Chapter 3 Experimental Methods 58-
4.3 Photoconductor Theory
The photoconductive detector, or photoconductor, is essentially a radiation-sensitive resistor, a schematic of which can be seen in Figure 4.7. Ohmic contacts are made to the semiconductor material, the conductivity of which depends on photogenerated carriers. A photon of energy hv greater than the bandgap energy of the semiconductor is absorbed to provide enough energy to excite an electron from the valence band to the conduction band, thus creating an electron-hole pair and increasing the conductivity.
Ohmic c o n t a c t \ ^
Figure 4.7 Schematic diagram of a photoconductive detector.
When a low resistance semiconductor is used a load resistance is usually placed in series with the detector. The value of this resistance is generally large compared to the resistance of the detector. This enables the photoconductor to be operated in a constant
Chapter 4: Highly W avelength Selective Photodetector
current mode. The circuit acts as a voltage divider and the signal is detected as a change in voltage. If a high resistance semiconductor, such as diamond, is used a constant voltage circuit is preferred and the signal is detected as a change in current in the bias circuit.
4.3.1 Photocurrent
The current density, J, through a semiconductor sample can be expressed as the product of its conductivity, a, and the applied electric field, E. The conductivity of a semiconductor is expressed in terms of the carrier concentrations, n and p, and their respective mobilities, Pe and Ph, thus:
J = gE = q{n\i^ + p\if^ )E (Equation 4.1) The carrier concentrations can described as the sum of their respective thermal equilibrium carrier concentrations, no and po, and the excess carrier concentrations due to optical excitation, ôn and Ôp:
M = Mg + ôn p = Po + Ô/7 (Equation 4.2)
The dark current density, Jq, and illuminated current density, J,, are respectively:
Jq = ctoE = ^(«o^e + (Equation 4.3)
and
J. = G^E = ^ { ( « 0 + 5n)p^ + (po + 8p)p;, }E (Equation 4.4)
The photocurrent density, Jph, can be defined as the difference between the illuminated current density and the dark current density, i.e. Jph = Ji - Jq. If this is multiplied by the
cross-sectional area of the device the photocurrent, Iph, can be obtained; from Figure 4.7 it can be seen that the cross-sectional area is A = wt. Also, the electric field can be expressed as E = Vg/l, giving:
Iph = -Iph^ = + W J - T Vb
w/ ^ (Equation 4.5)
/
This photocurrent is spectrally dependent and it is this which is divided by the incident optical power to obtain the spectral responsivity, R, which is expressed as Amperes per Watt.
4.3.2 Photoconductive Gain
The number of photons arriving at the detector is the photon flux, O, multiplied by the optical area of the detector, Aq. If the thickness, t, of the semiconductor is large enough
such that there is complete absorption of the incident light then the photocurrent can also be expressed as:
iph = (Equation 4.6)
where g is the photoconductive gain and r\ is the quantum efficiency or number of electron-hole pairs created per incident photon.
In Figure 4.7 the optical area of the detector is the product wl. The photon flux can also be described as the optical power density, Po, divided by the energy per photon:
O = — = (Equation 4.7)
hv he
The number of electrons photogenerated per unit volume per second is pO /t and the number of electrons per unit volume per second that recombine is ôn/xj, where xi is the excess carrier lifetime. Therefore the rate equation for excess electron concentration in the sample is:
^ = (Equation 4.8)
dt t T , V M y
Under steady state conditions Equation 4.8 is equal to zero and hence the excess carrier lifetime is given by:
tbn
T, = —^ (Equation 4.9)
C hapter 4: Highly W avelength Selective Photodetector
Equating equations 4.5 and 4.6 gives the following expression for the photoconductive gain:
g = (Equation 4.10)
In most semiconductors the conductivity is dominated by electrons as and hence the term in brackets in Equation 4.10 would be approximated by ôn|4e. However, in diamond |4e~|4h, so a better approximation would be to make the two mobilities equal. Also, equal concentrations of carriers are generated, as each excitation creates an electron-hole pair, so ôn=ôp. Equation 4.10 then becomes:
lY s
ri/^O
g = 28%p,; f (Equation 4.11)
Substituting the expression for Ôn from Equation 4.9 into Equation 4.11 gives:
g = (Equation 4.12)
Under reasonable electric fields the carrier velocity is simply the product of the carrier mobility and the electric field, Ve=peE. Using the classical velocity, distance and time relationship we can define the carrier transit time, Tt, as the time taken for a carrier travel between the Ohmic contacts.
