The results of Local Anodic Oxidation (LAO) on the thin GaMnAs layers are reported. The ferromagnetic GaMnAs layers were prepared by low temperature MBE growth in a Veeco Mod Gen II machine. The LAO process was performed with the AFM microscope Smena NT-MDT placed in the sealed box with the controlled humidity in the range 45-80%. The oxide was grown in the semi-contact mode of the AFM. Sample was positively biased with respect to the AFM tip with the bias from 6 to 24 V. The conductive diamond coated AFM tips with the radius 30 nm were utilized for the oxidation. The tip speed during the oxidation was changed from 400 nm/s to 1.5 μm/s. The tip force was also changed during the oxidation. The height of oxide nanolines increases with applied voltage from 3 to 18 nm. The width of these lines was approximately 100 nm at half of the maximum

Cukr, Miroslav

Jelinek, Michal

Komarnickij, Volodimir

Novak, Vit

Soban, Zbynek

Voves J.

I-Revues

ENS’07 Paris, France, 3-4 December 2007 
 
NANOSTRUCTURES DEFINED BY THE LOCAL OXIDATION  
OF FERROMAGNETIC GaMnAs LAYER 
 
 J. Voves, Z. Šobáň, M. Jelínek, V. Komarnickij 
 
Department of Microelectronics, The Czech Technical University FEE Prague,  
Technická 2, CZ-166 27 Praha 6, Czech Republic 
 
M. Cukr, V. Novák 
 
Department of Surfaces and Interfaces, The Institute of Physics of the ASCR v.v.i.,  
Cukrovarnická 10, CZ-162 53 Praha 6, Czech Republic 
 
 
ABSTRACT 
 
The results of Local Anodic Oxidation (LAO) on the thin 
GaMnAs layers are reported. The ferromagnetic GaMnAs 
layers were prepared by low temperature MBE growth in 
a Veeco Mod Gen II machine. The LAO process was 
performed with the AFM microscope Smena NT-MDT 
placed in the sealed box with the controlled humidity in 
the range 45-80%. The oxide was grown in the semi-
contact mode of the AFM. Sample was positively biased 
with respect to the AFM tip with the bias from 6 to 24 V. 
The conductive diamond coated AFM tips with the radius 
30 nm were utilized for the oxidation. The tip speed 
during the oxidation was changed from 400 nm/s to 
1.5 µm/s. The tip force was also changed during the 
oxidation. The height of oxide nanolines increases with 
applied voltage from 3 to 18 nm. The width of these lines 
was approximately 100 nm at half of the maximum.  
 
1. INTRODUCTION 
 
The fabrication and study of nanoelectronic devices 
demands the modification of metals and semiconductors 
on the nanometer length scale. Key elements for more 
complex structures such as transistors are narrow 
conducting wires and tunneling barriers. On the 
nanometer scale proximal probe based instruments, in 
particular the scanning tunneling microscope (STM) and 
atomic force microscope (AFM), have proven successful 
not only to image surfaces but also to modify them. AFM 
is well-known instrument used for surface topography 
measurements. Principle of the method lies in monitoring  
 
