St. J. Dixon-Warren

Two-dimensional profiling of carriers in a buried heterostructure multi-quantum-well laser: Calibrated scanning spreading resistance microscopy and scanning capacitance microscopy

We report results of a scanning spreading resistance microscopy (SSRM) and scanning capacitance microscopy (SCM) study of the distribution of charge carriers inside multi-quantum-well (MQW) buried heterostructure (BH) lasers. We demonstrate that individual quantum-well–barrier layers can be resolved using high-resolution SSRM. Calibrated SSRM and SCM measurements were performed on the MQW BH laser structure, by utilizing known InP dopant staircase samples to calibrate the instrumentation. Doping concentrations derived from SSRM and SCM measurements were compared with the nominal values of both p- and n-doped regions in the MQW BH lasers. For n-type materials, the accuracy was bias dependent with SSRM, while for SCM, excellent quantitative agreement between measured and nominal dopant values was obtained. The SSRM was able to measure the dopant concentration in the p-type materials with ∼30% accuracy, but quantitative measurements could not be obtained with the SCM. Our results demonstrate the utility of c...

Scanning spreading resistance microscopy current transport studies on doped III–V semiconductors

Two-dimensional (2D) carrier concentration profiling using scanning spreading resistance microscopy (SSRM) has been carried out on molecular beam epitaxy-grown GaAs and InP dopant calibration samples. The current transport mechanisms between the diamond-coated SSRM tip and the III–V semiconductor cleaved surface (110) was investigated as a function of semiconductor dopant concentration via current–voltage (I–V) measurement. A positive or negative tip bias was applied while scanning over each dopant concentration region (1016–1019 cm−3). The results were compared to simulated I–V curves based on thermionic emission theory. The best fits to the data obtained under forward bias indicated that the contact barrier heights, φb, were much lower than expected from conventional large area planar contacts to GaAs or InP. The effect increases with increasing doping concentration, as a result of a combination of barrier height lowering mechanisms such as image forces, thermionic field emission and minority carrier in...

Calibrated scanning spreading resistance microscopy profiling of carriers in III–V structures

Two-dimensional carrier profiling using scanning spreading resistance microscopy (SSRM) has recently been reported for Si- and InP-based structures. In this article, we report SSRM measurements solely on III–V material-based structures. We have studied GaAs and InP doping staircase structures, prepared using molecular-beam epitaxy. These structures were then used as calibration standards for the profiling of carrier density in state-of-the-art III–V-based optoelectronic devices. We discovered that SSRM data on GaAs can be obtained with either polarity; however, only one polarity (positive or negative sample bias for n- or p-GaAs, respectively) produces SSRM results that show quantitative correlation with dopant concentration as determined by secondary ion mass spectrometry (SIMS). In comparison, SSRM measurements using both bias polarities on n-InP correlates well with SIMS, while p-InP exhibits a similar polarity dependence to p-type GaAs. A physical model based on a Schottky junction is proposed to expl...

Direct imaging of the depletion region of an InP p-n junction under bias using scanning voltage microscopy

We directly image an InP p–n junction depletion region under both forward and reverse bias using scanning voltage microscopy (SVM), a scanning probe microscopy (SPM) technique. The SVM results are compared to those obtained with scanning spreading resistance microscopy (SSRM) measurements under zero bias on the same sample. The SVM and SSRM data are shown to agree with the results of semiclassical calculations. The physical basis of the SVM measurement process is also discussed, and we show that the measured voltage is determined by the changes in the electrostatic potential and the carrier concentration at the SVM tip with and without the applied bias.

Electrical Scanning Probe Microscopy: Investigating the Inner Workings of Electronic and Optoelectronic Devices

Semiconductor electronic and optoelectronic devices such as transistors, lasers, modulators, and detectors are critical to the contemporary computing and communications infrastructure. These devices have been optimized for efficiency in power consumption and speed of response. There are gaps in the detailed understanding of the internal operation of these devices. Experimental electrical and optical methods have allowed comprehensive elaboration of input–output characteristics, but do not give spatially resolved information about currents, carriers, and potentials on the nanometer scale relevant to quantum heterostructure device operation. In response, electrical scanning probe techniques have been developed and deployed to observe experimentally, with nanometric spatial resolution, two-dimensional profiles of the electrical resistance, capacitance, potential, and free carrier distribution, within actively driven devices. Experimental configurations for the most prevalent electrical probing techniques based on atomic force microscopy are illustrated with considerations for practical implementation. Interpretation of the measured quantities are presented and calibrated, demonstrating that internal quantities of device operation can be uncovered. Several application areas are examined: spreading resistance and capacitance characterization of free carriers in III-V device structures; acquisition of electric potential and field distributions of semiconductor lasers, nanocrystals, and thin films; scanning voltage analysis on diode lasers—the direct observation of the internal manifestations of current blocking breakdown in a buried heterostructure laser, the effect of current spreading inside actively biased ridge waveguide lasers, anomalously high series resistance encountered in ridge lasers—as well as in CMOS transistors; and free-carrier measurement of working lasers with scanning differential spreading techniques. Applications to emerging fields of nanotechnology and nanoelectronics are suggested.

