN could be achieved by irradiation with low energy electrons, low energy electron beam irradiation (LEEBI).

A7360L: Electrical properties of II-VI and III-V semiconductors (thin films/low dimensional structures)
A7920H: Electron-surface impact: secondary emission

This experimental result was followed by Nakamura, who clarified the annealing process.

A6170A: Annealing processes
B2550A: Annealing processes in semiconductor technology

The novel MOCVD technique enabled Nakamura to manufacture a blue LED. In 1992 Nakamura added Into the GaN crystal in order to create quantum wells for the electrons at the junction. The addition of In lowered the frequency of
the emitted photons to visible blue.

A7865K: Optical properties of II-VI and III-V semiconductors (thin films/low dimensional structures)

Nakamura's success helped direct attention to another application of GaN, high frequency and high power transistors.

B2560: Semiconductor devices


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