substrate
tip
water
oxide
 
 
Fig. 1    Local anodic oxidation by the AFM tip 
 
of the tip motion above the inspected surface. The tip is 
fixed on the end of elastic cantilever, which is deflected 
by attractive and repulsive forces between the surface and 
the tip. The deflection of the cantilever is optically 
scanned. Various approaches have been used to generate 
patterns by means of the AFM useful for device 
fabrication: mechanically modifying a resist layer that 
then acts as an etch mask for pattern transfer, local anodic 
oxidation and subsequent use of the oxide as the etch 
mask, and maskless techniques, i.e., those directly 
affecting the material’s electron system by the proximal 
probe itself. This can be done again mechanically or by 
using the probe for locally inducing chemical reactions. 
Early work on STM and contact AFM induced oxidation 
of Si was done by Dagata [1]. If the substrate surface and 
the tip are electrically conductive and the tip is biased 
negatively to the surface, local oxidation proceeds under 
the tip (see Fig.1). With this method it’s possible to 
fabricate oxide lines of any shape. Height of these 
© EDA Publishing/ENS 2007 ISBN :978-2-35500-003-4     85   
J. Voves, Z. Šobáň, M. Jelínek, V. Komarnicki , M. Cukr, V. Novák 
Nanostructures defined by the local oxidation of ferromagnetic GaMnAs layer 
formations reaches several nanometers and their width at 
half of the maximum is about 100 nm. Oxide lines can be 
written to form desired patterns on the surface by 
translating the tip in a controlled fashion. Several 
research groups have demonstrated the viability of this 
technique in both contact and tapping AFM modes. 
However, when LAO is performed in contact mode, the 
tip apex tends to degrade readily since the tip stays in 
physical contact with the surface. This subsequently 
shortens the life-time of the tip and sacrifices the 
resolution of oxide patterns. For these reasons, the non-
contact/tapping AFM mode is the most preferred to 
perform LAO. The AFM tip oxidizes the surface and 
forms an energy barrier for electrons in the metal or in the 
heterostructure. The LAO patterning is broadly reported 
on the metal (Ti) [2], Si [3] and GaAs layers [4,5]. 
Unsuccessful attempt has been realized with LAO on the 
GaN layer [6]. 
This method could be advantageous especially 
for the laboratory experiments. The sample surface can be 
examined before the oxidation. Consequently it’s possible 
to create oxide lines on the selected location. Oxidation 
results can be immediately investigated. In this work, a 
conducting AFM tip is used in tapping mode to oxidize 
GaMnAs surface producing nanometric constrictions in 
the thin ferromagnetic layer. 
Several spin-valve and spin-filtering effects have 
been reported in the ferromagnetic nanostructures with 
the domain-wall pinning on the nanoconstrictions. The 
giant magnetoresistance (GMR) up to several hundred % 
is produced by the opposite magnetisation of adjacent 
domain walls, the tunnel magnetoresistance (TMR) 
appears, when the constrictions are very narrow (sub 10 
nm), producing up to several thousand % [7]. Tunneling 
anisotropic magnetoresistance (TAMR) is present in the 
nanoconstricted structures with the strong spin-orbit 
coupling [8]. Application of the external magnetic field in 
the different directions produces both positive and 
negative MR. Most of nanoconstricted ferromagnetic 
structures are based on the GaMnAs layers patterned by 
the electron beam lithography. The LAO by AFM is 
attracting attention because of its relatively low cost and 
high resolution. Very few reports are presented about the 
LAO on the ferromagnetic GaMnAs layers. 
 
2. EXPERIMENTAL 
 
We present some results of our analysis of the LAO 
ability to produce lateral nanostructures in ferromagnetic 
semiconductor layers, namely in the thin GaMnAs layers 
grown by the Molecular Beam Epitaxy (MBE). 
 
 
 
Fig. 2  3D picture of the oxide lines prepared by LAO in the 
GaMnAs layer using semicontact mode. Set point was lowered 
to 40 % of original value, increasing the force applied on the tip. 
Humidity was 75%. Tip velocity during LAO was 400 nm/s. 
Negative bias applied on the tip was 5, 6, 7, 8 and 9 V  (from 
the left) for bottom lines and 10 V  for upper lines, where the tip 
force was varied without significant effect. 
 
 
2.1 MBE Grown GaMnAs Layers 
 
The layers were prepared by the low temperature (LT) 
MBE growth in a Veeco Mod Gen II machine using As4 
beam. The oxidation was performed on several samples. 
The first structure consisted of a conventional high 
temperature GaAs layer of about 200 nm thick followed 
by 5 nm LT GaAs layer and 10 nm GaMnAs layer grown 
at 200° C. The surface was atomically smooth with (1x2) 
reconstruction. The resulting Ga1-xMnxAs layer with xMn 
= 7 % had TC of  about 55 K (as grown) and about 150 K 
after annealing in air at 200°C for 2 hours. After 
annealing the layer was conducting with the conductivity 
200 Ω-1cm-1. The second sample consists of 200 nm GaAs 
buffer layer followed by 4.5 nm of LT GaAs layer and 
50 nm of GaMnAs.  
 
© EDA Publishing/ENS 2007 ISBN :978-2-35500-003-4     86   
J. Voves, Z. Šobáň, M. Jelínek, V. Komarnicki , M. Cukr, V. Novák 
Nanostructures defined by the local oxidation of ferromagnetic GaMnAs layer 
 
 
Fig. 3.  The AFM picture of three oxide lines after etching in the 
HCl (upper) and the 1D profile along the white line (bottom). 
 
