Rudra S. Dhar

Direct Nanoscale Imaging of Evolving Electric Field Domains in Quantum Structures

The external performance of quantum optoelectronic devices is governed by the spatial profiles of electrons and potentials within the active regions of these devices. For example, in quantum cascade lasers (QCLs), the electric field domain (EFD) hypothesis posits that the potential distribution might be simultaneously spatially nonuniform and temporally unstable. Unfortunately, there exists no prior means of probing the inner potential profile directly. Here we report the nanoscale measured electric potential distribution inside operating QCLs by using scanning voltage microscopy at a cryogenic temperature. We prove that, per the EFD hypothesis, the multi-quantum-well active region is indeed divided into multiple sections having distinctly different electric fields. The electric field across these serially-stacked quantum cascade modules does not continuously increase in proportion to gradual increases in the applied device bias, but rather hops between discrete values that are related to tunneling resonances. We also report the evolution of EFDs, finding that an incremental change in device bias leads to a hopping-style shift in the EFD boundary – the higher electric field domain expands at least one module each step at the expense of the lower field domain within the active region.

Direct charge measurements to read back stored data in nonvolatile memory devices using scanning capacitance microscopy

This paper describes a methodology for reading back electrical charges from nonvolatile memory (NVM)-based flash devices. These devices were programmed to store charges in the floating gates of the transistors. The primary goal is to identify and read back these static charges in the form of logic levels of “1 bit (1b)” and “0 bit (0b)” without destroying the data. Scanning capacitance microscopy (SCM) with ∼15 nm spatial resolution was used to directly probe the data stored in the floating gate transistors. SCM measures the on-site programmed charges in flash memory devices as transistor charges of ON/OFF or 1b/0b. Qualitative variation of charge carriers has also been observed. Both the sample preparation and SCM probing methods are also discussed. An application has been demonstrated on a Texas Instruments based microcontroller unit with embedded 512 KB nor flash devices.

Two-dimensional profiling of carriers in terahertz quantum cascade lasers using calibrated scanning spreading resistance microscopy and scanning capacitance microscopy

The distribution of charge carriers inside the active region of a terahertz (THz) quantum cascade laser (QCL) has been measured with scanning spreading resistance microscopy (SSRM) and scanning capacitance microscopy (SCM). Individual quantum well‐barrier modules with a 35.7‐nm single module thickness in the active region of the device have been resolved for the first time using high‐resolution SSRM and SCM techniques at room temperature. SSRM and SCM measurements on the quantum well‐barrier structure were calibrated utilizing known GaAs dopant staircase samples. Doping concentrations derived from SSRM and SCM measurements were found to be in quantitative agreement with the designed average doping values of the n‐type active region in the terahertz quantum cascade laser. The secondary ion mass spectroscopy provides a partial picture of internal device parameters, and we have demonstrated with our results the efficacy of uniting calibrated SSRM and SCM to delineate quantitatively the transverse cross‐sectional structure of complex two‐dimensional terahertz quantum cascade laser devices.

Electrical scanning probe microscopy of electronic and photonic devices: connecting internal mechanisms with external measures

Abstract The inner workings of semiconductor electronic and photonic devices, such as dopants, free charge carriers, electric potential, and electric field, are playing a crucial role in the function and performance of the devices. Electrical scanning probe microscopy (SPM) techniques have been developed and deployed to measure, with nanometric spatial resolution and high quantitative accuracy, the two-dimensional profiles of dopant, potential, electric field, and free carrier distribution, within unbiased and/or operating electronic and photonic devices. In this review paper, we summarize our latest SPM experimental results, including the scanning spreading resistance microscopy and scanning capacitance microscopy of terahertz quantum cascade lasers, scanning capacitance microscopy of non-volatile memory devices, scanning voltage microscopy of terahertz quantum cascade lasers, and scanning voltage microscopy of interband cascade lasers. Interpretation of the measured quantities are presented and calibrated, demonstrating that important internal physical quantities and inner mechanisms of device operation can be uncovered. It reveals that the novel SPM techniques would find more applications to the emerging semiconductor quantum devices and nanoelectronics.

Scanning Voltage Microscopy for Emerging Electronic and Photonic Devices: Integrating nanotips with a single atom end for SVM

Scanning probe microscopy (SPM) techniques have been developed and deployed to delineate the internal characteristics of electronic and photonic devices such as doping profiles, electric potential profile, electric field profile, and charge carrier profile, as they play a crucial role in the function and performance of biased and unbiased devices. Two of the most promising techniques, scanning spreading resistance microscopy (SSRM) and scanning capacitance microscopy (SCM), are now commercially available for two-dimensional (2-D) dopant profiling by the microelectronic and optoelectronic industries, but the techniques delineate only devices at equilibrium. Therefore, to directly observe the internal behavior of operating quantum optoelectronic devices, a new SPM technique, scanning voltage microscopy (SVM), has been developed.

Nanoscopic voltage distribution of operating cascade laser devices in cryogenic temperature.

A nanoscopic exploratory measurement technique to measure voltage distribution across an operating semiconductor device in cryogenic temperature has been developed and established. The cross‐section surface of the terahertz (THz) quantum cascade laser (QCL) has been measured that resolves the voltage distribution at nanometer scales. The electric field dissemination across the active region of the device has been attained under the devices lasing conditions at cryogenic temperature of 77 K.

SSRM and SCM study for doping concentration of THz QCL devices

Two-dimensional (2D) dopant profiling of the active region of THz quantum cascade laser (QCL) devices has been achieved with atomic force microscopy (AFM). Scanning spreading resistance microscopy (SSRM) and scanning capacitance microscopy (SCM) are shown as the two promising AFM techniques for 2D dopant profiling and mapping of dopant concentration for the sub-nanometer regime devices.