PRODUCTOS SUSTITUTOS DE LA CARNE

3.2.1 Photoconductivity

Photoconductivity is the process in which the absorption o f a suitably energetic photon o f wavelength A, by a semiconductor o f band gap Eg results in the creation o f an electron-hole pair, thereby increasing the conductivity o f the sample for the lifetime of the carriersT/ due to the increase in n andp in equation (2.4) [3.11]. Absorption can be

Chapter 3: The Applications o f Diam ond

either intrinsic, involving the excitation o f an electron across the bandgap, or extrinsic involving one or more impurity states within the bandgap as indicated in figure (2.6). For a bandgap expressed in eV , the low energy cut-off wavelength Xc for intrinsic photoconductivity is given in nanometers by:

where h is the Planck constant and c is the speed o f light. For diamond having a bandgap o f =5.5eV (§2.4.1) this corresponds to a threshold wavelength o f «225nm . If

Xi > Xc then the incident photons will be insufficiently energetic to excite an electron

into the conduction band and will pass unabsorbed through the crystal, hence diamond's optical transparency at visible wavelengths. If Xj < Xc then the photons will be absorbed strongly and photoconductivity w ill occur with the absorption being described at its simplest by Beer's law [3.12]:

/(%) = 4 e x p ( - m : ) (3.2)

where I(x) is the intensity o f light at a given depth x beneath the surface o f a material with absorption coefficient a when Iq is the intensity of light on that surface.

3.2.2 Photoconductivity in Diamond

The modem study o f photoconductivity in natural diamond was initiated by Nahum and Halperin in the early 1960's [3.4]. Analysis o f a stone described as 'intermediate', meaning that it had properties between types I and II, yielded a strong photoconductive response with a threshold o f «225nm corresponding to intrinsic absorption. Sub- bandgap photoconductive peaks were also observed and found to be strongly temperature dependent, appearing in spectra obtained at 300K but not at 80K. This characteristic was attributed to indirect transitions involving phonon interaction and thermally ionised carriers from states within the bandgap.

D enham et. al. [3 .5 ] in v estig a ted the in flu en ce o f nitrogen content on photoconductivity. The low temperature (90K) photocurrent spectra for samples of types la, Ilb and 'intermediate' are reproduced as figure (3.1). A single, strong photoconductivity threshold corresponding to the bandgap is evident for the low nitrogen type lib sample as expected, whilst the two nitrogen containing stones exhibit a considerable sub-bandgap response. The low energy threshold o f the nitrogen related feature occurs at =300nm, from which it may be deduced that the dominant impurity 55

Chapter 3: The Applications o f Diamond

based a b so rp tio n m ech an ism is likely to be the p resen ce o f single sub stitu tio n al 'C c e n tre s (§ 2 .5 .2 .4 ). A d d itio n a l stu d ie s h av e in v e s tig a te d th e p h o to c o n d u c tiv ity associated w ith the population o f states by longer w avelength light in the range 3.5pm - 350nm [3.6, 3.13, 3.14]. P hotoconductivity spectra exhibiting sharp sub-bandgap peaks in the range 440 -3 6 4 n m have been w idely reported for sam ples fo llo w in g high energy electron irrad iatio n [3.15, 3.16, 3.17] and is believed to be cau sed by the presence o f vacancies. lOOi 5 0 W 5 0 Type l a 5 0 4 0 SO E N E R G Y c V 6 0

Figure 3.1: The photoconductivity spectra of three natural diamonds having differing nitrogen contents (a) Type lib (b) 'Intermediate' (c) Type la. [Reproduced from 3.5].

T h e s p e c tra l p h o to c o n d u c tiv ity o f fre e s ta n d in g 1 9 0 -5 0 0 p m th ic k M P A C V D p o ly c ry sta llin e film s from fo u r in d ep en d en t sources has been stu d ied by A llers and C ollins [3.18] w ho identified a defect believed to be un iq u e to C V D m aterial w hich appeared as a broad band in the infra red at ~ 825nm (1.5eV ). A sim ilar observation was rep o rted for 1 0pm thick film s by G onon et. al. [3.19, 3.20] w ho rec o rd e d peaks at 1127nm ( l . l e V ) , 886nm (1.4eV ) and 653nm (1.9eV ) and a ttrib u ted th e low energy response to acceptor states due to the grain boundaries and the h ighest energy feature to acceptor states in the bulk diam ond.

Chapter 3: The Applications o f Diamond

In a study o f 15|im thick M P A C V D m aterial V aitkus et. al. [3.21] observed a m arked increase in the su b -b a n d g a p p h o to conductivity follow ing ex p o su re to deep ultraviolet radiation from a d euterium lam p. T he ultraviolet p hotoresponse current, reproduced as figure (3.2) w as fo u n d to evolve slow ly, taking alm ost l'/2 hours to approach saturation in the 'on' condition. A fter rem oval o f the ultraviolet source the current decayed slow ly to approxim ately 10% o f the saturated value and rem ained at that level during exposure to sub-bandgap (5 50nm ) green light. T he state o f p ersistent conductivity w as 'reset' by ex p o su re to a b rig h t w hite light. A sim ilar co n dition o f p e rsiste n t con d u ctiv ity was observed by G onon et. al. [3.22] and attributed to electron trapping at an ionised donor level 1.9eV below th e conduction band.

UV ILLUMINATION (1.5 W/cm WL OFF < _{UV OFF} o t H- Z LU c r oc ZD O -1 O) o — I WL ON

GREEN LIGHT PULSES

UV ON

(550 nm, 500 ^W/cm

1

0 5 0 0 0 10 0 0 0 1 5 0 0 0