 
Fig. 4.  1D profile of  four oxide lines prepared by LAO in 
GaMnAs layer with the external voltage source. Set point was 
lowered to 40 % of original value. Humidity was 75%. Tip 
velocity during LAO was 400 nm/s. Negative bias applied on 
the tip was 12, 16, 20 and 24 V. 3D picture with white 1D 
section line is in the insert. 
 
2.2 LAO Parameters and Results 
 
Several parameters are required to be set correctly for the 
preparation of homogenous and sufficiently high oxide 
lines. Ambient humidity is very important for optimal 
oxidation process [9]. The water condensates in the 
strong electrical field (1010 Vm-1) between the tip and the 
sample. Due to capillary forces condensated water creates  
 
 
Fig. 5. The Corbino ring with the diameter 200 nm produced by 
two LAO circles. 
 
 
  
 
Fig. 6. The 1D constriction realized by two LAO lines in the 
centre of Hall bar. 
 
a water junction, which sequentially dissociates and acts 
as electrolyte for following oxidation. The LAO process 
was performed with the AFM microscope Smena NT-
MDT placed in the sealed box with the controlled 
humidity in the range 45 – 80%. The oxide was grown in 
the semi-contact mode of the AFM. Sample was 
positively biased with respect to the AFM tip with the 
bias from 6 to 24 V. The conductive diamondcoated AFM 
tips with the radius 30 nm were used for the oxidation.  
The tip speed was changed from 200 nm/s to 2 µm/s 
during the oxidation. We used external voltage source to 
increase the applied bias above 10V. In the Fig. 2 the 
oxide lines prepared by LAO using internal voltage 
source are shown. Set point was lowered to 40 % of 
original value, increasing the force applied on the tip. 
Humidity was kept at 75%. Tip velocity during LAO was 
400 nm/s. Negative bias applied on the tip was 5, 6, 7, 8 
and 9 V  for bottom lines and 10 V  for upper lines, where 
the tip force was varied without significant effect.  
© EDA Publishing/ENS 2007 ISBN :978-2-35500-003-4     87   
J. Voves, Z. Šobáň, M. Jelínek, V. Komarnicki , M. Cukr, V. Novák 
Nanostructures defined by the local oxidation of ferromagnetic GaMnAs layer 
 
 
Fig. 7. AFM picture of the Hall bar centre with the 20 µm width. 
 
 
One sample was etched in the 3% HCl after the oxidation. 
Resulting profile shows, that the oxide depth in the 
susbstrate is approximately the same as the line height 
above the surface (Fig. 3). In the Fig. 4 four oxide lines 
are shown prepared by LAO with the external voltage 
source. Set point was lowered to 20 % of original value 
on the second sample. Humidity was 50%. Tip velocity 
during LAO was 500 nm/s. Negative bias applied on the 
tip was 12, 16, 20 and 24 V. The maximum height of the 
line is reached for the bias 24 V (18 nm). Different 
optimal oxidation speed of 400nm/s and ambient 
humidity of 75% have been found for the each sample. 
No significant change in the line height and homogenity 
has been observed by the tip speed up to 1 µm/s.  
 
3. SIMULATION 
 
The micromagnetic modeling can predict the magnetic 
behavior of materials on a short length scales of the order 
of nanometers. Individual magnetic moments will want to 
align with an external field, until the local energy 
minimum is reached. If the external field changes, the 
moments will rotate and the domain patterns change and 
move to a new configuration, corresponding to the new 
local energy minimum. The main focus of 
micromagnetism is to study the magnetization reversal 
process and the complex domain configurations, 
especially in thin films. 
 
  
 
Fig. 8.  The quantum dot produced by three interrupted lines 
with 38 nm spacing (dotted white lines). Simulated structure is 
defined by the white rectangle. 
 
