Title of Invention

AN APPARATUS FOR CARRYING OUT NON-DESTRUCTIVE MEASUREMENT OF ELECTROFLECTANCE AND SURFACE PHOTOVOLTAGE SPECTROSCOPIES OF A SEMICONDUCTOR SAMPLE IN SOFT CONTACT MODE

Abstract 1. An apparatus for carrying out non-destructive measurement of electroflectance and surface photovoltage spectroscopies of a semiconductor sample in soft contact mode comprising : a) a sample holder comprising - I) a front (4,5,6,8) and back portions; II) means for assembling a first (4) and a second electrode (1)on the front and back portion respectively; i) wherein said first electrode (4) in front is a transparent conducting electrode; ii) wherein said second electrode (1) is a metallic electrode or a transparent conducting electrode; III) means (3,10) for holding a sample (2) between the first and second . electrodes; IV) means (10) for adjusting and maintaining the spacing between the front surface of the sample and the first electrode, so as to reduce the spacing till Newton's rings become visible such that the front electrode softly touches the sample surface and the capacitive impedance of the whole two electrode assembly including the sample (i.e. first front transparent electrode, sample and second back electrode) is the same as that of the sample itself for the measurement and maintaining the same spacing during the measurement; V) means (10) for dissembling the first and second electrodes to retrieve the sample in an intact manner; b) means (8,9,11,12,13) for applying modulating voltage (14) from 10 volts to 10 milivolts depending on the sample through the said electrodes on said sample for electromodulation experiments; c) means for generating (15) and directing (16,17,19) a probe beam (18) centered at any selected wavelength of light with or without periodic chopping onto the sample through the first electrode as well as means for varying the said wavelength; d) means (21,22,23) for detecting the reflected or transmitted light beam (20) in phase with the applied modulation voltage and means (25) for separating as well as recording the modulating part (24) and dc part (26) of the detected electrical signal at each wavelength which together give electromodulation spectrum (27) which contains the information about the band structure of the material and related parameters; e) means (1,6) for picking up a photovoltage using a buffer amplifier (34) or picking up a photocurrent through the said electrodes in phase with the said chopping (32) of the said incident probe beam (31) and means (28,29,30,35,36) for recording the signal at each wavelength which gives SPV or photocurrent spectrum (36) respectively, which contains the information about the band structure of the material and related parameters as well as the information about the charge transport properties of the said sample.
Full Text FORM 2
THE PATENTS ACT 1970
COMPLETE SPECIFICATION
(See Section 10), rule, 13



TITLE
An Apparatus for/Electroreflectance and Surface Photovoltage
Spectroscopies/ in Soft Contact mode.




APPLICANT
Tata Institute of Fundamental Research., Homi Bhabha Road, Colaba, Mumbai 400 005, Maharashtra, India.
An aided autonomous institution under the administrative purview of the
Department of Atomic Energy, Government of India,
Anushakti Bhavan. Chatrapati Shivaji Maharaj Marg, Mumbai 400 001. Maharashtra, India.
846/MUM/2000 14 SEP 2000
The following specification particularly describes the nature of the invention GRANTED
and the manner in which it is to be performed:- 19-1-2005

FIELD OF THE INVENTION :
The present invention relates to an apparatus for canying out non-destructive measurements
of both electroreflectance ( ER ) and surface photovoltage ( SPV ) spectroscopies In particular the invention relates to a novel apparatus which offers many advantages such as it is non-destructive, simpler and more sensitive.

BACKGROUND AND PRIOR ART:
Of the many optical methods used to investigate the band gap and energy level structures of semiconductors and semiconductor microstructures, electromodulation (EM) and SPV
Spectroscopies are very powerful. EM and SPV spectroscopies have been reviewed by (i) M. Cardona, in Modulation Spectroscopy, ( Academic Press. New York. 1969 ) and (ii) L. Kronik and Y. Shapiro, in Surf. Sci. Rep. 37. 1 -206. (1999) respectively ).
Contactless electromodulation can be performed using photoreflectance (PR) or electron-beam electroreflectance (EBER). ( PR and EBER spectroscopies have been reviewed by (iii) Orest. J. Glembocki and Benjamin. V. Shanabrook, in Semiconductors and Semimetal. edited by D. G Seller and C. L Littler (Academic Press , New York, 1992 ), Vol 36, p. 221.and (iv) M. H. Herman. Proc. Soc. Photo-Optical Instrum. Eng., 1678, 296, (1992) respectively ). In PR, electromodulation is done by periodically chopping the pump light source
such as a laser or other light source. PR is not only contactless but also requires no special mounting as well as no special preparation of the sample. Modulation of the electric field in the sample is caused by photo-excited electron-hole pairs created by a pump light source, which is chopped at a


