The survey of dynamic procedures was limited to the scope of Micro seconds in a typical Scanning burrowing microscope.Now with the handiness of ultra-short optical maser pulsations combined usage of scanning burrowing microscopy Ultra short pulsation optical maser has enabled us to analyze ultrafast features of a stuff in an aroused province with ultimate spacial / temporal declaration, stuffs in aroused province exhibit unique belongingss which are frequently in the order of femtoseconds. The declaration of this technique is limited merely to the difference in the pulsation frequences which is in the order of femtosecond 10^-15seconds. The excitement is provided by agencies of short pulsation optical maser stimulation preferable of really high frequence.By analyzing the burrowing current part due to the optical maser stimulation it is possible to derive information about some of the surface and subsurface belongingss of the stuff with really high temporal and spacial declaration
Keywords – Scaning burrowing microscope, FemtoLaser stimulation, Ultrafast procedures
Over the old ages atom use was and its implicit in principals have been discovered, and the atom-manipulation and survey procedure has now become a reality.As early as 1959 Richard Feynman invited scientists to a new field of research with his address “ There is Plenty Of Room at the Bottom ” [ 1 ] : the field was focused on analyzing stuffs at atomic degree and to place a mechanism by which they can be manipulated. By changing the atomic agreement one could change many belongingss and behavior features of the stuff under survey, many jobs can be solved at an atomic degree, therefore giving us a lasting and more stable solution. The innovation of techniques such as scanning burrowing microscope ( STM ) [ 2, 3 ] and the atomic force microscope ( AFM ) [ 4 ] , have made this possible.
The STM works on the rule of Electron tunneling and is preponderantly used to analyze the topography of a metallic stuff at an atomic graduated table. They can besides be used to change the atomic agreement in order to absorb needed belongingss into the
stuff. Invented by Binnig, Rohrer, Gerber, and Weibel [ 2, 3 ] .Fig 1.1 describes the schematics of a STM. The STM is driven utilizing 3 reciprocally perpendicular Piezo electric transducers, it consists of a really crisp tip that scans the surface of the stuff. When a Bias electromotive force is applied a burrowing current is apparatus, the value of the current is amplified and compared with a preset mention value. Harmonizing to this difference the perpendicular place of the tip is varied ( alteration in size of the piezo electric transducer due to difference in current ) . Now when the tip is moved all over the surface in a systematic manner the tip continuously varies due to fluctuation in the tunneling current. the fluctuation in burrowing current is in line with the fluctuation in the distance between the tip and the surface. Therefore this perpendicular motion of the tip is the contour of the surface which is under scanning. This contour is normally displayed in gray graduated table in a computing machine screen or a CRT Fig 1.2a.Usually the image is besides supported with a contour secret plan for farther apprehension. The STM ‘s these yearss are computing machine controlled and have un paralleled truth degrees it besides allows to see the procedure as it happens. The installation to hive away and publish the information is besides available [ 5 ] , schematics in Fig 1.3.
Fig 1.1 -Scanning burrowing Microscope
Fig 1.2 ( a )
Fig 1.2 ( B )
We are now interested in analyzing the usage of this microscope when combined with an amplified femtosecond optical maser stimulation.In this instance we are more concerned in its ability to analyze the burrowing current instead than analyzing the topographical surface. When a part is energized by utilizing an external beginning such a optical maser the stuff is excited at an atomic degree, this influences the flow of charge bearers which in bend alters the burrowing current.The life span of these procedures are in the order of Femtoseconds ( Femtosecond is 1015 of a 2nd that is one or 1/1000th of a picosecond [ 8 ] ) . Along with the excitement a decay procedure is besides inevitable the survey of this procedure besides enables usage to analyze many bombers – topographic belongingss such as bearer kineticss, negatron soaking up, Raman sprinkling of the stuff. The survey of such ultra-fast dynamic procedures is now possible by the combined usage of STM and a femtosecond dual pulse laser stimulation. The dynamic belongingss were found to hold a direct relation with the optical maser frequence. Therefore enabling us to analyze these Ultra-fast procedures
hypertext transfer protocol: //research.ncku.edu.tw/re/articles/e/20080606/images/080519035652ixxP8e.jpg
This Concept of observing the nonlinear response of the burrowing junction ( due to laser stimulation ) straight in the current i¬‚owing through the contact was mentioned by Hamers and Cahill [ 7 ] The tunneling junction is excited by a two optical masers with hold in the order of femtoseconds and the nonlinear tunneling current constituent that depends on their temporal separation can be studied.
The ultimate ends of this method is to supervise dynamic surface procedures, surface topography, electronic surface provinces and defects with a much better spacial and temporal declaration when compared to the other available methods.
