|Advisor:||Paul R. Berger|
|Students:||Wei Gao (graduated with Ph.D. in September 1995)|
|Collaborators:||Matthew H. Ervin (Fort Monmouth Army Research Lab)|
|Jagadeesh Pamulapati (Fort Monmouth Army Research Lab)|
|Richard T. Lareau (Fort Monmouth Army Research Lab)|
|Stephen Schauer (Motorola)|
Importance of the Problem:
The In0.53Ga0.47As material system is very important for
modulation-doped field-effect transistors (MODFET) used in monolithic
microwave integrated circuits (MMIC) and avalanche photodiodes (APD),
p-i-n photodiodes and metal-semiconductor-metal (MSM) photodiodes used
in long wavelength (1.3 m
lightwave communications. For these applications, the
In0.53Ga0.47As material needs to be of high quality
and low carrier concentration. Liquid phase epitaxy (LPE) is a well
developed and proven technique to grow compound semiconductor materials.
High quality layers are possible with LPE with a minimum of capital
investment when compared to alternative techniques
such as molecular beam epitaxy (MBE) and metalorganic chemical vapor
deposition (MOCVD). LPE inherently provides high quality material
since growth is at reduced temperatures and closer to chemical
equilibrium, thus achieving near perfect crystallinity and purity.
LPE remains a viable option for the production of optoelectronic
materials and devices.
However, one problem with LPE growth is the unavoidable impurities which arise
from the melt and growth ambient. It has
now been recognized that Si and S are the two major background donor
impurity species present in the In and InP, and that C is the principal
acceptor, originating from the graphite boat. Various methods
have been used to reduce the concentration of the residual impurities.
Extended hydrogen baking of the In melt and the source solution
(formed by adding InAs and GaAs) is widely known to lower the levels
of both donor (Si and S) and acceptor
(Al, Mg, Zn) impurities in the grown epitaxial layer.
P. K. Bhattacharya et al. showed
electron mobilities significantly improved upon hydrogen baking the
entire melt after the In slug was individually prebaked with hydrogen.
Also, Bagraev et al. used a sapphire boat to
reduce the background doping and further enhance the electron mobility.
But Bagraev et al. also discovered that the addition of
trace amounts of rare earth elements to LPE growth melts could significantly
reduce the background doping level and increase the electron mobility.
Recent studies have shown that impurities (for example, S, Se, Si, C,
Te, O etc.) in III-V semiconductors form stable compounds with rare earth
elements and that these rare earth compounds are insoluble in indium (In),
the common solvent used in LPE growth of InGaAs.
Thus, rare earth elements can be used to remove these impurities from
the InGaAs LPE growth melts and reduce the background doping of grown epilayers.
Also, it was found that the rare earth elements do not incorporate into the
epitaxial layer at reduced growth temperatures, similar to those used in this
study. Thus, the reader is cautioned that the LPE melt is doped with rare
earth elements, but the epitaxial layer is not. The epitaxial layer will
henceforth be referred to as undergoing a rare earth treatment.
One dilemma of this study is the limited purity of commercially available rare
earth elements (see Table IV). This leads to new impurities being
introduced into the grown layers. The first rare earth elements studied
were La and Ba by Haigh, and examples of more recent
investigations of rare earth doped semiconductors concerning defects,
doping, growth, theory and optoelectronics can be found in the Materials
Research Society Proceedings.
Brief Description of Work and Results:
High quality In0.53Ga0.47As epilayers have been grown on
semi-insulating (100) Fe-doped InP substrates. The growths were performed
by liquid phase epitaxy (LPE) using rare earth doped melts in a
graphite boat. The rare earth elements studied were Yb, Gd and Er which act
as gettering agents of impurities. Hall measurements show an elevated
electron mobility for rare earth treated samples over undoped samples,
=11,470 cm2/Vs at 300K and reduced carrier
concentration (n-type), 9.33
cm-3. The Hall results
indicate an improvement in layer quality, but suggests that the treated
layers are compensated. Photoluminescence (PL)
studies show that the layers grown from rare earth doped melts
have higher integrated PL efficiency with narrower PL linewidths than the
undoped melt growths. The grown materials were fully characterized by
Fourier transform infrared spectroscopy, double crystal
x-ray diffraction, energy dispersive
spectroscopy, secondary ion mass spectroscopy and deep level
transient spectroscopy (DLTS). Compositional measurements reveal no
measurable incorporation of rare earth elements into the grown
epilayers. DLTS measurements indicate the creation of two deep levels
with rare earth treatment, which is attributed to either the rare earth
elements or impurities from within the rare earth elements.
Subsequent, glow discharge mass spectrometry measurements
revealed many impurities within the rare earth elements which
preferentially might lead to p-type doping centers and/or deep levels.
Thus, rare earth doping of LPE melts clearly improves epitaxial layer
quality, however, the purity of commercially available rare earth
elements hinders optimal results.
For further information contact:
Nanofabrication and Materials Processing Center (NanoMPC)
Nanoscale Patterning Laboratory
Nanoelectronics and Optoelectronics Laboratory (NOEL)
Polymer Device Laboratory (PDL)
201 Caldwell Laboratory
Department of Electrical and Computer Engineering
The Ohio State University
205 Dreese Laboratory
2015 Neil Avenue
Columbus, OH 43210 USA
Direct phone: (614) 247-6235
EE Dept. FAX: (614) 292-7596
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