I I f
Tr = — = ---= --- (Equation 4.13)
This simplifies the expression, given in Equation 4.12, for the photoconductive gain to the ratio of the free carrier lifetime, Xi, to the carrier transit time, Tt:
g = 2 — (Equation 4.14)
The quantum efficiency, rj, cannot be greater than unity; a single photon cannot generate more than one electron-hole pair. However, the photoconductive gain, g, can be less than or greater than unity. If the carrier lifetime is much larger than the carrier transit time (i.e. Ti» Xt) then a single photogenerated carrier can effectively make
several transits across the device. When it reaches a contact it is immediately replaced by injection of an equivalent carrier at the opposite contact in order to maintain charge neutrality. This continues until recombination takes place.
The gain of the detector can be increased by either increasing the carrier lifetime or, more easily, decreasing the carrier transit time. From Equation 4.13 it can be seen that the carrier transit time can easily be decreased by either increasing the bias voltage or reducing the inter-electrode distance, 1.
4.4 Design Criteria
There is a need for a simple UV photodetector to complement the detectors currently available. Such a detector should exhibit a strong response to UV radiation whilst being relatively insensitive to visible wavelengths; otherwise known as being "visible blind". This would enable detection of UV radiation within an environment which also contains significant amounts of visible and/or IR radiation, without the need for any filtering of the radiation before it reaches the detector. A good detector should possess a sharp cut off wavelength. The ability to tailor this cut-off wavelength would be a desirable quality.
For ease of use with standard integrated circuits (ICs) a low operating bias voltage would be a requirement. This would remove the need for high voltage power supplies, as needed for vacuum photodiodes and photomultiplier tubes. A bias voltage, Vg, of 10 V or less would enable direct connection to CMOS integrated circuits, popular op-amps and timers, but would exclude TTL circuits.
The output signal of the detector should be easily detectable. This can be helped if the dark current of the device is small, e.g. of the order of picoamperes. The output should then be seen as a large relative change in the current through the device. A large dynamic range may be possible and the signal shouldn't be hidden by the dark current. Another reason that a small dark current would be useful is that the device would then have low power consumption in its off-state; an important consideration for battery- powered applications.
Chapter 4: Highly W avelength Selective Photodetector
If the detector is to be made commercially available then cost of the detector is also an important consideration. Hence the use of a commercially available substrate material would be a requirement.
Other more application-specific criteria exist, such as speed of response, lifetime stability, sensitivity etc. However the initial need is for a detector which satisfies the criteria detailed above. Other factors are given consideration elsewhere in this thesis. The criteria considered here are summarised below:
♦ Sharp cut-off wavelength, Xc, with negligible sensitivity to wavelengths longer than Xc
♦ Low bias voltage (Vb < lOV) ♦ Low dark current (Id - pA)
♦ Use of commercially available material
4.5 Device Design
The energy bandgap of diamond is -5.5 eV at room temperature. This means that photons with energy greater than 5.5 eV are required to excite electrons from the top of the valence band to the bottom of the conduction band, in order to become available for conduction. This energy corresponds to a wavelength of < 225 nm. In intrinsic material, wavelengths longer than 225 nm should not have a significant effect on the resistivity of diamond. Intrinsic diamond is highly resistive ( - GQ), meaning that a photoconductive structure would be suitable, as both the "sharp cut-off wavelength" and the "low dark current" criteria could be satisfied.
Use of intrinsic diamond and a photoconductive design would also remove the need for any doping, as would be required for photodiode structures; this is an advantage, as diamond has proved to be rather difficult to dope reliably. In fact, most dopants which have been found for diamond have rather high activation energies (Ea for boron acceptors is -0.37 eV and for nitrogen donors is -1.7 eV), meaning low ionisation of the dopants at room temperature. Hydrogenated surfaces appear to display p-type characteristics; a photodetector on such material is discussed in Chapter 8.
