Ballistic electron emission spectroscopy of metals on GaP(110)

IBM T. J. Watson Research Center, Yorktown Heights, New York 10598
Journal of vacuum science & technology. B, Microelectronics and nanometer structures: processing, measurement, and phenomena: an official journal of the American Vacuum Society (Impact Factor: 1.36). 08/1991; DOI: 10.1116/1.585745
Source: IEEE Xplore

ABSTRACT Ballistic electron emission spectroscopy (BEES), a technique based on the scanning tunneling microscope (STM), was used to measure Schottky barrier heights of metals on cleaved n‐type GaP(110). The threshold voltages V 0 for current detection in the semiconductor were found to be uniform to within ±0.02 V over the sample surface for any given metal on GaP. A transport model for the current I c crossing the barrier, that includes both nonclassical transmission across the metal–semiconductor interface and electron scattering in the metal, yields I c ∝(V-V 0 )5/2 near threshold. The value of V 0 extracted from the data, which represents the Schottky barrier height, depends somewhat on the details of the transport model. Our best estimates of the Schottky barrier heights, within ±0.03 eV, are 1.07 (Mg), 1.11 (Ni), 1.14 (Bi), 1.25 (Cu), 1.31 (Ag), and 1.46 eV (Au).

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    ABSTRACT: Buried Al 0.4 Ga 0.6 As/GaAs superlattices on Au-GaAs Schottky diodes have been used as an energy filter to study the energetic current distribution in ballistic electron-emission microscopy BEEM at room temperature and T100 K. Due to the large difference in electron masses in Au and GaAs we find that parallel momentum conservation leads to considerable electron refraction at the Au-GaAs interface. As a consequence, the ener-getic distribution of the ballistic electron current is inverted beyond the Au-GaAs interface and an almost linear behavior of the BEEM spectrum is observed in the energetic regime of the superlattice miniband. S0163-18299851136-8 Ballistic electron-emission microscopy 1,2 BEEM is a useful tool to study the local properties of semiconductor interfaces and buried structures. BEEM is a three terminal extension of conventional scanning tunneling microscopy STM, where ballistic electrons are injected from a STM tip into a semiconductor via a thin metal base layer evaporated onto the sample. The corresponding ballistic electron current as a function of sample bias is called BEEM spectrum and is measured via a backside collector contact. In the BEEM spectrum, the onset bias of the BEEM current is only deter-mined by the Schottky barrier height at the metal semicon-ductor interface. The local resolution of these measurements can be as good as 10 Å. Originally, BEEM was only applied to determine metal-semiconductor Schottky barrier, heights and band-structure properties. 3,4 On GaP, e.g., the Schottky barrier heights for a large number of metals were determined by Ludeke, Pri-etsch, and Samsavar 5 and Prietsch and Ludeke. 6 Later, BEEM experiments were extended to higher bias voltages to study hot-electron effects such as impact ionization on GaP Ref. 7 and silicon. 8 In the following, even a BEEM current induced adatom generation was observed. 9 As such effects are beyond the validity of the original Bell-Kaiser model, 1,2 Monte Carlo calculations were used to analyze these data quantitatively. 10 After the Monte Carlo techniques worked well in the high bias regime, they were also applied to model the BEEM spectra in a voltage range close to threshold. Close to thresh-old, the Bell-Kaiser model 1,2 is usually a good description of the experimental data, however, it is not appropriate for samples on which elastic scattering in the metallic base plays a mayor role. This is of importance especially on the Au-Si 111 system, since here the electron needs a large transverse crystal momentum to enter the semiconductor. 11,12 It was also shown by Monte Carlo calculations that multiple elec-tron reflections inside the metal have a strong influence 13,14 and that elastic scattering processes are the reason why the spectra for Au on Si111 and Si100 look so similar. 15 In addition to surface properties, subsurface sample prop-erties were also investigated by BEEM. On a GaAs/Al x Ga 1x As double barrier structure, e.g., it was pos-sible to investigate the resonant states. 16 On self-assembled InAs quantum dots, 17,18 the BEEM current was found to be enhanced, and even fine structure in the BEEM spectrum was discovered and attributed to the quantized states inside the dot. Motivated by these results, Monte Carlo simulations of the BEEM spectra for such buried mesoscopic structures were carried out in the following experiments. 19 In our group, ballistic transport through the miniband of a GaAs-Al x Ga 1x As superlattice was studied recently. 20 A miniband in a short period superlattice covers a rather broad energy range compared to a double barrier resonant tunnel-ing diode and, therefore, electron transport through its states is more pronounced and better resolved. In our previous work, we have shown that the miniband results in a BEEM current threshold clearly below the height of the Al x Ga 1x As barriers and the peak in the second derivative of the BEEM spectrum was in good agreement with the calculated mini-band position in the GaAs-Al x Ga 1x As superlattice. In the present work, we analyze the spectral features of the mea-sured BEEM on Au-GaAs Schottky diodes with a buried GaAs-Al x Ga 1x As superlattice and show that due to parallel momentum conservation and electron refraction, the ener-getic distribution of the BEEM current is inverted beyond the Au-GaAs interface. To study the energetic distribution of ballistic electrons by means of a superlattice energy filter, a 10 period 25-Å/30-Å Al 0.4 Ga 0.6 As/GaAs superlattice SL was grown by molecu-lar beam epitaxy MBE on top of 600-Å undoped GaAs and a highly doped n-type collector region. To reduce the influ-ence of the interface, the SL was followed by 300-Å un-doped GaAs before finally capping it with an Au base layer. In order to provide ''flatband'' conditions at the Au/GaAs
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