# Presentation: 146984

## Identification of cation vacancies and complexes in AlGaN alloys

***Identification of cation vacancies and complexes in AlGaN alloys***

-  High quality and highly conductive AlxGa1-xN alloys with x > 0.5 are needed to achieve optoelectronic devices active in the deep ultraviolet region ( <  300 nm). Achieving highly conductive AlxGa1-xN alloys with high Al content has been very challenging. It is known that the presence of cation vacancies in AlxGa1-xN reduces n-type conductivity because cation vacancies act as compensating centers (or deep level acceptors) for n-type dopants.  However, identification of cation vacancies in AlGaN has been difficult.
-  By employing deep ultraviolet (UV) photoluminescence (PL) spectroscopy, we have identified two different deep level acceptors in AlxGa1-xN alloys: isolated cation vacancies (Vcation) with 3-negatively charged state and Vcation-complexes with 2-negatively charged state. These acceptor levels are pinned to two different energy levels universal to AlxGa1-xN alloys (0  x 1). The presence of prominent cation vacancy related emission peaks in Al-rich AlxGa1-xN alloys directly translates to reduced conductivities and thus the UV PL technique can be utilized for material and layer structural optimization.
- Much work remains to be done to enhance  p-type doping efficiency in Al-rich AlGaN alloys.

- Figure description: Deep acceptor (cation vacancies) energy levels (EA) as functions of x in AlxGa1-xN alloys, obtained from PL measurement. Ec and Ev are also included with ED0 being the shallow donor level. The valence band maximum of GaN is chosen as E=0. The well known yellow-line (YL) in GaN and violet-line (VL) in AlN are also indicated.

**Notes:**
Aim of the project:
Al-rich AlxGa1-xN alloys (x > 0.5), covering wavelengths from 300 to 200 nm, are ideal materials for the development of chip-scale UV light sources/sensors, because AlGaN is the only ultra-wide-bandgap semiconductor system in which the bandgap can be easily engineered through the use of alloying and heterostructure design. However, achieving highly conductive n-type and p-type AlxGa1-xN alloys with high Al contents (x > 0.6) is essential for the realization of high performance practical devices that are active in the deep UV region, since these materials are needed for current injection. There is an urgent need to develop new approaches to further improve material quality with reduced dislocation density and enhanced doping efficiency. Our NSF supported research program aims at addressing the aforementioned issues.  

Research results:
Achieving highly conductive AlxGa1-xN alloys with high Al contents is known to be very challenging. Prior to 2002, only insulating AlxGa1-xN (x > 0.5) could be obtained. It is known that the formation energy of cation vacancies decreases with increasing x in AlxGa1-xN and the presence of these cation vacancies AlxGa1-xN directly affect the ability for conductivity control because cation vacancies act as compensating centers or deep level acceptors.  Thus it is imperative to develop effective approaches to identify the presence of these compensating centers for materials grown under different conditions and hence methods to minimize their presence.  We have grown AlxGa1-xN epilayers of the entire alloy compositions by metal organic chemical vapor deposition (MOCVD) and have systematically investigated the impurity transitions by deep UV (down to 195 nm) picosecond time-resolved photoluminescence (PL) spectroscopy.  Two groups of deep impurity transitions were observed in AlxGa1-xN epilayers. The origins of two different acceptors involved in the deep impurity transitions have been identified as cation vacancy complex with 2-negatively charged state (Vcation-complex)2- in AlxGa1-xN alloys between x=0 and 1 and as isolated cation vacancies with 3-negatively charged state (Vcation)3- in AlxGa1-xN alloys between x=0.58 and 1. The well-known yellow-line (YL) and violet-line (VL) in GaN and AlN have been identified as special cases of these impurity transitions in AlGaN alloys. Our experimental results indicate that the formation of (Vcation)3- is more favorable in Al-rich AlGaN alloys, while (Vcation-complex)2- are more favorable in AlxGa1-xN alloys with lower x.  The two acceptor levels related with (Vcation)3- and (Vcation-complex)2- are pinned to the energy levels which are common to AlxGa1-xN alloys with an energy separation of 0.5 eV. The presence of Vcation related emission peaks in AlxGa1-xN alloys leads to reduced  conductivities and are detrimental to optoelectronic devices using AlxGa1-xN epilayers. Thus by monitoring/minimizing Vcation related emission peaks in AlGaN, optimal growth conditions and layer structures can be obtained. [Appl. Phys. Lett. 86, 222108 (2005) & 86, 261902 (2005).] 

In the next phase, we will focus on tackling the p-type doping issues in Al-rich AlGaN alloys by exploring new mechanisms such as tunneling, Mg-doped AlGaN supperlattice, and bandgap renormalization to further enhance hole conduction.

Significance of this work:
Efficient solid-state UV light sources/sensors are crucial in many fields of research and development. For instance, the 280 nm emission matches the absorption maximum of the naturally fluorescent amino acid tryptophan and can be used as a probe of protein structure and dynamics and hence its state of health. Thus, the availability of chip-scale UV light sources is expected to open up new opportunities for medical research and health care.  Solid-state UV light sources also have applications in water and air purification, equipment/personnel decontamination, and white light generation.  By identifying and gaining greater understanding of fundamental features of crystal growth and defect formation in AlGaN alloys we have been able to incorporate improved AlGaN layers into 280 nm deep UV light emitting diode structures, thereby achieving operational devices which so far are among the best in the world. 

## The P.I. (Lin) of this project serves as a faculty advisor for the Woman in Engineering and Science Program (WESP) at Kansas State University.  WESP provides outreach, recruitment, and programs for girls and women from middle school through post-graduate levels, to encourage them to consider and persist in careers in science, mathematics and engineering. Prof. Lin conducts demonstrations on LEDs, lasers and optical effects in local schools several times a year in order to reach out to the broader community.  The PI’s lab is a “hot” spot for illustrating “Science in Action” to local students.  She also participates in a yearly university open-house  demonstrating progress in III-nitride photonic devices through NSF supported materials research.   

- The P.I. (Lin) of this project serves as a faculty advisor for the Woman in Engineering and Science Program (WESP) at Kansas State University.  WESP provides outreach, recruitment, and programs for girls and women from middle school through post-graduate levels, to encourage them to consider and persist in careers in science, mathematics and engineering. Prof. Lin conducts demonstrations on LEDs, lasers and optical effects in local schools several times a year in order to reach out to the broader community.  The PI’s lab is a “hot” spot for illustrating “Science in Action” to local students.  She also participates in a yearly university open-house  demonstrating progress in III-nitride photonic devices through NSF supported materials research.

**Reaching out to local school kids**

**Figure description: *****“It’s so cool!  How did you do that?”***** Visitors from a local school observing a blue light emitting “wild-cat” - Kansas State University logo fabricated in the P.I.’s lab.  **