certain frequency. These photo-injected electron-hole pairs modulate the built-in electric field of the semiconductor or semiconductor structure. The photon energy of the pump source must be above the bandgap of the semiconductor material being investigated. Change in the reflected light of the probe beam in phase with the chopped pump beam is detected. By varying the wavelength of the probe beam and detecting the changes in reflectivity one records a PR spectrum. Although PR is non¬destructive, it suffers from a serious drawback at low temperatures due to photoluminescence background. This can be remedied by dual chopping PR, where both pump and probe beams are chopped at different frequencies and the reflected signal is detected at the sum frequency ( in (v) S. Ghosh and B. M. Aiora, IEEE. J. of Selected Topics in Quant. Electron. 1, 1108, (1995) ). In electron-beam electroreflectance, the pump beam is replaced by a modulated low energy election beam ( about 200 eV ) chopped at about 1 kHz. However, the sample and electron gun have to be placed in an ultra¬high vacuum chamber for the measurement.
In direct EM, called electroreflectance ( ER ), the periodic modulation of an applied electric field gives rise to sharp changes at particular wavelengths in the reflected light measured in phase
with the applied ac voltage. ( ER spectroscopy is reviewed by (vi) B. O. Seraphin, in Semiconductors and
Semimetals, edited by R. K. Willardson and Albert C. Beer (Academic Press, New York, 1972), Vol 9, p. 1 and (vii) D. E. Aspnes, in Handbook of Semiconductors, edited by M. Balakanski, (North Holland, New York, 1980), Vol 2, p. 109 and (viii) D. E. Aspnes and N. Bottka, in Semiconductors and Semimetals, edited by R. K. Willardson and Albert C. Beer (Academic Press. New York, 1972), Vol 9, p 457and (ix) Fred H. Pollak and H.Shen, Materials. Sci. & Eng R10, 275, (1993) ). Like PR, sharp features in the ER spectrum of the material occur at photon energies
corresponding to interband and intersubband transitions. These derivative-like features of the
spectrum can be used to study and characterize many of the important properties of semiconductors
(bulk or thin film), semiconductor surfaces/interfaces, semiconductor microstructures (single
quantum wells, multiple quantum wells, quantum wires and dots, superlattices and heterojunctions)
as well as actual device structures.

In general, ER spectroscopy can be broadly categorized in 1) longitudinal modes, and 2) transverse modes of electromodulation schemes. In the longitudinal methods, electromodulation can be achieved by various sample configurations like - 1) liquid electrolyte mode, 2) metal-insulator-semiconductor structure or MIS structure, 3) Schottky barrier method, 4) p-i-n structure, as well as 5) the COntaCtleSS method or CER ( CER is first demonstrated by (x) X. Yin and Fred H. Pollak, Appl.Phys.Lett. 59, 2305 (1991)). The liquid electrolyte method is simple to implement but can be used only over a limited temperature range around room temperature ( - 300 to 150 K ) and offers less
control of the space charge field owing to chemical passivation or dissolution effects. Rest of the above ER methods can also be used at low temperatures. The field in the space charge region of the semiconductor is modulated by applying voltage across an MIS configuration. But this MIS method requires careful sample preparation for electromodulation. Although, the Schottky barrier] configuration requires small voltage to operate, it needs metal electrodes to be deposited on the
sample surface, which by itself is a destructive technique. The p-i-n structure can be used only on



specially fabricated samples and produces a constant electric field as opposed to position dependent field of other longitudinal ER modes. In the transverse mode two metal electrodes are evaporated on the surface of the sample and electromodulation is produced by applying high voltages ( - 103 Volts or kilo Volts or kV ) across the thin ( ~ 1 mm ) gap between the electrodes. However, this technique can only be used on materials with resistivities greater than ~ 108 Q cm. Among all these various ways of ER spectroscopy, the most recent and most commonly used ER Spectroscopy is CER ( Patented by (xi) Fred H. Pollak and Xiaoming Yin, US patent 5,287,169. Date of patent 15"' Feb. 1994 ). In CER, a transparent Indium-tin-oxide ITO electrode is carefully separated from the front surface of the sample by a fraction of a millimeter with the help of insulating spacers, such that the thickness of the spacer is greater than the sample thickness in order to maintain the above mentioned spacing. This front ITO coated electrode is generally used for the application of

modulation voltage as well as the window for the incident and reflected light. But the use of CER is limited because of its experimental complexities, (i) CER needs high electric voltage ( ~ kV ) for electromodulation, which presents electrical shock hazards, (ii) depending on the magnitude of the applied high voltage and the spacing between the sample surface and the front electrode, the air may break down and it may be necessary to take special precautions to prevent that, (iii) in view of the small signal strength of the CER, careful electrical shielding may be needed to avoid pick up from the high voltage unit.
Surface photovoltage ( SPV ) spectroscopy ( Reviewed in Ref. (ii) by L. Kronik and Y. Shapira, in Surf. Sci. Rep. 37. 1-206, (1999) ) is a well known technique to map the electronic structure of bulk semiconductors near the band edge as well as at the sub band gap energies. In surface photovoltage ( SPV ) one measures the change in surface potential due to optically excited electron hole pair generation under periodic illumination and subsequent carrier redistribution and\or capture in the surface states. Recently, SPV has emerged as a powerful technique to study surface states, heterojunctions, quantum wells ( QWs ) and other nanostructures. Previously, various other techniques like Kelvin probe method, MIS structure, direct contact measurements like electrolyte method, depositing a semi transparent metallic contact, or depositing a transparent conductor on the sample have been used to measure the SPV. Kelvin probe technique uses a null capacitive method of measuring the SPV ( = contact potential difference ) by applying external dc bias. Although it has been widely used, Kelvin probe method suffers from noise related problems as a result of the residual electrostatic pick-up by the Kelvin capacitor as well as from the stray capacitances which require careful electrical shielding. SPV measurements using a fabricated MIS structure on a semiconductor sample are also useful but the technique requires special sample preparation. Alternatively, insulating spacer or air/vacuum gap can be used. But this reduces the capacitve coupling of the electrodes with the sample. As a result, the measured SPV is much less than the

