1 Introduction 2 The Haynes-Shockley Experiment

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( ) I01 A 0 = log . (44) I 02 Then, from expressions (41) and (43), α = 2.3 (A − A 0) (d 2 − d 1 ) . (45) - really need the schematic/principles of operation, because I think the taking of the baseline measurement involves the calculation and removal of I 01 I 02 in software, so that the quoted A is the (A − A 0 ) from the original notes... Have just transcribed them for now, until I can get hold of a proper manual for the machine. - don’t forget to talk about what an absorption edge actually *is*!! theory theory theory... all the stuff about the differnet thicknesses and linking A to alpha - yes, you do have to do different thicknesses because you have to be able to cancel off the reflection term from the simplified T equation! 3.4 Taking Spectra For this experiment you will be using the Varian 2215 UV-Vis Spectrophotometer. There are two main modes - an automatic mode, which will allow you to take spectra relatively quickly, and a manual mode, for when you want to take high resolution measurements. You will need to turn on the machine 5 to 10 minutes before you begin taking spectra. The switch is on the front left-hand side, next to the chart recorder. The spectrophotometer will run through a series of self-tests before the default screen (showing the instrument settings and the current absorbance measurement) appears. 3.4.1 Buttons Beneath the Screen! Menus are accessed via a series of buttons (eg Instrument Settings). Once you’re in a menu, you can change particular settings by pressing the number for that item on the menu (eg on the Instrument Settings screen, press 4 to change the chart resolution) and then pressing Enter. Most of the menus won’t concern you, but you should find the following basic controls (starting top left of console): 1. RAPID: ABSORBANCE vs WAVELENGTH 2. ADVANCED: INSTRUMENT SETTINGS 3. ENTER (bottom left of the numbers) 4. START, STOP (under the numbers) 5. BASELINE SETUP 6. GO TO WAVELENGTH 7. AUTOMATIC BALANCE 10
3.4.2 Loading Samples On the right of the machine there are two lids which you can remove to give you access to the sample chamber. Removing either of these lids results in a shutter automatically blocking the beam, but you should make sure you know where the beam path travels through the chamber. You will find a sample holder which contains 6 “turrets” (ie spots into which samples can be placed) - the holder can be rotated using the knob on the outside of the machine to bring particular samples into the beam path without opening the chamber, but this should only be done with care. Your demonstrator will show you how to mount your samples securely. 3.4.3 Low Resolution Spectra In the first instance (see section 3.5 below) you will be interested in taking spectra over a wide range of wavelengths in order to ascertain the position of the absorption edge for your particular sample. You can do this by using the “RAPID” mode. (i) Press “ABSORBANCE vs WAVELENGTH” (ii) Enter upper wavelength (e.g. 890 nm) (iii) Enter lower wavelength (e.g. 210 nm). Note that this machine claims to be able to scan down to 180 nm, but in fact it will be out of range (and will start complaining!) once you get below 200 nm. (iv) At this point the machine should ask you to take a “baseline” measurement - this is a measurement of the absorbance of a standard (in your case, the absorbance of air) over the specified wavelength range. The machine automatically subtracts this background from the measurements of your sample’s absorbance. It should just be a case of pressing “BASELINE SETUP”, and then “START”. (v) Rotate the sample holder (it may be safest to do this manually...) so that your sample is in the beam path. Ideally at this point you should zero your detector. The spectrophotometer should be at the high wavelength end (i.e. far from the absorption edge of the sample) where you would not expect it to absorb. Press “AUTOMATIC BALANCE”. Note that a couple of the samples are rather opaque, in which case the spectrophotometer will probably complain when you try to do this part of the process. Don’t worry too much. All that will happen is that there will be a constant offset introduced into your data, which shouldn’t ultimately affect your analysis. (vi) The spectrophotometer should now be showing the first screen (i.e. the screen from step (i) above) again - press “ABSORBANCE vs WAVELENGTH” if it isn’t. Now, press “START” to begin recording. 3.4.4 High Resolution Spectra The low resolution scan’s main purpose is to allow you to identify the wavelength range over which your samples absorb strongly. Once you have done so, you will need to take data at higher resolution in order to characterise the absorption process. Although for this section it is possible to simply record a series of charts and read the data from them, you will get more precise data if you digitise your results manually - that is, if you do the scan slowly, stopping and starting it and writing down the absorbance at a series of (well known) wavelengths. 11
Page 1 and 2: 1 Introduction PART III LABORATORIE
Page 3 and 4: D e = kT e µ e (9) and D h = kT e
Page 5 and 6: x f − x i = v d (t pf − t pi )
Page 7 and 8: (ii) valence band (iii) conduction
Page 9: By monitoring the absorption coeffi
Page 13: You can find a rough value for the

1 Introduction 2 The Haynes-Shockley Experiment

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