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 Introduction. The purpose of the investigation.
A wide set of modern electronic and light-emitting devices, e.g. transistors or injection laser diodes, is based on the semiconductor heterostructures.
These heterostructures contain various potential barriers- n+-n, n-p junctions etc.- whose position should be known with a high precision relative to the nanometer-thin layers the heterostructure may consist of. Thus, to characterize the heterostructures constituting the base elements of the devices or the whole devices, one should measure the thickness of heterolayers and determine the localization of the potential barriers within the heterostructure. These objectives may be reached via SPM studies on the structure cleavages.
What is attractive and of paramount importance, the SPM studies can be carried out not only under equilibrium conditions, but also under applied external bias, when high current flows though the operating devices. In this note we present several examples of the semiconductor device heterostructures characterization in different SPM modes with the use of NT-MDT tools
Getting ready the measurements. Sample preparation.
The semiconductor heterostructures are usually epitaxially grown on thick substrates (a few hundredths of microns) and appear on the sample cleavages in the close proximity to their edges (1 or 2 microns or less). Optical viewing system of SPM device is necessary to position the tip of the SPM cantilever on the heterostructure layers of interest. The heterostructure samples of different semiconductor systems were studied: ZnSSe/GaAs, ZnBeMnSe/GaAs, GaAlAs/GaAs and GaInSbAs/GaSb. The samples with freshly prepared cleavages were griped in a miniature vice or fixed by a double-side scotch on the vertical side of the small block. In the case of GaAs-based laser diodes, the freshly cleaved rectangular resonators (typical sizes 100x300x1000 microns) were soldered on In-coated heat sinks. If necessarily, an external bias was applied between p- and n-contacts of laser diodes, both external power supply and SPM setup having common earth. A crucial point for successful SPM studies of heterustructure samples is preparation of the mirror-like cleaved faces (for details see results on topography study of ZnSSe and ZnBeMgSe-based laser structures).
SPM mode choice.
Both contact and resonant modes were used for the topography acquisition. In many cases the surface relief of the heterostructure cleavage contains sufficient information to measure the thickness of individual layers and the positions of the heterointerfaces. Additional data of the heterostructure inspection with high lateral resolution permitted the usage of the local elasticity mode (force modulation technique). To directly demonstrate chemical contrast of the constituent layers we applied lateral force imaging. The information of practical significance for the device structures- electrostatic potential distributions - was obtained by the combination of the above-described approaches with two pass techniques such as SKM and EFM. A separate attention, from the vantage point to grade up the instrumental function in electrostatic measurements, was given to the EFM data in contact mode obtained by the measurements of vibration amplitude of electrostatically excited cantilever (force modulation technique).
Cantilever selection.
The majority of results were obtained by contact cantilevers CSC12. These soft sensors have rectangular shape and long (~15 micron) tip and are made of highly doped Si of p-type conductivity. Rectangular shape and small force constant of these cantilevers make them excellent candidates for friction measurements, while slim, long and conductive tip permits to detect electrostatic forces with minimal influence of the parasitic capacitance of the tip side and of the flat part of the cantilever. When applying two pass techniques such as SKM and EFM, we used resonant cantilevers NSG11/Pt of similar design.
Special conditions of measurements.
To suppress 50 Hz interference, when measuring the potential distributions by SKM or EFM with applied to the device contact biases, an external power supply and SPM should have common earth. To avoid coupling between topography relief and SKM or EFM signals the cleavage surface of the studied device should be almost atomically flat. Studying an operating light emitting device when high current flows through a sample, the cantilever may get warm, that results in the shift of the resonant frequency. One should wait until new resonance is stabilized.
Measurements realization.
Optical viewing system of SPM device should be used to position the tip of the SPM cantilever on the heterostructure layers of interest.
On the very edge of the cleavage the cantilever tip can be easily damaged due to large height variation in surface relief. To save the tip and time, one should choose a scan frame with one side almost parallel to the edge and use slow scan rates (about one micron per second).
To study the potential distributions on the cleavage of device heterostructure, first the SKM mode should be applied with its ability to measure absolute value of the contact potential difference (CPD). Then, to determine the individual peculiarities in the CPD profile with better lateral resolution, a non-contact EFM may be used.
Obtained results: analysis, processing, representation and application.
The obtained data on topography, lateral force and local elasticity studies illustrate wide abilities of SPM for layer thickness characterization in semiconductor heterostructures. We also present results that permit to consider SKM and EFM modes as powerful tools for the investigation of the electrostatic potential distributions in the layered semiconductor devices. More detailed discussion of the obtained results can be found in scan descriptions.
Acknowledgments.
We are very grateful to our colleagues Dr V.P. Evtikhiev, Dr S.V. Ivanov and Dr V.V. Lundin from A. F. Ioffe Institute, as well as Dr H. Riechert from Infineon Technologies AG (Germany) for providing the samples for the investigations. We would particularly mention Dr V.P. Evtikhiev who has, by his discussions about the operation of high power laser diodes, maintained our interest in the subject of the presented work.
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