intrinsic SPV generated within the sample. Electrolyte method suffers from surface passivation and dissolution effects as well as limited temperature range of operation. All other methods of depositing a semitransparent metal or transparent conducting film on the sample are useful but these are obviously destructive techniques.
OBJECT OF THE INVENTION :
The main object of the present invention is to provide a simple as well as a compact apparatus for doing measurements of ER and/or SPV, overcoming aforementioned shortcomings and drawbacks encountered with the previous conventional experimental systems.
Another object of the present invention is to provide an apparatus for doing ER and/or SPV, which is relatively simple in structure and easy to use without any necessary requirement of special sample preparation, like depositing some oxide layer or thin films or coatings on the surface of the sample, thus providing intact recovery of the sample without any destruction/darrmge/ impairment.
Yet another object of the present invention is to provide an apparatus for ER measurement, which requires no high voltage units and related experimental complexities.
A further object of the present invention is to provide an apparatus for ER and/or SPV measurements over a wide range of temperatures without resorting to destructive techniques.
A still further object of the present invention is to provide an apparatus for carrying out these ER and/or SPV measurements at room temperature with or without high vacuum and in any ambient to which the sample surface may be subjected / exposed.


846/MUM/2000
Summary of the Invention:
With a view to fulfill the objects, the present invention provides an apparatus for carrying out non-destructive measurement of electroflectance and surface photovoltage spectroscopies of a semiconductor sample in soft contact mode comprising :
a) a sample holder comprising -
I) a front (4,5,6,8) and back portions;
II) means for assembling a first (4) and a second electrode (1)on the front and back portion respectively;
i) wherein said first electrode (4) in front is a transparent conducting
electrode; ii) wherein said second electrode (1) is a metallic electrode or a transparent conducting electrode;
III) means (3,10) for holding a sample (2) between the first and second electrodes;
IV) means (10) for adjusting and maintaining the spacing between the front surface of the sample and the first electrode, so as to reduce the spacing till Newton's rings become visible such that the front electrode softly touches the sample surface and the capacitive impedance of the
' whole two electrode assembly including the sample (i.e. first front transparent electrode, sample and second back electrode) is the same as that of the sample itself for the measurement arid maintaining the same spacing during the measurement; V) means (10) for dissembling the first and second electrodes to retrieve the sample in an intact manner;
b) means (8,9,11,12,13) for applying modulating voltage (14) from 10 volts to 10 milivolts depending on the sample through the said electrodes on said sample for electromodulation experiments;




c) means for generating and directing a probe beam centered at any selected wavelength of light with or without periodic chopping onto the sample through the first electrode as well as means for varying the said wavelength;
d) means for detecting the reflected or transmitted light beam in phase with the applied modulation voltage and means for separating as well as recording the modulating part and the dc part of the detected electrical signal at each wavelength which together give electromodulation spectrum which contains the information about the band structure of the material and related parameters;
e) means for picking up a photovoltage using a buffer amplifier or picking up a photocurrent, through the said electrodes in phase with the said chopping of the said incident probe beam and means for recording the signal at each wavelength, which gives SPV or photocurrent spectrum respectively, which contains the information about the band structure of the material and related parameters as well as the information about the charge transport properties of the said sample.
The first electrode is a transparent conducting electrode. Preferably it is substantially transparent with an at least semi-transparent conducting coating on the side facing the sample. The second electrode is disposed at the rear of the sample and it is a metallic electrode or a transparent conducting electrode such that the sample is in good thermal and/or electrical contact with this second electrode.

According to the invention, means are provided for adjusting or reducing the space between the from surface of the sample and the first electrode such that the spacing is reducible till Newton's rings become visible so that the front electrode softly touches the sample surface and the capacitive impedance of the whole two electrode assembly including the sample ( i.e front transparent electrode, sample and back electrode ) is approximately the same as that of the sample itself for the measurement and maintaining the same spacing during the measurement and after the measurement said means for assembling the two electrodes provide for dissembling the electrodes to retrieve the sample unimpaired. Such means are constituted by a set of two screws which are electrically insulated from the second electrode. The screws fix the first and the second electrodes together with the said sample placed in between the said electrodes.
The sample holder is provided with means for carrying out measurements of ER and/or SPV within a wide range of temperatures such as from about near zero degree Kelvin to about 400 degree Kelvin or more.
There is further provided means for the safe upper limit of the said modulating voltage which is set below the voltage level at which the electroluminescence signal which is insignificant and is carried out by means of blocking the probe beam while applying the modulation voltage and detecting the electroluminescence light generated as a result of the modulation voltage and thus minimizing the applied voltage to a level below which the electroluminescence signaJ is insignificant
Alternatively, the safe upper limit of the said modulating voltage is also set below the voltage level by means of optimizing the strength of the modulating voltage by stopping the probe beam while applying modulation voltage by monitoring the current passing through the sample to avoid unnecessary heating of the sample by keeping it less than about 10 microampere. In general,