The material chosen was free-standing randomly-oriented polycrystalline diamond grown by microwave plasma assisted chemical vapour deposition (MPACVD). The diamond layer is grown on a silicon substrate which is removed as the diamond layer is strong enough to support its own weight. The diamond film used was of about 100 pm thickness. A scanning electron micrograph (SEM) of the film can be seen in Figure 4.8.
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Figure 4.8 Scanning electron micrograph of a cross-section of a CVD grown diamond film.
In Figure 4.8 it can be seen that at the nucléation side of the film (bottom edge in the figure) there is a very high density of very small grains, whilst at the growth side (upper edge in the figure) there are much fewer grains which are much larger. The average grain size on the growth side is of about 20-40 pm. It can also be seen that there is considerable surface roughness, with a peak-to-trough distance of 10-15 pm.
Raman spectroscopy has previously been carried out on this material. A red He-Ne excitation source was used and only a sharp peak at 1332 cm'^ was seen, with a very low featureless background. This indicates a high quality diamond structure and no graphitic regions; graphite gives a broad Raman peak centred at about 1560 cm '\
Pan et al. [1993] carried out mobility measurements on a 360 pm thick MPACVD diamond film and found that the mobility on the growth (large grain) surface was 50 times greater than on the nucléation (nanocrystalline) surface. Plano et al. [1994] compared the surface collection distance, ds, for planar electrode structures with the bulk collection distance, dy, for sandwich electrode structures on both 500 pm thick MPACVD and flame grown material. Their results showed that ds = 2db. Such results suggested that use of a planar structure on the growth surface of the diamond film would result in better device performance than a sandwich structure across the thickness _ _
Chapter 4: Highly Wavelength Selective Photodetector
of the film. Another consideration is that photons with energy just above the bandgap energy would be absorbed near the surface which is illuminated. Attempts to collect carriers at an electrode placed on the other surface of the film may not be very successful.
From the photoconductive theory it can be seen that one way of increasing the gain of a photoconductive device is to reduce the distance between the electrical contacts. This would reduce the carrier transit time, Tj, allowing a photogenerated carrier to make more passes through the detector during its lifetime, Xi, although a particular photocarrier does not actually make several transits.
As a polycrystalline film is being used here, grain boundaries have to be considered. Grain boundaries can contain dangling bonds, interstitials, vacancies and regions with possess short range order typical of all possible carbon phases. The term "non-diamond carbon" is often used to refer to such material. These may trap carriers and cause recombination, therefore reducing the gain of the detector. There are various ways in which grain boundaries can affect the electrical characteristics: carriers may get trapped at the grain boundaries, recombination may occur, or a path of lower resistance may cause a short circuit.
To minimise the loss of photogenerated carriers at grain boundaries it was decided that the electrode spacing should be matched to the mean crystallite dimensions on the growth surface of the film. This should achieve a "pseudo-single crystalline" effect, as illustrated in Figure 4.9. Carriers generated in some crystallites, such as those illustrated by the white outline arrows, will reach the electrodes before reaching a grain boundary.
I
E lectrodes
Figure 4.9 Schematic diagram demonstrating the principle of close electrode spacing to achieve pseudo-single crystalline carrier transport.
It would be impractical to fabricate electrodes across just one grain, so there will be areas which exhibit a "pseudo-single crystalline" nature and other regions which will be dominated by the grain boundaries. This may cause there to be "hot-spots" and "cold- spots" in the device's response to the UV radiation. In order to average out the effect of this and provide a reasonable detector performance, the detector should have a reasonably large area. An additional benefit of this is that the detector should exhibit a higher responsivity due to a larger number of photons being incident on the device. This may increase the dynamic range of response of the detector, allowing lower photon flux levels to be detected.