Two-dimensional transverse cross-section nanopotentiometry of actively driven buried-heterostructure multiple-quantum-well lasers

We report results of two-dimensional local potential measurement of the transverse cross-section of operating buried-heterostructure (BH) multiple-quantum-well lasers. The measured two-dimensional image of potential distribution resolved clearly the multiquantum-well active region and the p-n-p-n current-blocking structure of the BH laser, showing close correlation to the scanning spreading resistance microscopy image. Nanopotentiometry measurements were also performed on the p-n-p-n current-blocking structure of a BH laser under different forward bias voltages. The nanopotentiometry results provide direct insight into the behavior of p-n-p-n current-blocking layers intended to minimize current leakage. Our results demonstrate the application of nanopotentiometry to the delineation of complex buried structures in quantum optoelectronic devices.

Direct observation of lateral current spreading in ridge waveguide lasers using scanning voltage microscopy

We report results of two-dimensional (2D) local voltage measurement of the transverse cross section of operating multiquantum-well ridge-waveguide (RWG) lasers. We observe lateral nonuniformity of local voltage in the n-cladding layers of the laser and attribute the voltage variation to 2D carrier transport effect within the RWG lasers. The quantitative evaluation of this effect indicates the local vertical current density to be ∼40% smaller at the edge of the ridge than at its center. Our results demonstrate the strength and application of scanning voltage microscopy technique in quantitatively delineating 2D current flow in operating optoelectronic devices.

Scanning voltage microscopy on buried heterostructure multiquantum-well lasers: identification of a diode current leakage path

We report scanning voltage microscopy (SVM) results on actively driven buried heterostructure (BH) multiquantum-well (MQW) lasers that exhibit current blocking failure at high current injection operation. The measured two-dimensional image of local voltage distribution delineates the buried structures of the BH laser. The results, in combination with light-current-voltage (L-I-V) measurements, connect macroscopic external performance to measurements on the nanometer scale. Our experimental results suggest that the current blocking breakdown observed in the MQW BH lasers correlates with the turn-on of a diode leakage path when the devices are biased at high current injection.

Scanning voltage microscopy on active semiconductor lasers: the impact of doping profile near an epitaxial growth interface on series resistance

We apply scanning voltage microscopy to actively biased multiquantum-well ridge-waveguide semiconductor lasers. We localize the source of a major and hitherto unexplained sample-to-sample difference in current-voltage characteristics to the responsible junction. This is found to correspond to the regrowth interface, subsequently confirmed through secondary ion mass spectrometry to have different doping profiles in the two cases. By comparing the internal voltage profile of the operating lasers, we found that a voltage difference of 0.44 V occurred within /spl sim/100 nm of the regrowth interface in these laser structures, accounting for 88% of the difference in the measured series resistance. Additionally, 75% of the total device series resistance is associated with the structures heterobarriers. These results relate nanoscopic measurements to macroscopic performance and are of significance in improving device understanding, design, and reliability.

Scanning differential spreading resistance microscopy on actively driven buried heterostructure multiquantum-well lasers

We have developed a new scanning probe microscopy-based technique, scanning differential spreading resistance microscopy (SDSRM), which enables the determination of free carrier distribution inside operating electronic and optoelectronic devices. The results of our SDSRM study of multiquantum-well (MQW) buried heterostructure (BH) lasers under zero and forward biases are reported. Individual QW-barrier layers can be resolved in high-resolution SDSRM. The SDSRM results show different internal carrier distribution within the MQW active region in BH lasers with and without biases and provide direct experimental evidence of electron overbarrier leakage. Our results demonstrate the utility of SDSRM to delineate quantitatively the transverse cross-sectional structure of complex two-dimensional devices such as MQW BH lasers under operating conditions, in which traditional probing such as secondary ion mass spectroscopy, scanning spreading resistance microscopy, and electron beam-induced current microscopy can either apply only to devices under zero bias or provide only qualitative pictures.