depending on the sample, the applied modulation voltage is from about 10 volts to about 10 millivolt.
There is further provided means for measuring photovoltage adapted to measure weak SPV signals ( ~ microvolts in case of SPV measured with the apparatus of the present invention at room temperature) of sample like p-GaAs and n-InP.
The apparatus of the present invention allows measurements of ER and/or SPV with or without vacuum ( DESCRIPTION OF THE INVENTION :
The present invention involves a novel apparatus for measuring both ER and SPV spectra. These techniques of the present invention utilize a sample holder consisting of a two electrodes assembly, where a sample having a thickness of about 0.5 mm. is sandwiched between the electrodes till Newton's rings become visible so that the front electrode softly touches the sample surface and the capacitive impedance of the whole two electrode assembly including the sample (i.e first front transparent electrode, sample and second back electrode ) is approximately the same as that of the sample itself for the measurement. The front electrode is substantially transparent substrate like glass/quartz with transparent coating of a conducting material such as ITO on the side facing the sample or a transparent conducting electrode. The back electrode is a metallic electrode of a suitable shape such as a regular metal plate or a 'L' shaped metal plate. The back electrode can also be a transparent or a semitransparent conducting electrode like front electrode. The sample is in

thermal and /or electrical contact with the back electrode. These new techniques of doing ER and SPV are termed hereafter as soft contact electroreflectance ( SCER ) and soft contact surface photovoltage ( SCSPV ) respectively. These techniques are useful for studying the material characteristics of semiconductors and semiconductor structures within a wide range of temperatures from near zero degree Kelvin to about 400 degree Kelvin or higher.
a) In case of SCER, in order to reduce this modulation voltage, the front transparent ITO electrode is made to softly touch the semiconductor surface. Contrary to the CER, the front transparent electrode is in direct contact with the sample surface in SCER. The modulation voltage is applied between the transparent conducting electrode and the back electrode. The probe beam is incident on the sample through the front transparent electrode. This transparent electrode also acts as a window for the light reflected from the sample to come out of electrode assembly. It is now found that in this way the required modulation voltages can be reduced considerably to a few volts and in some cases to few tens of milli Volts. In general, the preferred voltage range for most of the samples is from about 10 volts to about 10 millivolts. The change in the reflectivity ( AR(X) ) is measured with the lockin amplifier, in phase with the applied ac. The dc part of the signal (R(A.)) is electrically separated and fed to the analog to digital port of the same lock in. After measurements, the values of AR(X)/R(V) vs X. are calculated numerically to obtain the ER spectrum in SCER mode. It is shown in this work that ER spectrum obtained by SCER is spectroscopically equivalent to that obtained using CER. In order to know the upper limit of the modulation voltage which can be safely applied to sample, electroluminescence ( (xii) Sanctip Ghosh and Thomas J. C. Hosea, Rev. Sci. lustrum 71, 1911, 2000) background from the sample under the applied voltage is monitored. The safe limit is set below a voltage level where the electroluminescence signal becomes insignificant. Alternatively, current through the sample is also monitored to avoid heating and to set a voltage limit below which the current is less than about 10 microampere.

b) The same soft contact mode arrangement as described in case of SCER is also useful for sensitive SPV measurements. In this SCSPV, the light incident on the sample is periodically chopped and the photovoltage produced due to separation of photo generated carriers under surface electric fields is measured. The same sample holder ( as described in fig. la & fig. lb ) is also used for the SPV measurements. Instead of applying ac voltage from outside as in SCER, the above mentioned ac SPV signal is picked up with the same set of electrodes and leads. Finally, by recording the SPV signal S( X ) vs X in phase with the chopped light, as the wavelength (A,) of the incident light is varied, the SPV spectrum is obtained. In this case, appropriate order sorting filter just after the monochromator is used to prevent unwanted light from falling on the sample. The most important aspect of SCSPV is the increase in the magnitude of the signal strength as compared to SPV signal in the non contact mode. It is observed that in the method of the present invention that there is around thousand times ( ~ 1000) increase in the SPV signal strength as compared to contactless methods. Thus weak ( ~ microvolts in case of SCSPV measurements done at room temperatures ) signals of samples like p-GaAs and n-InP can be measured. This is explained in terms of the increase of the capacitive coupling.


