Very low density two-dimensional hole gas in an inverted GaAs/AlAs
interface
Y. Hanein, Hadas Shtrikman, and U. Meirav
a)
Braun Center for Submicron Research, Weizmann Institute of Science, Rehovot 76100, Israel
Received 4 October 1996; accepted for publication 8 January 1997
We utilize an inverted heterostructure grown on 311A GaAs to realize a two-dimensional hole gas
2DHG with a built-in back gate. The density of the 2DHG is easily and reproducibly varied
between 5 10
9
and 5 10
11
cm
-2
. The mobility of the 2DHG is highly anisotropic in the 311A
plane. © 1997 American Institute of Physics. S0003-69519701311-9
Two-dimensional electron or hole gas 2DEG and
2DHG, respectively structures of AlGaAs/GaAs are widely
used for the study of physics of low dimensional electronic
systems and quantum transport. A particularly versatile real-
ization of 2DEGs is the inverted-semiconductor-insulator-
semiconductor ISIS structure,
1
where the carriers are accu-
mulated in an undoped GaAs layer on top of an undoped
AlGaAs barrier thus ‘‘inverted’’, grown over an n
+
con-
ducting layer. In ISIS devices, the sheet carrier concentration
can easily be modulated by the underlying conductor layer
and by surface Schottky gates, thereby increasing the range
of possible measurements and allowing patterned gate struc-
tures on the surface while having additional and separate
control of n by means of the back gate.
In this work, we study ISIS structures grown on 311A
oriented GaAs substrates where a 2DHG is formed in an
analogous fashion. This p-ISIS structure allows us to vary
the sheet hole density p over a wide range, and in particular
to achieve and to measure extremely low densities.
In conventional modulation doped 2DEGs 2DHGs,
placing the donors acceptors far from the channel generally
leads to improved mobility, particularly at low densities. In-
deed, very low density 2DEGs with high mobility have been
realized using spacers of order 300 nm.
2
In an ISIS structure,
since the carriers are generated by field effect rather than by
modulation doping, the spacing between the channel and any
intentional doping can be increased at will. Furthermore, in-
creasing the depth of the channel below the surface does not
lead to major difficulty in the formation of Ohmic contacts,
due to the absence of an AlGaAs barrier between the surface
and the channel. For this reason, the ISIS is an attractive
device for the realization of high quality, low density 2DHG
systems. Given that the carriers are generated by a field ef-
fect, one might ask why 311A substrates are at all neces-
sary for accumulating a 2DHG in a p-ISIS. The answer lies
in the pinning of the surface potential and the resulting need
for a p-type cap layer which, although depleted by the sur-
face states, has a crucial role in bringing the valence band
close to the Fermi level. This point will be elaborated below.
Work on 100 n-type ISIS structures
1,3,4
failed to match
the high mobility of conventional heterostructure 2DEGs at
the low densities. The inferior mobility was attributed to the
relatively poor quality of the interface due to the incorpora-
tion of background impurities which tend to ride towards the
surface and accumulate at the inverted interface, as well as
enhanced interface roughness. However, during molecular
beam epitaxy MBE growth on the 311A plane, there is a
reduction in background impurity incorporation, particularly
carbon,
5
in comparison to growth on the more common 100
plane. Moreover, the height of single monolayer fluctuations
in the 311A direction is smaller due to the tilt angle with
respect to the cubic axes 25°. Both effects should give rise
to a superior inverted interface and thus to a better quality of
the 2DHG in 311A ISIS devices.
Si is invariably the n-type dopant used to produce high
mobility 2DEG in 100 GaAs. Nevertheless, Si is an ampho-
teric impurity in GaAs, and it is well established that it can
be used either as a p - or an n-type dopant on 311A GaAs
substrates, depending on the MBE growth conditions.
6,7
This
method has already been successfully utilized in producing
various hole devices, such as high mobility 2DHGs
8,9
and
p-channel transistors.
10
Figure 1 shows a schematic layer profile of the p-ISIS
structures studied. The samples were MBE grown on semi-
insulating 311A substrates, oriented to within 0.1 degree,
using Si as the p-type dopant. Growth was therefore carried
out at a relatively high temperature of 640 °C and low
As
4
/Ga flux ratio of 4. The first layer grown was a 1
micron thick p
+
GaAs buffer. As indicated above, this layer
serves as a built-in back gate. Next a 300 nm barrier layer,
essentially of AlAs, was grown, followed by a 150 nm un-
a
Electronic mail: hmeirav@wis.weizmann.ac.il
FIG. 1. Schematic presentation of the p-ISIS structure grown on semi-
insulating 311A GaAs substrates, consisting of a thick p
+
buffer, a 300
nm undoped AlAs barrier, a 150 nm undoped GaAs channel layer, and a top
50 nm GaAs layer which is p doped. The 2DHG forms at the lower in-
verted interface of the channel layer upon application of a negative bias to
the buffer.
1426 Appl. Phys. Lett. 70 (11), 17 March 1997 0003-6951/97/70(11)/1426/3/$10.00 © 1997 American Institute of Physics