The construct behind accomplishing the higher declaration is the multi photon emmisivity of stuff when subjected to pulsed optical maser stimulation. In order to acquire past the negatron bandwidth restriction we use repeated short pulsation of the optical maser straight in the tunneling junction and analyze any non additive behavior that is reflected in the tunneling current.Therefore the electronic response is now straight dependent on the hold between the pump and the investigation optical maser pulses that are delayed in the order of femtoseconds Fig 1.4
This being the instance now the sidelong declaration of the STM is non due to the topography or diffraction, it I due to the non additive constituent of the burrowing current due to the optical maser stimulation. This paves the manner to to analyze the ocular reading of the negatron kineticss when in an aroused province.
Though we find this a straighforward instance. Many restrictions have to be overcome. To get down off with, placing the nonlinear constituent of the burrowing current is a challenge by itself as the current is fast and little, thermic enlargement of the tip can do a immense fluctuation in the tunneling current, nevertheless this is overcome by taking a optical maser frequence which is much faster than the tip thermic response clip. But still the quasi inactive addition in temperature of the system has shown to increase the tip length in the scope of 10-10 m. This by itself requires a close monitoring. The tip requires careful readying in order to guarantee that it scans merely the needed country. Small Variation in Laser Source besides creates a challenge in acquiring accurate stimulation. Beyond all this the experiments have proved to give out information on charge bearer recombination, surface provinces and other atomic graduated table information [ 26,27 ]
The construct can be by experimentation verified. A electrochemically engraved tip was used to scan a clean GaAs sample. Output of an amplified TiSaphire Laser was used to excite the tunneling junction at a rate of 50hz. Michelson inferometer provided the needed hold of 40fs.A short extremum form in the tunneling current was noticed in line with the optical maser frequence. Pulse length was about 50fs in breadth. Due to the short rise and autumn of this current there is no possibility that this is influenced by the dynamic distance accountant of the STM ( Z piezo ) as it has a much greater response clip. The current was besides found non to depend on the tip to come up distance as the same current peaking was observed even when the tip was retracted. The regulating factor for the current extremum was noticed to be the existent optical maser topographic point place. Thus we can reason and corroborate that we are able to analyze the current constituent due to the optical maser stimulation which carries information at an atomic degree [ 28 ] .
Fig 1.5 shows the nonlinear fluctuation of the of the current with regard to the incident optical maser flux, the graph besides indicates that the sprinkling of values reduces with higher frequence of optical maser stimulation. Furthermore the current was besides controlled by the prejudice electromotive force a negative electromotive force reduced flow of current and Vice Versa. This proves that a tunneling constituent of current exists with the laster stimulation
Further to this in order to understand the decay feature of these aroused negatrons a pump and investigation optical maser stimulation is required. Here two short optical maser pulsations that are delayed at a femtosecond degree illuminate the tunneling junction ( for the same GaAs stuff ) and the current is studied against by changing the hold “ The pump excites the negatrons to a hot province and the investigation takes them to a continuum province ” .The below graph represent such an experiment
Note in fig 1.6 that the current ranges lower limit when hold is increased and easy once more reaches upper limit. This gives us information on the Photo negatron dynamica i.e “ the current decrease can either intend that the pump optical maser depletes the charge bearer surface or that the charge bearers are moved to provinces which have less efficient emanation coefficient ” . Furthermore, shorter pulse continuance will increase the comparative strength ( between pump and investigation ) of the optical maser induced tunneling constituent doing it a really desirable status for our survey [ 28 ] .
As we already saw there are many restrictions and barriers such as little magnitude, non-linearity, ultrafast nature to analyze this nonlinear constituent of the burrowing current that carries the needed information.These obstructions have created a figure of characteristics that are required for the apparatus. The three chief divisions of the apparatus are
Femtosecond optical maser system
Scaning Tunneling Microscope
Fig 1.7 depicts the schematics of such a apparatus
The yoke quality in footings of harmonics and mechanics between the systems has a really critical function to play in guaranting the truth of our readings.