BRIEF DESCRIPTION OF THE DRAWINGS :
Salient features and important advantages of the present invention will be illustrated with the help of the accompanying drawings. The drawings are for illustration only and do not restrict the scope of the invention :
Fig. 1 Schematic diagram of the sample holder ( side view ) used in both SCER and SCSPV measurements.
Fig. 2 Schematic diagram of the sample holder ( cross-sectional view ) used in both SCER and SCSPV measurements.
Fig. 3 Block diagram of the SCER experimental setup.
Fig. 4 Block diagram of the SCSPV experimental setup.
Fig. 5 CER and SCER spectrum of the GaAs/Ino.26Gao.74AS/GaAs QW.
Fig. 6 A comparison of the maximum (AR) signal of erhh| transition from SCER and the EL background at various ac amplitude. EL background vanishes below ~ 1.25 V.
Fig. 7 SPV spectrum in the contactless mode and SCSPV spectrum of the GaAs/In0.26Ga0.74As/GaAs QW.

DETAILED DESCRIPTION OF AN APPARATUS AS ILLUSTRATED IN THE ACCOMPANYING DRAWINGS & ITS OPERATION :
A typical apparatus of the present invention essentially comprises a sample holder of this invention comprising an assembly of two electrodes as shown in fig. 1 and fig. 2; gadgets for applying modulation voltage in case of SCER ( about 10 Volts to about 10 millivolts ); gadgets for sending the probe beam of required wavelength; gadgets for detecting the reflected and transmitted light beams; gadgets for converting the detected light signals to electrical signals; gadgets for separating the modulating or ac part of the signal and the dc part of the signal; gadgets for measuring the signals and recording the data ; also gadgets for picking up photovoltage and/or photocurrent and other accessories; gadgets for measuring the photovoltage and/or photocurrent signal and recording the data as shown in the block diagrams depicted in fig. 3 and fig. 4 for SCER and SCSPV experiments respectively.
Figure 1 ( side view ) and fig. 2 ( cross-sectional view ) show the construction of a typical sample holder. Sample holder consists of a two electrodes assembly comprising a grounded L' shaped back electrode 1 ( made of copper ) on which the sample 2 is mounted and a front transparent conducting electrode as shown by the shaded area 4 in fig. 1. The base of that 'U shaped back electrode 1 is clamped on the cryostat cold head/finger when measurements need to be done at lower than room temperature or on a stage which can be heated for measurements done at higher than room temperature as required. The front electrode 4 is an Indium-Tin-Oxide (ITO) coated on the glass slide 5. The uncoated side of the glass plate 5 is fixed on a bakelite frame 6 having a rectangular window 7. The front electrode 4 and the back electrode 5 are assembled keeping a sample 2 in between, such that the bakelite frame 6 is screwed to the back electrode 1 so that the ITO coated surface 4 softly touches the sample 2 surface till Newton's rings become visible so that

the capacitive impedance of the two electrodes assembly ( i.e front transparent electrode 4, Sample 2 and back electrode 1 ) is approximately same as that of the sample 2. As mentioned above, this methodology of assembling the two electrodes and the sample is termed as the soft contact electroreflectance (SCER) and soft contact photovoltage (SCSPV) during measurements on ER and SPV respectively. A fine gold wire 8 is electrically connected to the ITO coated surface on one side and is connected to the metal screw holder on the other side 9 of the bakelite plate. Two metal screws 10 in fig.2 fix the front 4 and back electrode 1 assembly together. The screws are electrically insulated from the back electrode 1 with threaded teflon rings 3, embedded inside both the holes in the 'L' shaped back electrode. A shielded cable is connected between the other end of the screw 11 and the central part of the BNC connector 12. The back electrode 1 is connected to the ground of the BNC connector 12.
In case of SCER spectroscopy ( schematically described in fig. 3), an ac voltage source 13 is used to apply modulating ac voltage 14 between these electrodes ( 1 and 4 ). A 150 watt quartz tungsten halogen lamp 15, lens 16 and monochoromator 17 arrangement is used as a light source and the light output 18 centered at some wavelength is focused on the sample 2 with a lens and mirror arrangement 19 as shown in the block diagram given in fig. 3. Window 7 in the bakelite frame 6 and the transparent ITO coating 4 on the glass 5 allow the incident light to fall on the sample 2 and also allow the reflected light 20 to go out. This reflected light 20 is focused with lens 21 and collected with a Ge photovoltaic detector 22. Proper order sorting filters are used before the Ge detector to block any unwanted light. The current from the detector is transformed into a voltage signal by a current to voltage converter 23 and the modulating or ac part 24 is fed into a SRS830 Lock:in amplifier 25. The change in the reflectivity ( AR(λ) ) is measured with the Lock-in amplifier 25, in phase with the applied ac voltage 13. The dc part 26 of the signal (R(λ)) is separated using a filter arrangement 23 and fed to the analog to digital port of the same Lock-in

