Growth and Properties of III-V Nitride Films, Quantum Well Structures and Integrated Heterostructure Devices
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7'T F - -
I II 'I
I I I I I I I I
I I I . . . .'I 'I
. - I . . . .
i-vi >0 r-V
AIN A6
* Ivs MgS
W 4- LiGaO2 Wj
MgSe
Cubic GaN
ZnO
GaN
ZnSe rdS ZnSe GaP O O CdSZ0
( O 2
SiC(6H) t,•
m 1
Lattice Constants
InN10s
Z
AlAsY
3.0
3.5
CdSe
GaAs 0
Basal Plane 2.5
Zne InP
Si 0 4.0
4.5
5.0
LATTICE CONSTANT
5.5
6.0
6.5
(A)
Figure 1. Energy bandgap versus lattice constant for selected semiconductors. 123
Mat. Res. Soc. Symp. Proc. Vol. 395 01996 Materials Research Society
beam epitaxy (MBE) of high quality GaN, AIxGal-xN, and AIN films on GaN/SiC substrates. The GaN films were grown homoepitaxially by rf plasma-assisted MBE on 3 jim thick GaN buffer layers previously grown on 6H-SiC substrates prepared by MOCVD at Cree Research, Inc. By using this homoepitaxial approach, we have circumvented the problems associated with heteroepitaxial nucleation of GaN on highly lattice-mismatched substrates such as sapphire or SiC and have instead concentrated on the issues associated with the MBE growth process itself. We employed radio frequency (rf) plasma-assisted MBE instead of the commonly used ECR plasma-assisted MBE to generate beams of active nitrogen for film growth. Using an rf plasma source, MBE growth of very high quality GaN and AlxGal-xN layers, AIxGal-xN/GaN multiple quantum well (MQW) structures, and blue-violet light emitting diodes based on double-heterostructures of AIxGal-xN/GaN have been achieved at growth rates of -0.3 jim/hr. Recently-measured valence band offsets for Ill-V nitride materials pose fundamental problems for vertical transport devices such as laser diodes. New approaches to deal with vertical transport are discussed. Integrated heterostructures are proposed, which incorporate graded nitride layers to eliminate band offsets, for the development of a variety of vertical transport devices such as light-emitting diodes, laser diodes, photocathodes, electron emitters based on the negative-electron-affinity of AIN, and certain transistor structures. EXPERIMENTAL DETAILS The III-V nitride films were grown using a three-chamber MBE system. The first chamber is equipped with an ECR plasma source for plasma cleaning of substrates using various gas mixtures. The second chamber is equipped for surface analysis of substrates and epilayers at temperature up to 800 0 C using Aiger electron spectroscopy. The third chamber is the main MBE film growth chamber which accepts substrates up to 75 mm in diameter, has provisions for up to nine MBE source ovens, is equipped with reflection high energy electron diffraction (RHEED), and includes an optical pyrometer for measuring substrate temperatures. An Oxford Applied Research MPD21 radio frequency (rf) plasma source was used to generate an active nitrogen flux. The plasma source was operated at nitrogen pressures ranging form 5 x10- 6 to 4 x 10-4 Torr using rf powers of 150 to 400 W. The nitride films synthesized by MBE during this study were grown on 2-3 jim thick highquality GaN buffer layers prepared by MOCVD on basal plane 6H-SiC substrate
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