3.1 Micromagnetic Simulator 
 
The micromagnetic Simulator SimulMag [10] is an easy-
to-use PC-based magnetic design tool. The simulator 
allows one to simulate the system response of a magnetic 
device or circuit under the influence of external magnetic 
fields, currents, or local field sources. The analysis is 
based on a collection of single-domain elements whose 
size and position can be specified by the designer. The 
elements may be magnetic or nonmagnetic, a conductor, 
or an insulator. The element’s magnetic properties, such 
as magnetization, anisotropy, pinning fields, and 
resistivity must be specified or selected from a library of 
materials. Electrical circuits can be specified and currents 
applied. The elements interact through magnetostatic 
interactions and user specified exchange interactions. The 
elements are arranged in groups to allow different 
structures to be built up and manipulated independently. 
Relative motion between the groups can be specified. 
 
3.2 Simulated Structure 
 
Three-dimensional nanostructure with two 
constrictions was approximated by the rectangle shaped 
oxide regions (rectangle in Fig. 8) constricting thin 
GaMnAs layer on the semi-insulating GaAs buffer. 
Analysis of the structure in the external magnetic field 
with different orientation was performed by means of 
micromagnetic simulation software SimulMag. This 
simulator minimizes the Gibbs free energy of the 
micromagnetic system and uses semi-empirical models 
for GMR and AMR calculations. Input structure for the  
© EDA Publishing/ENS 2007 ISBN :978-2-35500-003-4     88   
J. Voves, Z. Šobáň, M. Jelínek, V. Komarnicki , M. Cukr, V. Novák 
Nanostructures defined by the local oxidation of ferromagnetic GaMnAs layer 
 
 
Fig. 9    The input structure for the micromagnetic simulation. 
 
Fig. 10  Results of micromagnetic simulation by the external 
magnetic field. The field was changed from 30 mT to -30 mT 
and back. Resulting magnetisation of the sample shows different 
levels of hysteresis with different in-plane field orientations 
(magnetic field B paralell to the current J and with the angle 45˚ 
between B and J). 
 
Fig. 11   Qualitative semi-empirical values of GMR with the 
same magnetic field orientations as in the Fig. 9. 
 
simulation is shown in the Fig. 9. Magnetic easy axis was 
oriented along the z direction. The field was changed 
 
 
Fig. 12   Oxide line height vs. voltage applied on the AFM tip. 
 
from 30 mT to -30 mT and back. We performed the static 
calculation of the structure magnetisation. Saturated 
magnetisation of the central quantum dot was chosen a 
little higher (40kA/m) than in other regions (20 kA/m) to 
simulate the retarded switching of the central part 
magnetisation during the magnetic field sweep. Resulting 
magnetisation of the sample shows different levels of 
hysteresis with different field orientations (Fig. 10) as 
expected. The hysteresis loops show several 
nonlinearities due to retarded domain switching of the 
central dot by the magnetic field change. These 
nonlinearities produce the sharp GMR changes, as shown 
in the Fig. 11. 
 
4. CONCLUSION 
 
The LAO can be applied to very thin ferromagnetic 
semiconductor layers to define quantum point contacts, 
quantum wires, rings, dots and other devices. We 
analyzed the influence of the different conditions of LAO 
by AFM tip applied on the thin ferromagnetic GaMnAs 
layer. The maximum height of the obtained oxide lines 
was 18 nm by 24 V applied on the layer in respect to the 
AFM tip (Fig. 12). Oxidation speed of 400-500 nm/s and 
ambient humidity of 50-75% have been used. Optimal 
parameters should be found separately for the each 
GaMnAs layer in the dependence of its resistivity and the 
surface quality. Nanostructures with the quantum dot in 
between two constrictions were prepared. The analysis of 
the prepared nanostructure by means of micromagnetic 
simulation software SimulMag was performed. The 
simulation can be used for the qualitative estimation of 
the ferromagnetic nanostructure behavior. Low 
temperature magnetotransport experiments are started for 
the further analysis of LAO patterned ferromagnetic 
nanostructures. 
 
© EDA Publishing/ENS 2007 ISBN :978-2-35500-003-4     89   
J. Voves, Z. Šobáň, M. Jelínek, V. Komarnicki , M. Cukr, V. Novák 
Nanostructures defined by the local oxidation of ferromagnetic GaMnAs layer 
ACKNOWLEDGEMENT 
 
The work was supported by the grant No. 102/06/0381 
GACR, by the grant No. KAN400100652 GA CAS and 
by the grant No. MSM 6840770014 The Ministry of 
Education CR. 
 
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Nanostructures Defined by The Local Oxidation of Ferromagnetic GaMnAs Layer

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