amplifier 25. The value of AR(λ)/R(λ) vs X are calculated numerically to obtain the ER spectrum 27 in SCER mode.
Similar soft contact mode arrangement is also useful for sensitive measurements of surface photovoltage ( SPV ) spectra. The same sample holder ( as described in fig. 1 and fig. 2 ) is also used for the SCSPV measurements ( schematically described in fig. 4 ). A 150 watt quartz tungsten halogen lamp 28, lens 29 and monochoromator 30 arrangement is used as a light source and the light output 31 centered at some wavelength is periodically chopped with an optical chopper 32 and is focused on the sample 2 with a lens arrangement 33 as shown in the block diagram given in fig. 4. Window 7 in the bakelite frame 6 and the transparent ITO coat 4 on the glass 5 allow the incident light to fall on the sample 2. In SCSPV, the periodically chopped light incident on the sample 2 produces photovoltage due to separation of photo generated carriers under surface electric fields. Instead of applying ac voltage from outside as in case of SCER, the above mentioned ac SPV signal is picked up with the same set of electrodes ( 1 and 4 ) and leads ( 8 - 12 ), a buffer amplifier 34 and fed into a SRS830 Lock-in amplifier 35. Finally, the SPV signal S( λ ) vs λ in phase with the chopped light is recorded while the wavelength {X) of the incident light is varied to get a SCSPV spectrum 36. Appropriate order sorting filter is used just after the monochromator 30 to prevent unwanted light from falling on the sample.
EXAMPLES :
Following examples are the results of some experiments carried out to compare the ER and the SPV spectra as obtained by our new SCER and SCSPV techniques respectively using the apparatus of the present invention with those done in the conventional contactless techniques. Also described is a new experiment to set the safe upper limit of the applied voltage in case of

electromodulation measurements. These new SCER and SCSPV experiments were done in the above mentioned assembly of two electrodes including the sample as discussed in the detailed description of the apparatus and accompanying drawings to illustrate the use of the apparatus of the present invention.
EXAMPLE I : SCER AND CER SPECTRA OF A 100 A QUANTUM WELL ( QW ) OF GaAs/In 0.26 Ga 0.74 As/GaAs:
ER spectroscopy is done on a 100 A quantum well ( QW ) of GaAs/Ino.26Gao.74As/GaAs sample both in SCER mode and in CER mode. Respective ER spectra of the QW are plotted in fig. 5. Dotted curve 37 is the ER spectrum of the e1-hhi transition as obtained by the conventional contactless mode ( CER ) and the dotted curve 38 is the ER spectrum as obtained in the new SCER mode under similar experimental settings. The only difference between the two is the amplitude of the ac voltage applied during the measurement. In case of CER, an ac amplitude of ~ 1.25 kV is applied for electromodulation, while in the SCER configuration only 0.5 V ac amplitude is applied. The continuous curves in 37 and 38 are the Aspnes's

first derivative lineshape function ( m = 2 , assuming lorentzian broadening ) to the experimental data. ( 77?w lineshape function is described in Ref. (vii) by D. E. Aspnes and N. Bottka, in Semiconductors and Seinimetals. edited by R. K. Willardson and Albert C. Beer ( Academic Press, New York. 1972), Vol 9, p 457 ) The e|-hh| transition energy values as obtained from the lineshape fitting from the two curves are respectively Eo = 1.1768 eV in contactless mode and Eo = 1.1774 eV in SCER ( with experimental resolution AE - 3 meV ). The fitted values of the broadening parameter ( Y ) are also nearly the

same within experimental error ( 5.69 meV in the contactless mode and 6.07 meV in the soft contact mode. ). This shows that SCER is reliable spectroscopic technique as the conventional CER.
EXAMPLE II : METHOD TO SET THE UPPER LIMIT OF THE MODULATING VOLTAGE:
One experiment is performed to set the upper limit of the modulating voltage that can be applied during SCER spectroscopy. Figure 6 shows a plot 39 of measured change in the modulated reflectance ( AR ) for a erhh| transition in the SCER experiment on a sample where two QWs as mentioned in example I are sandwiched between p and n type layers of GaAs. AR varies with the modulation potential and in this case it is possible to reduce the applied ac amplitude to ~ 15 mV and still get a good ER lineshape. At this point we would like to comment on the upper limit on the ac amplitude that can be-applied during an SCER measurement. Applying large modulating voltage may heat up the sample or produce unwanted electroluminescence ( EL ) signal which can distort the ER lineshape. EL background of the same sample is measured under various applied ac voltages in the same SCER set up with the light from the monochromator blocked. As shown in fig. 6, reduction in the applied ac voltage reduces this EL background 40 sharply. In this case, we see that below ~ 1.25 V ac amplitude, the EL background 40 vanishes totally and merges with the noise level. Therefore, for this sample it is preferable to apply ac volts below this value while doing SCER measurements. Therefore, a preliminary measurement of the EL background is performed for each sample and the maximum ac voltage which is allowed without any unwanted EL background contribution is determined before the actual SCER measurement. Alternatively, in case sample does not emit EL, series current can be monitored to avoid excessive heating of the sample.

