# Presentation: 051940

## Nanoscale physics of nitride semiconductor heterostructures for electronic and optoelectronic devices

- Our principal research objective is to elucidate the relationships among nanoscale structural morphology, electronic properties, and device behavior in Group III-nitride semiconductor heterostructures.  To this end, proximal probe techniques offer unique capabilities for materials characterization at the nanometer scale.
- In collaboration with researchers at Emcore/Veeco, we investigated the nanoscale structure of InGaN/GaN quantum-well structures designed for application in visible light emitters, as shown in Figure 1.  Researchers at Emcore/Veeco observed that modification of epitaxial growth conditions (“improved” vs. “baseline”) in nominally identical quantum-well structures led to large increases in luminescence efficiency, as shown in Figure 2.
- Imaging by scanning capacitance microscopy (SCM), as shown in Figure 3, revealed the origin of the increased luminescence efficiency.  The “improved” structure exhibited highly localized carrier accumulation, evident as circular features in Figure 3(b), whereas carrier accumulation in the “baseline” structure was much more uniform, as shown in Figure 3(a).  Numerical simulations and nanoscale capacitance spectroscopy reveal that this carrier localization occurs in nanoscale In-rich clusters within the quantum-well region, leading to improved radiative recombination efficiency as localized carriers are shielded from defects in surrounding material.  These experiments provided direct experimental confirmation of a widely postulated correlation between In clustering and high luminescence efficiency in InGaN/GaN quantum well structures - a topic of considerable controversy within the community of researchers investigating nitride-based light emitters for displays, solid-state lighting, and other applications.

- Edward T. Yu	**University of California, San Diego**	DMR-0405851

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- [X. Zhou, E. T. Yu, D. I. Florescu, J. C. Ramer, D. S. Lee, S. M. Ting, and E. A. Armour, *Appl. Phys. Lett.* **86**, 202113 (2005).]

- Figure 1.  InGaN/GaN quantum-well structure for visible (green) LED, and experimental geometry for SCM measurement.

- Figure 2.  Room-temperature photoluminescence spectra, revealing increased luminescence efficiency in “improved” vs. “baseline” sample.

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- Figure 3.  SCM images of (a) “baseline” and (b) “improved” structures, revealing localized carrier accumulation in “improved” structure correlated with increased luminescence efficiency.

**Research**

**Notes:**
Research Objective:
Research under this program focuses on the investigation of nanoscale structural morphology and electronic properties in Group III-nitride semiconductor heterostructures, which are of intense current interest for a broad range of applications including high-power, high-temperature microwave electronics, and visible light emitters for displays and solid-state lighting.  Emphasis is placed on the application and advancement of scanning probe microscopies, which provide unique and powerful capabilities for characterization of structure and electronic properties at the nanoscale, and on the establishment of a comprehensive understanding spanning both fundamental material properties and their impact on device performance in nitride heterostructures.

Introduction:
Group III-nitride semiconductors have, due to their large energy band gap, prominent spontaneous and piezoelectric polarization effects, and related properties, been of intense current interest for application in visible light emitters and high-power microwave electronics.  However, the heteroepitaxial nature of most nitride semiconductor epitaxial growth leads to the presence in nearly all nitride epitaxial material of very high densities of defects, most notably threading dislocations, and other material inhomogeneities at nanometer and larger length scales.  The presence of such defects and inhomogeneities, combined with the strong coupling between structure and electronic properties via spontaneous and piezoelectric polarization fields, necessitates a comprehensive understanding of defects, their electrical behaviors, and their impact on various aspects of device performance.

Technical Approach:
Experimental investigations of these issues emphasize the use of various scanning probe characterization techniques, including atomic force microscopy (AFM), electrostatic force microscopy (EFM), scanning Kelvin probe force microscopy (SKPM), scanning capacitance microscopy (SCM), and conductive atomic force microscopy (cAFM).  Using these techniques individually and in combination, a comprehensive picture of the structure and electronic properties, at the nanoscale, of defects and material inhomogeneities can be developed.  The resulting knowledge and understanding is then combined with device-level studies and, in many cases, detailed computations and numerical simulations to develop a detailed understanding of the impact of nanoscale electronic behavior associated with such defects and material inhomogeneities on behavior of macroscopic devices, and in many cases to guide and inspire strategies for mitigation of these effects.

