Characterization of Thin Films   Modern techniques employed in the characterization of electronic thin films and devices Thickness - Structure and Morphology surface bulk irrespective of ultimate applications - chemical composition surface bulk

1.  Film Thickness 1.1  Optical methods Interference of two or more beams of light whose optical path difference is related to film thickness Optical Mechanical

Fizeau fringes of equal thickness (FET)

Quartz crystal oscillation Deposition rate Water-cooled shroud 1 Hz corresponds to 1.24x10-8 g of Al (0.46Å) for f=6MHz and A=1cm2

2.  Structural characterization TEM, SEM, XRD, STM, AFM, LEED, RHEED 3.  chemical characterization

Electron Spectroscopy – (EDX), AES, XPS

X-ray emission Auger electron energy

AES & XPS spectrometer

Quantification by AES XPS (Angle resolved) Determination of chemical state by chemical shifts of XPS - no damage on sample surface - angle resolved XPS h e-  I0 x

XPS Surface of AlGaAs etched in a CF2Cl2 + O2 plasma

Rutherford Backscattering Spectrometer (RBS) From energy & momentum conservation : E0, M0: incident ion E1: after elastic scattering M: surface atom : scattering angle  = 170o Kinematic factor  depends just on the atomic weight of target atom Kpt=0.922 Ksi=0.565

Peak height (H) is directly proportional to atomic concentration Peak width (E) is directly proportional to film thickness The concentration ratio From figures

Measurement of Film Resistivity J = nq J : electric current density g : charge  : velocity  =   : mobility  : electric field  J = nq by Ohm’s law (J = ) : conductivity  : resistivity Film resistance R = /wd

If  = w “sheet resistance” with units of “ohms per square” Ohms/ Lithographic patterning method To avoid contact resistance problem 4 points probe method for sheet resistance van der Pauw method For resistivity

sheet resistance by 4-point probe method: Resistivity : van der Pauw method for arbitrary points Hall measurement : resistivity, carrier concentration, mobility

The electric field in the y-direction produces the Hall voltage VH

The Hall coefficient RH is defined as Similarly, for n-type samples The Hall mobility is defined as Resistivity

Photoluminescence (PL) - One of the most sensitive nondestructive methods Characterization of the centers responsible for shallow donors and acceptor species which control the electrical properties and luminescence Low temp. PL reduces the spectral broadening due to vibronic processes

DA: donor acceptor pairs CA: conduction band to acceptor X: free excitons D0X: excitons bound to neutral donors A0X: excitons bound to neutral acceptors

Study of Interface roughness by PL Exciton is used as probe dBohr > Ls dBohr < Ls