5.4.3 Introduction to III-V Nitrides - Growth Methods - MBE

Molecular Beam Epitaxy (MBE) using exited N_2 or Reactive Molecular-Beam Epitaxy (RMBE) is another approach that proved successful in obtaining high quality III-Nitride material. One of the hopes that went into this approach was that lower substrate temperatures could be used than required for CVD or MOVPE, increasing the incorporation rate of volatile nitrogen species. RMBE usually employs ammonia (NH_3) and a pure group III element to obtain III-Nitride growth. However, RMBE was unable to produce high-quality material at substrate temperatures in the 600°C range believed to be due to the low reaction rate between NH3 and the group III element (Ref. 1). RMBE does produce material of high quality though at higher substrate temperatures (above 700°C for GaN and above 900°C for AlN) as verified by our group and discussed later.

Using exited N_2 is another popular approach in MBE III-Nitride growth. Several methods are employed, the most popular one being plasma excitation of the N_2 specie to increase its reactivity, or using a Kaufmann ion source (Ref. 1). Plasmas are usually generated by a Radio Frequency (RF) or Microwave (ECR) source. Our group attempted to grow GaN by exciting N_2 using a jet with a heated nozzle, as discussed in a later chapter.

A review of MBE and RMBE growth is given in a paper by M. J. Paisley and R. F. Davis (Ref. 2), and a short summary of their paper is in place here. RMBE techniques were first used by Gotah et. al. (Ref. 12) in the growth of GaN on c-plane sapphire, using a substrate temperature of 615ºC and NH_3. Their films had a high n-type carrier concentration though on the order of 1019 cm^-3.

Significant improvements in the properties of GaN were achieved by Yoshida et. al. (Ref. 13,14), using substrate temperatures of 700ºC and a thin buffer layer of AlN between the sapphire and the GaN. The AlN buffer layer significantly improved the electrical and optical properties of the subsequent GaN layer, but the n-type carrier concentration was still on the order of 10^19 cm^-3.

Notable increase in the resistivity of GaN films was reported by Powell et. al. (Ref. 15), using a ion source for nitrogen and substrate temperatures between 650-750ºC. The resistivity of the films was as high as 10^14 cm^-3.

Later Powell et. al. (Ref. 16) reported on MBE grown GaN on c-plane sapphire using substrate temperatures between 700-850ºC. They used a gallium effusion cell and a dosing tube at NH3 pressures of 3x10^-4 torr. Growth rates were 0.4-0.5 um/hour, except at substrate temperatures >825°C where rates dropped to 0.2mm/hour. The FWHM of X-ray rocking curves dropped by one half when the substrate temperature was raised across the interval of 760-780°C, while remaining relatively constant at higher substrate temperatures. The n-type carrier concentration drifted upward with growth temperature and was in the neighborhood of 10^18 cm^-3. Hall mobilities changed dramatically from 5 cm2V-1s-1 at a substrate temperature of 700°C to 20 cm^2V^-1s^-1 at 760°C to >100 cm^2V^-1s^-1 at temperatures > 780°C.

M. J. Paisley et. al. (Ref. 17) reported on GaN grown on c-plane sapphire using a compact ECR source for the deposition of molecular nitrogen. Substrate temperatures of 650ºC were used as well as an AlN buffer layer of about 15 nm. Resistivity and Hall measurements of these typically 0.5mm films could not be made owing to the high resistivity of the samples.

AlN was first successfully grown by Yoshida et. al. (Ref. 14) using NH_3 and c-plane sapphire. They used substrate temperatures between 1000-1200°C and growth rates of about 1 um/hour.

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