Fabrication and characterization of Ag-BaF 2 /GaSb Schottky diode

The Ag-BaF 2 /GaSb Schottky diode measurement has been investigated by using voltage versus current (I-V) at different temperatures, voltage versus capacitance (C-V) and hotoelectric measurements on n-type GaSb carrier per cm -3 , Current – voltage measurement were used to study the interface layer. Near ideal characteristics were observed for Ag-BaF 2 /GaSb contact with ideality factor values (1.21-1.451). The Schottky barrier height and donor concentration (N d ) have been obtained from C-V characteristics. Barrier height results from these methods were the same approximately with the results obtained from current – voltage measurement and were in the same range which suggests that the Fermi level is pinned in the lower half of the band gap. Photoelectric measurements were used for barrier height measurements by applying the fowler method. The surfaces of sample was analyses using scanning electron microscopy (SEM) and also the x-ray is used to investigate the reasons for the non-ideal behavior of Ga contacts and to study the surface of Ag-BaF 2 layer on the air-cleaved n-GaSb surface.


Introduction
"Schottky barrier-type devices are rectifying metalsemiconductor (M-S) structure.This device is used in microelectronics, in solar cell applications, and in chemical sensing" [1].In the Schottky model the amount of band bending is equal to the difference between the work functions,  m and  s of the metal and semiconductor respectively.Thus Where (q) is the contact polenta of the junction and(V i ) is the potential barrier which an electron moving from semiconductor into the metal has to surmount.The barrier height, looking from the metal towards the semiconductor is given by  b = qV i + (E c -E f ) ...….. (2)  b = m - s ...….. (3) Where ( s ) is the electron affinity of the semiconductor.Most practical Schottky contacts do not appear to obey the above equation in that the barrier height is found to be independent of the difference between the work functions [2].In the past several authors remarked that the schottky barrier height determined from C-V measurements by assuming a simple model was different from the one obtained from I-V measurements.Crowell and Rideout [3], working on Si and GaAs contacts, attributed this difference to the influence of thermionic field emission on the charge transport through the interface.Later on, similar observations were made on GaAs Schottky barriers in which other contact metals were used [4][5][6][7].Recently effects were reported for several metal / GaSb barriers [8].The present paper reports the study of the I-V measurement, C-V measurement and photoresponse measurement at room temperature.The cleaving was done immediately before transferring to a high vacuum system so that the air interacted with the faces for a period of only 5 to 10 minutes.When etching was also used, it was carried out after preparing an ohmic contact, as explained, on one side of the crystal only.The ohmic contact was covered with 'Lacomit ' a chemically resistant varnish which was allowed to dry then the other side was cleaned and polished to a fine finish with 0.25 m diamond paste.The surface was then cleaned ultrasonically in methanol for 5 minutes then chemically etched using a solution of 1% bromine in methanol.Finally the crystal was cleaned ultrasonically in methanol again for 5 minutes.A metal mask with 0.5 mm diameter circular holes was placed on the prepared surface to produce contacts in the form of small dotes.An Edward model E610 vacuum coater was used to prepare method and BaF 2 layers.This system is intended primarily for the production of thin film coatings which can be used in the microelectronics.This ideal was tested by four different techniques viz.I-V measurement, C-V measurements, photoreponse measurement, XRD and SEM image.

Electrical properties 2.2.1 Currentvoltage measurements
The currentvoltage measurements were measured using a D. C. method in which the voltage was preset in steps and the corresponding current measured.Currents were measured with a Keithley while a Keithley electrometer (Model -2000) programmable voltage supply provided the supply voltage.The current as a function of applied bias V is given by the relation [9][10].: Where, the saturation current Here n is the diode ideality factor, A* is the modified Richardson constant for the semiconductor (A*= 5.08x10 4 Acm -2 K -2 ).For forward values of V in excess of 3KT/q, a plot of log I against V gives a straight line.The value of I o can be obtained by extrapolating the straight line to V = 0. Knowing Io, A*, S is the diode cross-sectional area, and T is the temperature, the barrier height  B can be determined.
The ideality factor, n, can be calculated from the equation n =

Currentvoltage measurements
C-V measurements for Ag-BaF 2 /GaSb Schottky diode at frequency of 1 MHz were determined by using precision LCR Bridge (model HIOKI 3532-50 LCR Hi TESTER) at room temperature.The reverse bias applied was the range ( 0 to 1.6 ) volt.The capacitance C as a function of applied bais V is given by the relation [11]: Where S is the diode cross-section area,  s is the permittivity of the semiconductor, V R is applied reverse voltage, and all other symbols have their usual meaning.It is seen from the above equation that a plot of 1/C 2 versus V R gives a straight line with slope 2/S s qN d and an intercept on the voltage axis V O = (V i -KT/q).The slope of the straight line can be used to determine the do pant concentration N d and The KT factor comes from the contribution of majority carriers to the space charge.