Femtosecond optical maser system and quiver isolation
In order to obtain the needed consequence we require a optical maser which has the ability to pulsate at frequence in line with the relaxation clip of
the system which is in the femtosecond scope therefore we require a really speedy pulsation mechanism. This demand is translated into the apparatus by utilizing a focussing lens to direct the optical maser towards the burrowing junction.The positional truth of the optical maser is really critical as we already saw that the burrowing current constituent is straight relative to the optical maser place. “ Normally focal length of 20 centimeter that is positioned outside the Ultra High Vacuum chamber. The angle of incidence of the optical maser beam with regard to the surface normal is about 60 – 80A° . This place of the optical maser focal point on the burrowing tip is controlled by a maneuvering mirror that can be moved parallel to the tip axis utilizing a interlingual rendition phase. Movement of optical maser topographic point across the vertex causes a diffraction form to look, as we move it further down the tip vertex this form alterations. When the optical maser is focused at the junction the diffraction form is least influenced by the tip and by this we guarantee that the optical maser is focused precisely in the junction A focal point diameter of 50 millimeter is typically achieved. The size of the focal point ~ 50 millimeter diameters is sufi¬?ciently big to supervise the place of the optical maser focal point on the tip or on the sample utilizing a binocular microscope with big working distance ” .
The typical optical maser beginning we use is normally the direct end product of a Ti: sapphire femtosecond oscillator or amplii¬?ed laser pulsations after chirped pulse amplii¬?cation. Femtosecond oscillations is produced by a femtosecond oscillator capable of presenting ultra-short optical maser pulsations of 40 fess continuance at a wavelength of 800 nm ~80 MHz repeat rate, 4 NJ pulse energy. The amplii¬?ed optical maser pulsations are so compressed and pulsations of 60 fs continuance and 2-3 mJ energy are obtained at a repeat rate of up to 300 kilohertz. “ Second harmonic coevals in a thin beta-barium borate ~BBO Crystal is used to bring forth short optical maser pulsations at 400 nanometer utilizing either the end product of the femtosecond oscillator or the amplii¬?ed optical maser pulsations. A Mach-Zehnder interferometer generates pump and probe pulsations with variable clip hold ” [ 6 ] .This describes a typical optical maser apparatus that is used for such experiments ( all values are taken from the mention and are for intent of account )
Scaning burrowing microscope
The UHV-STM for the discussed experiment has to full-i¬?ll several demands. First, the first-class quiver isolation so that the concentrating capableness of the optical maser system is non hindered. A 2nd demand is that the STM should supply sufficient infinite to enable optical entree for the optical maser which means that the STM should be able to mensurate the burrowing junction from a larger distance. A 3rd of import demand is the geometry of the burrowing tip. The tip should hold a really crisp vertex so that it picks up merely the needed tunneling current although this can be guaranteed by the tip readying methods, the slow alteration in tip geometry due to the thermic enlargement over a period of uninterrupted pulsating can non be avoided. This alteration is dimension though will be utile while analyzing the topography of the specimen, it leads to error when we are interested in the nonlinear tunneling, Therefore our apparatus should hold a installation to replace the tip and attain the same positional truth every clip we do so. The STM therefore must let a fast and dependable tip replacing process and in “ situ tip word picture ” [ 6 ] .
The STM is extremely sensitive to external perturbations and great attention must be taken to maintain quivers off from the STM to guarantee good declaration under optical maser operation. We use a Short-term memory that is coupled over a “ Viton stack ” ~see Fig. 1.8 this allows us to change muffling belongingss based on the external conditions. It acts as a low base on balls filter for external quivers and cut off frequence Is about 100hz
Now this damping is sufficient when there is no optical maser operation. With the inclusion of the optical maser system we are faced with another job. The optical maser system requires a chilling system to be run in order to maintain the optical maser in a stable status. This system creates quiver during optical maser operation and this hampers the measuring procedure. The chilling system is really powerful and the quivers are of higher magnitude ( normally centered around 40hz ) . This is overcome by adding extra H2O reservoirs and extra-long hosieries.To a big extent these quivers are damped and therefore enable us to hold the needed truth. [ 6 ]
“ Study Of Hot Electron burrowing [ 29 ] ”
Introduction- ” The development of semiconducting material natural philosophies and devices has been come oning with the development of atomically controlled fiction engineerings that enable us to drastically change the belongingss of the stuff “ [ 18-20 ] . “ However, boulder clay day of the month the bearer kineticss in the stuffs have been analyzed merely with techniques that provide spatially and/or temporally
Resolution merely in the scope of microseconds ” [ 28 ] . With the debut of the above processs we can now analyze the behaviour of these bearers on a femtosecond graduated table.Fig 1.9 shows the schematics for such an experimental setup.The pump and investigation pulsation trains are generated by two Synchronized Titanium: sapphire optical masers ( Mira and Chameleon, Coherent, cardinal wavelength = 800aˆ‰nm, repeat rateaˆ‰aˆ‰= 90aˆ‰MHz, pulse width = 140aˆ‰fs ) they were guided to a pulsation chooser, which consisted of Pockels cell and polarizers. The hold clip between the two pulsation trains was continuously varied in the femto and nano scodne scale.The pulsation train repeat rate was set at 1Mhz.The two pulse trains from the two optical maser beginnings were combined on to the same optical apparatus and guided into the STM scanning the specimen. The light topographic point, diameter of was measured to be around 10aˆ‰I?m and an mean strength of up to several mill Watts was recorded. This was focused on a sample surface below the STM tip utilizing lenses placed outside the UHV chamber. The burrowing current signal from the STM preamplifier with a bandwidth ofA ~10aˆ‰kHz was phase-sensitively detected utilizing a lock-in amplifier so that merely the needed constituent was measured. In this application we are interested in analyzing the a Gallium Phosphide surface. GaP being a broad spread semiconducting material turns out to be a really suited stuff for our experiments. With an indirect spread of 2.27 electron volt and a direct set spread of 2.9 electron volt, therefore soaking up of visible radiation at 800 nm ~1.55 electron volt is merely possible via a two-photon
procedure ( 231.55 eV53.1 electron volt ) . For GaP, the intermediate provinces lie in the set spread, but GaP
is known to hold a assortment of surface provinces at these energies with life-times in the picosecond scope. [ 18 ] ( all values are taken from the mention and are for intent of account ) .
