Measurements of contact potential differences and external voltage drop profiles.
The unique information of practical significance for the device structures may be obtained by the combination of the above-described approaches with two pass techniques such as SKM and EFM. We used for these modes Cantilevers NSG11/Pt. These sensors has slim and long tip that is optimal design decreasing the parasitic capacitance of the tip side and of the flat part of the cantilever.
In Fig. 1 AFM/SKM study of GaAlAs/GaAs laser diode is presented. Topography (left image) and SKM data in figure below were obtained under equilibrium conditions, when both n- and p- contacts of the laser diode were grounded (n-p junction of the device is closed). Data in fig.1c were taken under external positive bias applied to p-contact (n-p junction of the device is open). In topography image (see fig.1a) all the layers of the laser diode are resolved. There are two bright horizontal lines in topography image between n-GaAs substrate and n-GaAs buffer and in the middle of the undoped waveguide. The upper line corresponds to the peripheral thin GaAlAs layer used to adjust the growth parameters of the following Ga0.5Al0.5As emitters and Ga0.8Al0.2As waveguide. The lower line is compressively strained InGaAs QW.
SKM images in Fig. 1 permits to localize n-p junction as the place when the contact potential difference (CPD) undergoes the main changes. SKM data on Fig. 1 demonstrate unambiguously that the surface electric field of n-p junction is inverted when applying high forward biases. The bulk electric field must have, however, the same sign for any biases and only reduce its absolute magnitude under forward biases. To find quantitative correlation between obtained SKM data and potential distribution in the bulk, generally, one have to account for the profile of the surface band bending across the structure. Assuming that the surface band bending does not depend significantly on the bias conditions, it is possible to exclude the band bending contribution in CPD by subtraction equilibrium SKM data from SKM data taken under applied bias. As a result of this procedure, a surface voltage drop profile will be obtained that have to conform to the bulk voltage drop profile.
Fig. 1 Topography (a) and SKM data images of GaAlAs/GaAs laser diode (b - nonbiased, c - forvard biased).
Fig. 2 Surface voltage drop profiles.
In Fig. 2 the surface voltage drop profiles are displayed over the cleavage surface topography image of the studied device. Three experimental profiles obtained for 500, 800 and 1200 mV biases increase monotonously in the waveguide region, that is in a good agreement with bulk voltage drop modeling (for simulations one can use device simulator SimWindows available by internet). One should underline, however, that the measured surface voltage drops are about half of the applied biases. Besides, experimental profiles reveal a nonzero potential on the grounded n-substrate and some voltage drop in both highly doped emitters. These deviations from the expected picture of the voltage drop results from the convolution of the instrumental function with true voltage drop.
AFM images are obtained by commercial SPM P47H with CSC12 cantilevers.
On the next pages description of the used lateral force mode and the details of image obtaining are presented. For more information see also paper NN.
Additional information about laser structures were received with usage of Contact EFM, Kelvin mode.