EXAMPLE III :

SCSPV AND CONTACTLESS SPV SPECTRA OF A 100 A



QUANTUM WELL (QW) OF GaAs/In 026 Ga 0.74 As/GaAs:
SPV spectroscopy is done on a 100 A quantum well ( QW ) of GaAs/In0.26Gao.74 As/GaAs both in SCSPV mode and in contactless mode. Respective SPV spectra of the QW are plotted in fig. 7. The plot 41 is the SPV spectrum as measured in the usual contactless mode and the plot 42 is the SPV spectrum as measured in the soft contact mode ( SCSPV ). Main features corresponding to the band edge of GaAs and Ino.26Gao.74As QW are reproduced in both experiments. For further comparison of the transition energies we calculate the numerical derivative spectrum ( d(ExSPV(E)/dE vs E, where E is the incident energy ) and fit the Aspnes lineshape function with m = 2. The transition energies of e1-hhi line as obtained by above mentioned lineshape fitting are E0 = 1.1757 eV and E0 = 1.1764 eV ( with experimental resolution AE - 3 meV ) for contactless and soft contact modes respectively. The broadening parameters are l = 7.26 meV and T = 7.02 meV for contactless and SCSPV modes respectively. So within the experimental accuracy, the spectroscopic signatures of the e1-hhl transition energy are similar in both of these experimental methods of SPV. These transition energy values are also comparable with the values as reported from the ER experiments on the same sample as mentioned in example I.




ADVANTAGES OF THE INVENTION :
The apparatus of the present invention provides a new ER spectroscopic technique, enjoying all the plus points ( e.g a nondestructive experiment and one that can be used at low temperatures as well as without any special sample preparation like deposition of some predesigned layer or thin film or coating on the sample surface ) of the conventional CER which is so far the most recent and most popular method of doing ER, with the added advantages of -
• It requires low ac voltage for electromodulation ( e.g at least thousand times
lower ac voltage than that of CER ). So the apparatus of the present invention does not
need any high voltage unit and related instruments. This considerably simplifies the
electronics accessories required in the ER setup.
• It reduces experimental hazards such as electrical shock and sample
damage.
• No extra care is necessary to reduce the possibility of electrical discharge
and measurements can be done at atmospheric pressure in air or in vacuum at room
temperature.
• No extra care is necessary for electrical shielding to avoid pickups from high
voltage units as required in CER.
• No insulating spacer between the two electrodes is needed to maintain a
spacing of about a millimeter between the top surface of the sample and the front ITO
electrode as required in CER.


• As described in the summary and in example II, a technique is provided to set the upper limit of the modulation voltage that can be applied safely without any unwanted electroluminescence background and/or unwanted heating of the sample during electromodulation measurements.
The apparatus of the present invention also provides a new SPV spectroscopic technique, which has the following advantages -
• Nearly a thousand times increase in the signal strength than the conventional
contactless method. This makes it possible to measure the weak ( ~ microvolts in case of
SCSPV done at room temperature ) SPV signal of p-GaAs and n-InP as well as to study
their properties, which are very difficult to measure in contactless mode of SPV.
• It is also a non-destructive technique and requires no special sample
preparation.
To summarize, present invention relates to a novel and simple as well as a compact apparatus for doing measurements of ER and/or SPV, which avoids by simple means the aforementioned shortcomings and drawbacks encountered with the prior conventional experimental systems.

WE CLAIM:

1. An apparatus for carrying out non-destructive measurement of electroflectance and surface photovoltage spectroscopies of a semiconductor sample in soft contact mode comprising :
a) a sample holder comprising -
I) a front (4,5,6,8) and back portions;
II) means for assembling a first (4) and a second electrode (1)on the front and back portion respectively;
i) wherein said first electrode (4) in front is a transparent conducting
electrode; ii) wherein said second electrode (1) is a metallic electrode or a transparent conducting electrode;
III) means (3,10) for holding a sample (2) between the first and second
. electrodes;
IV) means (10) for adjusting and maintaining the spacing between the front surface of the sample and the first electrode, so as to reduce the spacing till Newton's rings become visible such that the front electrode softly touches the sample surface and the capacitive impedance of the whole two electrode assembly including the sample (i.e. first front transparent electrode, sample and second back electrode) is the same as that of the sample itself for the measurement and maintaining the same spacing during the measurement;
V) means (10) for dissembling the first and second electrodes to retrieve the sample in an intact manner;
b) means (8,9,11,12,13) for applying modulating voltage (14) from 10 volts to 10 milivolts depending on the sample through the said electrodes on said sample for electromodulation experiments;


c) means for generating (15) and directing (16,17,19) a probe beam (18) centered at any selected wavelength of light with or without periodic chopping onto the sample through the first electrode as well as means for varying the said wavelength;
d) means (21,22,23) for detecting the reflected or transmitted light beam (20) in phase with the applied modulation voltage and means (25) for separating as well as recording the modulating part (24) and dc part (26) of the detected electrical signal at each wavelength which together give electromodulation spectrum (27) which contains the information about the band structure of the material and related parameters;
e) means (1,6) for picking up a photovoltage using a buffer amplifier (34) or picking up a photocurrent through the said electrodes in phase with the said chopping (32) of the said incident probe beam (31) and means (28,29,30,35,36) for recording the signal at each wavelength which gives SPV or photocurrent spectrum (36) respectively, which contains the information about the band structure of the material and related parameters as well as the information about the charge transport properties of the said sample.

2. An apparatus as claimed in claim 1 wherein said first electrode (4) is a transparent conducting electrode.
3. An apparatus as claimed in claim 2, wherein said first electrode (4) is a substantially transparent substrate (5) with an at least semi-transparent conducting coating (4) on the side facing the sample.
4. An apparatus as claimed in claim 1, wherein said second (1) electrode disposed to the rear of the sample is a metallic electrode (1) or a transparent conducting electrode such that the sample is in good thermal and/or electrical contact with this second electrode.