The tunnel procedure under light is ini¬‚uenced by the presence of aroused negatrons in the stuff and the strong optical i¬?eld in the signifier of the optical maser. These effects can increase the effectual tunnel chance due to two different mechanisms: coherent exposure assisted tunneling and tunneling of hot negatrons. Therefore If the excitement of negatrons occurs via a two-photon Procedure, the time-dependent current constituent due to the tunneling of these negatrons carries information about the kineticss of the intermediate degree. The life of the negatron in each phase has information which antecedently could ne’er be studied Now a modulated optical maser beginning in the signifier of Pump and investigation pulsations are used to light the junction a bias electromotive force is applied and can be varied as good. On puting up a prejudice electromotive force the burrowing current is generated. The curve of the burrowing current will now incorporate the nonlinear fluctuation due to the pulsating optical maser every bit good.
Fig 2.0 denotes the current fluctuations due to the Piezo scans. We are now able to see both the tunneling current due to the topography, normally denoted by the bigger extremums and the smaller extremums that denote the fluctuation due to the optical maser emanation. These extremums have a unvarying tallness and therefore are non caused by some instability in the junction.Therefore we can reason by stating that these little negligible extremums induced by the optical maser stimulation represent the local belongingss of the Charge bearers [ 28 ] .
Band Structure probe [ 9 ] ”
The technique was besides used to analyze the set construction of a cleaved LT-GaAs/AlGaAs and AlGaAs/GaAs interfaces. The current was studied for the illuminated an non lighted conditions as shown in Fig 2. By outlining the graphs between the prejudice electromotive force and the burrowing current we were able to analyse the charge bearers for each junction. Fig 2.1 denotes the assorted graphs that were drafted from the experiment
Fig 2 schematics of the experimental apparatus
Fig 2.1 end product graphs of the experiment
Results – As already disclosed the consequences of this experiment is greatly utile in analyzing the semiconducting material composing such as bulk, minority bearers, drosss, concentration of negatrons and holes. The flow of current against the bias electromotive force was studied. The relation between the current magnitude and the bias electromotive force way could state us what was the charge carries that was bring oning the burrowing current. For illustration we could notice from the graph we could see that for non-illumination that the burrowing current does non flux unless a prejudice electromotive force much larger that the set spread is applied this indicates that no drosss are present in our sample. We could besides see that the onset electromotive force is negative and positive for different beds. Negative oncoming indicates the presence of negatrons and positive oncoming indicates the presence of holes as charge bearers. Further the fluctuation of the current can besides be studied in lighted status. It is normally found to be higher in magnitude and the onset electromotive force helps us to place whether the bed has “ n ” type or “ p ” type behavior. Further we can analyze the burrowing current with regard to disintegrate clip, this gives us a whole new set of information and avenues to experiment on the nature of flow of the charge bearers in the semiconducting material, their decay clip can besides be studied at a femtosecond graduated table [ 9 ] .Though this technique is non direct, careful survey of the information and comparing of consequences of other known methods give us a good apprehension of the UHV STM applications.
The ultimate end of an ultrafast optical maser assisted STM is to to supervise the electronic and structural surface kineticss on an atomic graduated table with femtosecond clip declaration. The above experiments and analogies proves that it can be achieved. However this needed careful designation of the nonlinear tunneling current part due to the optical maser stimulation. This current is dependent merely on the frequence fluctuation of this ultra-fast optical maser therefore enabling us to make the femtosecond declaration. Thus we can reason that this new technique is really powerful and its utilizations are yet to be to the full explored.