Photoelectric measurement
The photoelectric method is the most accurate band direct method of determined barrier height.When a monochromatic light is incident on a metal in contact with the semiconductor and the photon energy h is large than the barrier height but smaller than the band gap of the semiconductor, the incident photons will excite some electrons from the metal over the barrier.
The resulting photocurrent I ph for (h - b ) 2 3KT is given by the Fowler theory [12]..
Where B is contact of proportionality.If

Structural analysis
The x-ray diffraction patterns of the sample were recorded using x-ray diffactometer (model Bruker D8 Advance), with CuKα radiations (λ=1.5418Ǻ) at a scanning rate of 0.4 sec/step.The x-ray to investigate the reasons for the non-ideal behavior of Ga contacts and study the surface of Ag-BaF 2 layer on the aircleaved n-GaSb surface.

Surface morphological studies
Scanning electron microscope (SEM) (JEOL, Model JSM-6360A) was used to investigate the surface of the fabricated schottky devices. .

Current-voltage measurement
The I-V characteristics (forward bias and reverse bias) of Ag contact on air-cleaved GaSb, presented in Fig. 1 against applied reverse voltage in Fig. 3 shows straight line behavior up to -1.7 V. giving barrier height in the range These are thus in very good agreement with values from I-V method.Carrier concentration obtained from the slope was in the range.C-V characteristics of Ag contact on aircleaved GaSb, presented in Fig. 3. Reverse bias voltage (Volt) Fig. 3 The relationship between 1/C 2 with reveries bias voltage Ag-BaF 2 -GaSb Schottky diode

Photoelectric measurement
The barrier height of a Schottky contact can be determined from the spectral dependence of photovoltaic.Photo-threshold measurements have long been accepted as the single most reliable method of determining Schottky barrier heights [2].Fig. 4 shows a typical photoresponse plot for Ag-BaF 2 -GaSb contact which gives a barrier height of 0.553 eV from Fowler method.In general, the result from this method lower value for the barrier height without correction than those barrier height values observed from the I -V and C -V methods.
The general photoresponse equation due to Fowler can be written as [16]: .. (10) Where R is the response of the diode per light . Equation 10 suggests that, for large µ , R  µ 2 or R  µ which is applied in the classic Fowler method used in this study.The third term, , is negligible for µ3.
However the term, De Sousa Pires [17] has put forward a correction method involving a correction diagram for the photoelectric measurement of metalsemiconductor barrier heights.Fig. 5 shows the correction produce on Ag-BaF 2 -GaSb where the least squares fit was made from 4Kt (lower limit) to 9.5Kt (upper limit).For this range the diagram gives a correction of about 0.6Kt which is 0.015 Ev.The corrected barrier height is thus 0.553 + 0.015 = 0.568 Ev which is more reasonable and comparable to the values obtained from the I-V and C -V methods [13-15].The transport properties, while of interest in themselves, require theoretical modeling in order for them to be linked to the measurement of the wave length of excitation and to the electronic yield as a function of the metal.Vasoya1 et al [18] have derived a modified expression in the absence of scattering to explain the enhanced photoemission observed with thin films.The modified equation can be written as ..... ............ ( 11) The photoresponse measurements were also corrected using the cohen modified equation.Fig. 6 show plotted against the photo energy, hυ, for Ag-BaF 2 -GaSb schottky contacts respectively.The barrier height result for Ag-BaF 2 -GaSb diode was measured as 0.460 eV.Barrier height results obtains from the Cohen modified equation were in good agreement with those obtained using the de Sousa Pires correction method and both sets of corrected results show reasonable and comparable agreement with barrier height values obtained from I-V and C-V methods for same diode [19].3.4 X-ray analysis X-ray analysis, represented in Fig. 7 (a-b), Fig. 7(a) shows the X-ray energy dispersive spectrometer (EDS) analysis, focused within the Schottky contact region, of a studied MS-type device.X-ray analysis shown in Fig. 7(a) , did not show any oxygen or carbon at the surface, using a BaF 2 layer between the Ag and GaSb causes clustering to disappear from the surface , thus showing the important of BaF 2 in preventing chemical and physical interacting between metal and semiconductor.The X-ray analysis, shown in Fig. 7(b).Obviously, the important elements, e.g., Ag, Ba, F, Ga, and Sb, are found due to contamination of the surface.Based on the work reported by Hashizume et al., the chemical composition of this thermally grown oxide layer is possibly the InPO -like component [20].

Conclusions
The Fabrication and characterization of Ag-BaF 2 -GaSb Schottky diode is discussed in this paper.Barrier height results were measured using I-V and C-V methods and were comparable and in general were less than 0.5 eV.The I-V method at low temperatures was used to measure barrier hights and to establish the current transport mechanisms.The results suggest the more than one mechanism was responsible for current transport.Photoresponse measurements were used for barrier height measurements by applying the Fowler and modified Fowler methods.The scanning electron micrograph and x-ray of the Ag-BaF 2 -GaSb Schottky diode to investigate the reasons for the non-ideal behaviour of Ag contacts and to study the surfaces of BaF 2 , and Ag-BaF 2 layers on the air-cleaved n-GaSb surface.
function of h, a straight line is obtained whose intercept on the h axis directly given the barrier height  b .

Fig. 1 I
Fig. 1 I-V characterization of Ag-BaF 2 -GaSb Schottky diode into two types (a) forward bias and (b) Reverse bias at different tempersure

Fig. 5 A
Fig.5 A correction diagram for the experimental Fowler plot.