Minority carrier diffusion length measurements in 6H–SiC
Alexander Y. Polyakov, Qiang Li, Sung Wook Huh, and Marek Skowronski
Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh,
Pennsylvania 15213-3890
Olena Lopatiuk and Leonid Chernyak
Physics Department, University of Central Florida, Orlando, Florida 32816
Edward Sanchez
Dow Corning Compound Semiconductor, Midland, Michigan 48686
͑Received 5 August 2004; accepted 1 December 2004; published online 10 February 2005͒
Minority carrier diffusion lengths were measured as a function of temperature and position along the
growth axis of lightly nitrogen doped boules of 6H–SiC grown by the physical vapor transport
technique. It is shown that the diffusion lengths increase from 1 to 2 microns in the seed portion of
the boule to about 4 microns in the tail portion of the boules. Deep levels transient spectroscopy
measurements revealed the presence of deep electron traps with the activation energies of 0.35 eV,
0.5 eV, 0.65 eV, and 1 eV. The densities of all these traps decrease when moving from seed to tail
of the boules. A good correlation between the change of the lifetime values and the density of the
0.65 eV and 1 eV electron traps was observed. The measured lifetimes show an increase with
temperature following a power law that suggests that the hole capture could be determined by
cascade capture process. © 2005 American Institute of Physics. ͓DOI: 10.1063/1.1853501͔
I. INTRODUCTION
Silicon carbide is an important semiconductor material
with a variety of applications in high-power/high-
temperature electronics.
1
For many applications, such as
power p-i-n rectifiers, power thyristors, bipolar transistors,
the control of minority carriers lifetimes, and diffusion
lengths is of utmost importance. However, at present the un-
derstanding of the recombination mechanisms in SiC is in-
complete.
The techniques most widely used to determine the life-
time values in SiC include the measurements of the switch-
ing characteristics of the SiC diodes,
2–5
studying their
current–voltage characteristics,
2–5
measurements of the re-
laxation time of photoluminescence or photoconductivity.
6–9
Diffusion lengths were most commonly calculated by fitting
the dependence of the electron beam induced current ͑EBIC͒
signal on electron energy in SiC Schottky diodes and p-n
junctions.
9,10
In Refs. 9 and 10 a variant of this method al-
lowing local diffusion length measurements near defect sites
was described. Also, Anikin et al.
5
measured the diffusion
lengths from the bias dependence of Schottky diodes photo-
current taking into account the known absorption coefficient
of the used light.
The results of the measurements show a strong scatter,
but some trends are consistent. It appears that for samples
grown by the same technique the lifetime values are consid-
erably higher for the 4H polytype than for the 6H
polytype.
6–8
Also, the lifetimes measured in good quality
thick films prepared by chemical vapor deposition ͑CVD͒ are
by at least an order of magnitude higher than in bulk crystals
grown by physical vapor transport ͑PVT͒ technique.
5–9
The lifetimes of holes in n-type material were observed
to increase with temperature for temperatures above
300 K.
5–8
The diffusion lengths of minority carriers were
shown to decrease dramatically near dislocations and
micropipes.
9,10
The highest room temperature hole lifetimes
reported were close to 1
s for CVD grown epitaxial layers
of 4H–SiC and about 0.4
s for 6H–SiC.
6–8
For bulk crystals
the hole lifetimes were usually lower, from several nanosec-
onds to some tens of nanoseconds.
4,5,9,10
The measured dif-
fusion lengths of holes in n-SiC vary from below 1
mto
about 10
m.
2,9,10
For p-type samples the lifetimes of elec-
trons are usually quite short, close to 1 ns, and the diffusion
lengths are close to 1
m.
10
The nature of recombination centers in SiC crystals is
not well understood. There is general agreement that recom-
bination proceeds via deep centers in the gap. It has been
observed that, for CVD grown material, the lifetimes de-
crease with increased nitrogen concentration and with in-
creased density of compensating shallow acceptors suggest-
ing that the formation of deep recombination centers could
be facilitated by increased concentration of shallow
dopants.
6–8
The correlation of lifetime data and deep levels spectra
͓measured by deep levels transient spectroscopy ͑DLTS͔͒ is
not yet complete. Anikin et al.
5
suggested that the dominant
recombination centers in 6H–SiC crystals grown by either
PVT or container-free liquid phase epitaxy could be the traps
located near 0.4 eV and 1.3 eV from the conduction band
edge. Combined DLTS and lifetime measurements on CVD
grown 4H–SiC epilayers indicate an existence of a correla-
tion between the lifetime values observed and the density of
deep electron traps located at 0.65 eV ͑Refs. 11 and 12͒ and
1.6 eV below the conduction band.
11
However, despite all the previous work it seems that
much still has to be done to improve our understanding and
control of recombination processes in SiC. For example, it
would be very interesting to monitor changes in the minority
JOURNAL OF APPLIED PHYSICS 97, 053703 ͑2005͒
0021-8979/2005/97͑5͒/053703/6/$22.50 © 2005 American Institute of Physics97, 053703-1