PROCESOS ESTRUCTURALES APRENDERES

The reflectance spectra o f D and E are shown in the graph o f fig. 8.8. The spectra for structure D

shows resonance at a wavelength o f 1748nm which is not very clear fi’om the fig. 8 . 8 and is more

discernible in the photocurrent spectra v^ ich is shown later in the chapter. The reflectance at this wavelength is —30%. At the wavelength for vdiich this structure was designed for, X=1680nm, the reflectance is —58%. For structure E, the resonance minima occurs at a wavelength o f 1650nm which is closer to the value desired for the operating wavelength and the reflectance at resonance

is -8%. At a wavelength o f 1680nm, the reflectance for E is measured to be -39% .

E exhibits a lower reflectance than D and this is due to the fact that both have different values o f cavity thickness. This in turn changes the point at which resonance will occur and also the value o f the reflectance due to the fact that the absorption coefficient o f the material used in the cavity

CIIAI^TER .S’ O p tic a l R e su lts: R C E R h o to d e te c to r

region varies witli wavelength D has a resonance wavelength greater tlian tlie corresponding wavelength for the bandgap o f GaSb [Eg = 0.72eV, A,~l 73pm] It was seen in chapter 2 that the absorption coefficient drops rapidly when the wavelength exceeds the wavelength corresponding to the bandgap energy o f the semiconductor. Since D shows resonance at 1748nm, then the absorption coefficient will be decreased. The structure was designed to have minimum reflectance at X,= 1680nm which corresponds to a=5000cm ‘’ and the cavity thickness was designed to be some multiple o f À/2n. This is not tlie case with structure D, instead tlie cavity is some multiple o f X,/2n for A,= 1748mn This then requires a larger front mirror in order to match the front and

back mirrors as tlie value o f a has decreased at this longer wavelength

The situation, as far as minimisation o f the structure reflectance is concerned, is better in tlie case o f structure E where the reflectance is lower. The wavelength is still not that which was designed for, but has now shifted to the left o f 1680nm to ^=1650nm This corresponds to an increase in the absorption coefficient from the value o f a at X.= 1680nm Tlie reflectance is low because the mirrors are more closely matched at 1650nm despite both having approximately the same

reflectivity as D Tlie only parameters which have changed are a and the cavity tliickness. Tlie

cavity tliickness now corresponds to some multiple o f l/2 n where X= 1650nni.

I 0.6 S

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Figure 8.8Reflectamce spectra of samples D and E /resonant cavity de\’icesj showing, the different wavelengths at which resonance occurs

Fig 8 9 plots the measured reflectance spectra o f both D and E as well as tlie measured Bragg stack on which these cavities were grown. As can be seen the reflectance o f the Bragg stack is still high over the resonance wavelengths corresponding to D and E. Hence the back mirror

C lf.iP T E R S O p tic a l Results: R(. 'E P h o to J e tec to r

reflectivity for wavelengths at which resonance occurs for structures D and E will still be constant 10 OK 0 4 0 2 0 ,0 I— 1400 1500 lAXi 1700 Wavelength |nni| 1900

F igu re 8.9 Reflectance spectra showing sam ples D and E and comparing these to the reflectance spectra of the Bragg reflector oJM 2

As was stated in table 8-1 tlie tliickness o f the GaSb absorber region was both measured and calculated from the reflectance spectra The calculated values for the epilayer thickness was

modelled using tlie multilayer model mentioned in botli chapters 5 and 6 Tlie best fit to D was

obtained by using a value o f cavity thickness equal to Q2 0nm for tlie cavity thickness and an

absorption coefficient o f I400cm '\ see fig. 8.10. This value o f absorption coefficient agreed witli measured values obtained on a GaSb epilayer In order to optimise the device for a wavelength o f I748nni, i.e. to achieve zero reflectivity, the front mirror reflectivity needs to be -75% . For a structure designed for operation at I680nm, the value o f the front mirror for tlie structure is -53 % which is much closer to the value for the air-seniiconductor interface

CHAPTER S O p tic a l R esults: R C E P h o to J etecto r D |c a k u la (e d | — — D (m caJiured) s

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F igu re Measured and modelled rejlecfaiice spectra f o r sample D For the modelled spectra the absorption coejjicient is assttmed to be NOOcm'' at a wavelength of 1748nm and

the cavity thickness is taken to be ~920nm

The best fit for structure E was obtained using a value o f cavity tliickness equal to 1470nm with a value o f the absorption coefficient o f ~ 5 4 0 0 cn f’ at X,= 1650nm, see fig 8.11 This also agrees well witli measured values obtained on low doped GaSb epilayers. Zero reflectance o f the

structure requires a front mirror reflectivity o f ~17% for a=5400cm '% Rb=08% and d=1470nm.

This low value o f the front mirror reflectivity means that the air-semiconductor reflectivity [~34%] needs to be reduced The use o f a dielectric mirror with a suitable refractive index would be an option to try and obtain tliis sort o f reflectivity.

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F igu re 8.11 M eastired and moiielled reflectance spectra f o r sample E. For a Jit to the measured data the absorption coejjicient is assumed to be 6000cm ' a t a wavelength o f

I650nm and the cavit\> thickness is taken to be ~1470nm

CH APTER 8 O p tic a l R esu lts: R C E P h o to d e te c to r