Results:
In the context of electronic devices, this approach has led to a detailed understanding of electrical properties (charge density and conductivity) associated with threading dislocations, from which important insights into the origin of leakage currents in Schottky (gate) contacts to nitride semiconductor heterostructures for field-effect transistors emerged.  On this basis of these insights, a suite of approaches based on processing and heterostructure design has been developed that allows gate leakage currents in transistors to be reduced by a factor of approximately 1000.  These advances have a major impact on power consumption, noise, and other properties of nitride-based transistors.  In the context of optoelectronic devices, SCM imaging and spectroscopy have been employed to observe and characterize monolayer fluctuations in InGaN/GaN quantum-well structures for visible light emitters via their effect on carrier accumulation behavior at the nanoscale.  In addition, SCM studies of InGaN/GaN quantum-well structures have recently revealed a direct connection between In clustering the quantum well, i.e., the formation of In-rich clusters in the InGaN quantum well, within which mobile carriers are localized, and increased luminescence efficiency (see slide 1).  Because features such as surface morphology and roughness were observed to be very similar for both “baseline” and “improved” sample structures, which were nominally identical in structure but exhibited large differences in luminescence efficiency, the ability to probe subsurface electronic and morphological structure of the InGaN quantum well by SCM was essential.  Carrier localization is observed directly as highly inhomogeneous contrast in SCM images obtained near the threshold voltage for electron accumulation in the quantum well (bright regions in image correspond to presence of electrons).  Spatially localized capacitance spectra, i.e., measurements of localized capacitance properties as a function of applied bias voltage, are combined with numerical simulations of capacitance-voltage profiles for varying In concentration and varying quantum-well and cap layer thickness to demonstrate that the SCM contrast shown in slide 1 can arise only from localized, nanoscale variations in In concentration, specifically the presence of In-rich clusters within which carriers preferentially accumulate.  By comparison of SCM contrast and luminescence spectra in “baseline” and “improved” samples, a direct correlation between the presence of In-rich clusters in the quantum well and increased luminescence efficiency is demonstrated.   This observation helps to confirm a widely postulated explanation for the high luminescence efficiency of nitride-based light emitters despite the presence of very high defect densities:  carrier localization within In-rich clusters allows carriers to be “shielded” from nearby defects that can act as non-radiative recombination centers, thereby increasing the radiative recombination efficiency.  The fact that formation of these clusters can be controlled via use of appropriate epitaxial growth conditions is a significant advance in the development of highly efficient nitride-based light emitters.

## Nanoscale physics of nitride semiconductor heterostructures for electronic and optoelectronic devices

- Our education and outreach efforts incorporate, among other activities, participation in a yearly offering of a Freshman Seminar in Electrical and Computer Engineering at UCSD.  Each year, the seminar focuses on different aspects of electronics, photonics, and related technologies and their impact on students’ everyday lives.  Shown at right is the PI teaching of a Freshman Seminar class in April 2006.  The topic for this lecture was “Why Size Matters: What is Nanotechnology and Why Is It Different?”
- Graduate students involved in this project are active participants as mentors to secondary school students from the UCSD Preuss School.  The Preuss School is a San Diego charter school targeting underprivileged, high-potential students from the San Diego Unified School District.  UCSD students act as mentors to Preuss School students, providing tutoring, academic and career advice, and guidance in extracurricular activities.  Shown at right is a graduate student from the PI’s laboratory, Sourobh Raychaudhuri, assisting a Preuss School student in his studies.

- Edward T. Yu	**University of California, San Diego**	DMR-0405851

- PI Edward Yu teaching a Freshman Seminar class at UCSD.

- Graduate student researcher Sourobh Raychaudhuri assisting a student at the UCSD Preuss School in his studies.

**Education and Outreach**