Page 175 - Electrical Properties of Materials

Preparation of pure and controlled-impurity single-crystal semiconductors 157

What was largely an effort to pacify safety officers and stop the use of large
quantities of arsine and phosphine, came into its own with the advent of ni-
trides. The nitrogen source dimethylhydrazine was used, but it was found that
using a similar r.f. discharge source to that used in MBE was preferable. In
1992, several laboratories started work on dilute N additions to semiconductors
such as GaAs, InAs, and GaP.
Initially the fact that N added to GaAs reduced the bandgap, although GaN
has a much wider gap, surprised researchers, but soon the large bowing of
the conduction band became well known. A large number of long wavelength
devices have been made including lasers in the 1.3 μm range (GaInNAs) and
solar cells. In 2002 a 10 Gbit s –1 MOVPE grown Vertical Cavity Surface Emit-
ting Laser (see Section 12.7.3) went onto the market. This work has been
done in parallel with MBE and MOCVD usually in the same labs, the former
is for research and quality, the latter is preferred for production, especially
multilayered structures.
The great success of this technique in the past decade has been in the diffi-
cult area of producing reasonable quality InGaN for LEDs. There are no nitride
substrate crystals, so growth has usually been on sapphire (Al 2 O 3 ). The re-
actants are trimethyl gallium and trimethyl indium and ammonia (NH 3 )asa
source of nitrogen. The two-flow MOCVD method used at Nichia Chemical
◦
Industries first heats the sapphire substrate to 1050 C in a stream of H 2 . Then
the temperature is lowered to 510 C to grow a buffer layer of GaN approxim-
◦
ately 30 nm thick. Then the main growth of GaN film of approximately 2 μm
at a temperature of 1020 C takes about 30 min. At a reduced temperature of
◦
about 800 C the InGaN active layer is grown 0.3 μm in about 60 min. The
◦
active layer has a pn detailed structure. InGaN is naturally n-type but the con-
ductivity can be controlled by silicon impurity via a stream of silane (SiH 4 )in
the reactant gas. Then there is a very thin layer of InGaN with the In content
controlled so that band edge recombination gives the required LED colour.
Finally, there is the p layer, which took a while to design. It uses Mg im-
purity, deposited via a metal-organic compound called bis-cyclopentadienyl
magnesium, and has to be followed by an anneal in nitrogen to activate the im-
purity. The resulting semiconductor slice is riddled with threading dislocations
10
–2
originating at the lattice mismatch with sapphire, typically about 10 cm .
However, once it is metal contacted and cleaved or cut into diodes, usually
2
just under 1 mm in area, the LEDs work well with a quantum efficiency
of 10–50%.
8.11.7 Hydride vapour phase epitaxy (HVPE) for nitride devices
The HVPE technique has attracted great interest because it can produce thick
layers at high growth rates and comparatively low cost. The basic reactants are
obtained from the following chemical reactions, where the bracketed (g), (l),
and (s) refer to gas, liquid, and solid phases, respectively.
2Ga(l) + 2HCl(g) = 2Ga Cl(g) + H 2 (g)

and for the higher chloride,
GaCl(l) + 2HCl(g) = GaCl 3 (g) + H 2 (g).