Choice of the device geometry should take into account the following requirements; a planar structure to be fabricated on the growth surface of the diamond film, electrode spacing should be matched to the mean crystallite dimensions, the device area should be reasonably large, and the active detector region should be maximised. An interdigitated transducer (IDT) structure was chosen, as illustrated in Figure 4.10. The structure has external dimensions of 2 mm x 2 mm, and consists of 15 pairs of electrodes of width 25 |4m and spacing 25 pm. This spacing has been chosen to match the mean crystallite grain size, whilst also taking into account the fact that the surface roughness of the diamond film may limit the resolution possible during the photolithography stage during fabrication of the device. Contact pads are provided to allow easy probing of the detector and also for wire bonding should the detector need to be packaged.
Figure 4.10 Interdigitated transducer (IDT) structure chosen for the photoconductor.
Chapter 4: Highly Wavelength Selective Photodetector
Gold was chosen for the contact metallisation because it makes an Ohmic contact to diamond. It is suitable for use with both etching and lift-off lithographic techniques. Only a single layer metallisation would be needed and it does not require annealing.
4.6 Device Realisation
Devices were fabricated on material whose polycrystalline nature can be seen in Figure 4.9. Photolithography was used to pattern the device structure; described in greater detail in Section 3.1.1.3. An etching process was used. The fabricated device can be seen in Figure 4.11. Good correlation between the fabricated structure and the device design used on the photolithography mask, as seen in Figure 4.11, can be seen.
f t
Figure 4.11 Interdigitated transducer photoconductor structure on diamond.
Care was taken to avoid the formation of any short circuits between the two electrodes; an example of such a defect can be seen in Figure 4.12. Any metallisation which makes contact to two electrodes of different polarity would render the device unusable as a path of low resistance would be made in parallel to the highly resistive path through the diamond film. Any incident radiation on the detector would not affect the current through the detector.
Short circuit
r y
^Figure 4.12 An example of an electrical short circuit on an IDT structure.
Although on the lithographic mask the finger-to-spacing ratio is 1:1 with both being of width 25 pm, it is unlikely that this will be the case on the fabricated device. As can be seen in Figure 4.12, the width of the fingers is likely to be less than the spacing width; however, the finger-to-finger distance will remain about 50 pm. The reason for this is that there is likely to be some developing of the unexposed photoresist and maybe some lateral etching of the metallisation under the photoresist. Due to the surface roughness of the polycrystalline diamond film the edges of the fingers are unlikely to be straight, as in Figure 4.10, but rougher as in Figure 4.12. The photoresist on the peaks of the crystallites is likely to be in close contact with the lithographic mask whereas the photoresist in the troughs between crystallites will not be.
4.7 Results
Electrical and spectral measurements were carried out on the as-fabricated device. The current-voltage characteristics are shown in Figure 4.13; the solid line shows the dark current and the dashed line shows the current measured with a standard incandescent light bulb shining on the device. The dark current is less than a picoampere for voltages up to ±10V; this is really below the resolution of the analysis equipment. This would indicate that the "low dark current" specification has been satisfied.
Chapter 4: Highly Wavelength Selective Photodetector 2 10-11 1 10-11 c t 3 o -1 10 -2 10 -11 -11 1---1---1---1---1---1---1---1---1---1---1---1--- r I I I I I I I I : I I -10 -5 0 5 10 Voltage (V)
Figure 4.13 1-V characteristics of a diamond photoconductor in the dark (solid line, along x-axis) and when illuminated with an incandescent light bulb (dashed line).
The spectral responsivity for the device (Figure 4.14) shows a responsivity of about O.lmA/W at 200nm and a low responsivity of about lO'^A/W for visible wavelengths. It can be seen that the "cut-off wavelength" criterion has been satisfied. This spectrum was taken for a bias voltage of lOV, satisfying the "low bias voltage" condition.
<
> o Q. CO 10 ' 100 200 300 400 500 600 700 800 900 Wavelength (nm)F igure 4.14 Spectral responsivity o f a diamond photoconductor. Bias voltage is lOV.
The roughness of the spectrum in the visible wavelengths is due to the small current levels being measured here; of the order of picoamperes. This is at the limit of the analysis equipment and any small amount of noise could significantly alter the current level measured for both the dark current and the illuminated current.
The external quantum efficiency (EQE, riext) can be described as the ratio of number of electrons in the circuit to the number of photons incident on the device. The number of