5. An apparatus as claimed in claims 1 to 4, wherein means for adjusting or
reducing the space between the front surface of the sample and the first
electrode comprises a set of two screws which are electrically insulated from the
- second electrode being adapted to fix the first and the second electrodes together with the said sample placed in between the said electrodes.
6. An apparatus as claimed in claims 1 to 5, comprising means for carrying out measurements of ER and SPV within a wide range of temperatures.
7. An apparatus as claimed in claim 6, wherein the said temperature range is from about near zero degree Kelvin to about 400 degree Kelvin or more.
8. An apparatus as claimed in claims 1 to 7, wherein means for the safe upper limit of the said modulating voltage is adapted to be set below the voltage level at which the electroluminescence signal is insignificant and adapted to be carried out by means of blocking the probe beam while applying the modulation voltage and detecting the electroluminescence light generated as a result of the modulation voltage and thus minimizing the applied voltage to a level below which the electroluminescence signal is insignificant.
9. An apparatus as claimed in claims 1 to 8, wherein means for the safe upper limit of the said modulating voltage is adapted to be set below the voltage level by means of optimizing the strength of the modulating voltage by stopping the probe beam while applying modulation voltage and monitoring the current passing through the sample to avoid unnecessary heating of the sample by keeping it less than 10 microampere.
10. An apparatus as claimed in claims 1 to 9, means for measuring photovoltage adapted to measure weak SPV signals (~ microvolts in case of SCSPV done at room temperature) of sample like p-GaAs and n-lnP.


11. An apparatus as claimed in claims 1 to 10, wherein the means for SCER and SCSPV measurements being adapted to carry out measurement in air without a vacuum for measurements done at normal room temperature(~ 300 degree Kelvin).
12. An apparatus as claimed in claims 1 to 10, wherein the means for SCER and SCSPV measurements being adapted to carry out measurement with a vacuum ( 13. An apparatus as claimed in claims 1 to 12, wherein the means for SCER and SCSPV measurements being adapted to carry out measurement on any sample with or without any predesigned top layer or thin film of some other material deposited on said sample or an ambient to which the samp)e surface may be subjected/exposed.
14. An apparatus as claimed in claim 1 substantially as herein described with reference to accompanying drawings.
Dated this 14th day of September, 2000
S. MAJUMDAR of S.MAJUMDAR & CO. Applicant's Agents



Documents:

846-mum-2000-cancelled pages(19-01-2005).pdf

846-mum-2000-claims(granted)-(19-01-2005).doc

846-mum-2000-claims(granted)-(19-01-2005).pdf

846-mum-2000-correspondence(14-08-2007).pdf

846-mum-2000-correspondence(ipo)-(28-12-2004).pdf

846-mum-2000-form 1(14-09-2000).pdf

846-mum-2000-form 13(16-08-2007).pdf

846-mum-2000-form 19(30-10-2003).pdf

846-mum-2000-form 2(granted)-(19-01-2005).doc

846-mum-2000-form 2(granted)-(19-01-2005).pdf

846-mum-2000-form 3(14-09-2000).pdf

846-mum-2000-other document(14-09-2000).pdf

846-mum-2000-power of attorney(14-09-2000).pdf


Patent Number 204169
Indian Patent Application Number 846/MUM/2000
PG Journal Number 43/2008
Publication Date 24-Oct-2008
Grant Date 12-Jan-2007
Date of Filing 14-Sep-2000
Name of Patentee TATA INSTITUTE OF FUNDAMENTAL RESEARCH
Applicant Address HOMI BHABHA ROAD, COLABA, MUMBAI - 400 005, MAHARASHTRA, INDIA, AN AIDED AUTONOMOUS INSTITUTION UNDER THE ADMINISTRATIVE PURVIEW OF THE DEPARTMENT OF ATOMIC ENERGY, GOVERNMENT OF INDIA, ANUSHAKTI BHAVAN, CHATRAPATI SHIVAJI MAHARAJ MARG, MUMBAI-
Inventors:
# Inventor's Name Inventor's Address
1 DATTA, SHOUVIK, RESEARCH SCHOLAR, TATA INSTITUTE OF FUNDAMENTAL RESEARCH, HOMI BHABHA ROAD, COLABA, MUMBAI - 400 001
2 GHOSH SANDIP RESEARCH SCHOLAR, TATA INSTITUTE OF FUNDAMENTAL RESEARCH, HOMI BHABHA ROAD, COLABA, MUMBAI - 400 001
3 BRIJ MOHAN CONDENCED MATTER PHYSICS AND MATERIAL SCIENCE, TATA INSTITUTE OF FOUNDAMENTAL RESERACH HOMI BHABHA ROAD, COLABA, MUMABI-400 005.
PCT International Classification Number G01N 21/17
PCT International Application Number N/A
PCT International Filing date
PCT Conventions:
# PCT Application Number Date of Convention Priority Country
1 NA