1. Comparative study of the properties of ZnO
layers grown by MBE and RF sputtering
techniques
BY
MUHAMMAD FAISAL MALIK
DEPPARTMENT OF PHYSICS
The Islamia University of Bahawalpur

2. Properties of ZnO
 ZnO is a wide band gap
semiconductor, band gap (3.37 eV).
 High exciton binding energy of 60
meV
 Having the ability to remain stable in
harsh environment.
 Have High thermal conductivity.
 Have High breakdown voltage.
 Have high radiation tolerance

3. Application of ZnO
solar cells
transparent conducting films
Sensors
Varistors
light emitting diodes
 laser diodes

4. Deposition Techniques
The growth technique played a significant role in
controlling the properties of ZnO films, because
the same material deposited by two different
techniques, usually had different physical
properties. This was due to the fact that the
electrical and optical properties of the films
strongly depended on the structure, morphology
and nature of impurities present. Moreover the
films grown using any particular technique might
have different properties due to the variation of
the deposition parameters and hence the
properties can be tailored by controlling the
deposition parameters. It was, therefore,
important and necessary to make a detailed
investigation on the different techniques used for
the deposition of ZnO films.

5. Molecular beam epitaxy
Molecular beam epitaxy (MBE) is a method of growing
epitaxial films first pioneered in the 1970’s[20]. Growth is
carried out in an ultra high vacuum environment, which
minimises the potential for contamination of films. Growth
occurs when diffuse molecules or atoms from a material
source (effusion cell) are directed at a substrate,
MBE is capable of layer-by-layer growth with excellent
control of the purity and crystalline quality of the resulting
film. The main advantage of molecular-beam epitaxy (MBE)
is its precise control over the deposition parameters. With
the feedback from reflection high-energy electron diffraction
(RHEED), the growth mode of ZnO epilayer can be
monitored in real time dynamically. For ZnO thin-film
deposition by MBE, Zn metal and O2 are usually used as
the source materials. MBE grown ZnO films can vary
considerably in surface morphology, displaying columnar
growth, hexagonal island formation, “brain” like structures.

6. RF sputtering
Sputtering is a technology in which the
material is released from the source at much
lower temperature than evaporation. The
substrate is placed in a vacuum chamber with
the source material, named a target, and an
inert gas (such as argon) is introduced at low
pressure. A gas plasma is struck using an RF
power source, causing the gas to become
ionized. The ions are accelerated towards the
surface of the target, causing atoms of the
source material to break off from the target in
vapour form and condense on all surfaces
including the substrate.

7. Current- Voltage
Measurement
The dominant current transport mechanism in the Schottky diode is thermal
emission of majority carriers over the potential barriers between the metal
and the semiconductor. A quantitative analysis shows that the ideal current-
voltage (I–V) characteristics have the form:
)1(  nKT
qV
s eII
Where n is the ideality factor and Is, is the reverse saturation
current
Where
KT
q
s eTAAI

 2*
Where A is the diode area, A* is the effective Richardson constant
(32 A K-2 cm-2) ; q is the charge on an electron, k is the Boltzmanns
constant

8. Deep Level Transient
Spectroscopy
DLTS measurements have been
widely used in the study of deep levels
in materials and devices. The tool has
been used to assign ‘thermal
signatures’ to certain deep levels,
characterized by an activation energy
and its corresponding capture cross-
sectional area. It is also used in
comparing how these ‘thermal
signatures’ vary when processing or
growth conditions change.

9. Deep Level Transient
Spectrometer
DLS- 83 D

10. DLTS: principle of operation
DLTS uses capacitance transient signals resulting from relaxation processes
following an abrupt change of bias voltage or light applied to the sample being
investigated.

11. Summary about DLTS
A standard protocol, described below briefly,
was adopted for DLTS measurements:
 Acquiring/preparation of samples: Sb-Ge
samples were acquired with essential details
about samples.
 Fabrication and I-V characterization of
Schottky contacts to ensure its good I-V
characteristics or low leakage current.
 C-V characterization of Schottky barriers to
study voltage dependence capacitance
response of diodes.
 DLTS scans for analysis of samples.

12. Photoluminescence
Spectroscopy
In this experiment, the energy levels in a
semiconductor quantum well structure
are investigated using the technique of
photoluminescence (PL). A laser is used
to photoexcite electrons in a
semiconductor and when they
spontaneously de-excite they emit
luminescence. The luminescence is
analyzed with a spectrometer and the
peaks in the spectra represent a direct
measure of the energy levelsin the

13. Facilities
 In semiconductor lab (IUB) the following characterization
techniques are available
 Burker Tensor 27 (for Fourier transform infrared
spectroscopy)
 Hitachi Model S-3000H (Scanning electron microscopy)
 Keithley 6487 Picoammeter/voltage source (for current-
voltage)
 7200 Boonton capacitance meter ( for capacitance- voltage
measurement)
 DLS-83D deep level spectrometer, Hungary (for deep level
transient spectroscopy)
 4285A LCR meter ( 75kHz- 30MHz) (for capacitance voltage
measurement at different levels and frequencies)
 Thermal evaporation coating unit
 Raman and PL

14. Collaboration
 In semiconductor lab (IUB), the facilities of any growth
technique and optical measurements are not available due to
lack of funds. In order to overcxome the problem the
semiconductor lab is collaborated with other universities.
 The collaborators are listed below
 Prof. Dr. M. Willander (ITN, Linköping University, Campus
Norrköping, Norrköping Sweden)
 Prtof. Dr. Q. Wahab (IFM, Linköping University, Linköping
Sweden)
 Prof. Dr.M-A Hassan (Department of Electrical and computer
Engineering and the Center for optoelectronic and optical
communication, University of North Carolina Charlotte, USA)
 Prof. Dr. Shehzad Naseem (MERC, University of the Punjab,
Lahore Pakistan)
 Dr. A. Mahmood (National Institute of Optoelectronics of
Pakistan, Islamabad, Pakistan)
 Dr. A. S. Bhatti (Department of Physics, COMSAT Institute of
Technology, Islamabad, Pakistan)

15. Present Study
In this study we will grow the ZnO wafers
by MBE and RF sputtering technique
and then we will characterize it optically
(SEM, XRD, and PL) and electrically (I-V,
C-V, and DLTS). These characterization
techniques will give us the structural
study as well as electrical and optical
characterization. This will help us to
characterize growth related defects in
ZnO. After this study we will be able to
achieve the following goals and
objectives.

16. Expected Goals and Objective
 To realize the potential of high performance ZnO
based optical and electronic devices.
 It will help in understanding the deep levels in
ZnO, so the long life and high performance
devices can be achieved
 Correlation between the main technological
parameters and material properties.
 By the reduction in cost of ZnO based devices,
eventually industry and common man will get
benefit from these devices.
 The better understanding of characterization of
defects in ZnO helps to increase the efficiency of
solar cells, transparent electrodes and blue/UV
light emitting devices.