25nm

Dilute nitride semiconductors are promising for long-wavelength light-emitters and detectors, high-performance electronic devices, and high efficiency solar cells. For GaAsN and InGaAsN alloys, the electron mobility and optical emission efficiency decrease as the nitrogen incorporation increases. Conflicting results have been reported regarding the mechanisms of nitrogen incorporation, and the relationship between nitrogen incorporation mechanisms and the properties of InGaAsN alloys remains unknown. Schematics of possible nitrogen lattice sites and N-Si complexes are shown in (a) and (b). This project aims to understand and control the atomic to nanometer-scale structure of dilute nitride semiconductor alloys, in order to tailor the properties of heterostructures for a variety of applications.

FRG: Tailoring the Properties ofDilute Nitride Semiconductor Alloys

Rachel S. Goldman, University of Michigan Ann Arbor, DMR 0606406

(a)

(b)

FRG: Tailoring the Properties ofDilute Nitride Semiconductor Alloys

Rachel S. Goldman, University of Michigan Ann Arbor, DMR 0606406

We recently identified and quantified the origins of the transitions from layer-by-layer to multilayer growth during GaAsN alloy film growth. At the lowest growth temperatures, limited adatom surface mobility leads to layer-by-layer growth. As the growth temperature increases, the interplay between surface and step-edge diffusion leads to multilayer growth. At high growth temperatures, adatoms have sufficient kinetic energy to overcome the step-edge barrier, leading to layer-by-layer growth once again. Interestingly, the temperature range for multilayer growth is influenced by the Ga flux and the arsenic source, As2 or As4. The observed surface reconstructions are summarized on plots of growth rate versus temperature. The shaded regions indicate the conditions which lead to significant surface roughness, referred to as the “forbidden temperature window.”

350 450 550 350 450 550

0.4

1.2

0.8

0.4

0.8

1.2

Substrate Temperature (°C)

Growth Rate (m/hr)

(a)

(b) (c)As2/Ga=20 As2/Ga=10

As4/Ga=30

FRG: Tailoring the Properties ofDilute Nitride Semiconductor Alloys

Rachel S. Goldman, University of Michigan Ann Arbor, DMR 0606406

In silicon-doped dilute nitride alloys, it has been suggested that nearest-neighbor Si and N atoms form NAs

- SiGa complexes, which act as deep electron traps.

We compared the free carrier densities in Si-doped bulk-like GaAsN films, which contain both Si and N atoms, with modulation-doped GaAsN-based heterostructures, where Si and N atoms are spatially separated by several 10s of nm.

In (a), for bulk-like films, the decreasing free carrier density with increasing N composition indicates that N-related defects are acting as electron trapping centers. On the other hand, for the modulation-doped heterostructures shown in (b), the free carrier densities are apparently independent of N composition, suggesting that N atoms in the channel layers are not acting as charge traps. Since the electron trapping is observed in bulk-like films but not in the modulation-doped heterostructures, the dominant electron trapping mechanism is likely N-Si complexes.

F

ree

carr

ier

den

sity

(x10

18cm

-3)

N composition (%)0

00.5 1.0 1.5 2

0.5

GaAsN:Si Room TGaAsN:Si 77KGaAs:Si Room T

1.0

1.5

2.0

GaAs:Si 77K

Carr

ier

Den

sit

y (

x10

11 cm

-2)

AlGaAs/GaAs 2DEGAlGaAs/GaAsN 2DEG

0.5

1.0

2.0

1.5

Temperature(K)

2.5

1.5 4 300

(a)

(b)

International Collaborations:The project involves a collaboration

between U-Michigan, Notre Dame, and University College Cork. (The work of the Cork group is supported by the Research Frontiers Programme of the Science Foundation Ireland). To this end, we have had several joint meetings during the past year. For example, Prof. Stephen Fahy visited the University of Michigan as a Visiting Fellow in the Physics Department at the University of Michigan in March and December 2006. In December 2006, Prof. Fahy served on the PhD candidacy exam of Ms. Yu Jin, providing valuable comments about her work. In addition, Notre Dame University and the Tyndall National Institute, Cork, have established a formal visiting arrangement for staff and students between the two institutions.

FRG: Tailoring the Properties ofDilute Nitride Semiconductor Alloys

Rachel S. Goldman, University of Michigan Ann Arbor, DMR 0606406

On January 11th, 2007, a Michigan-Cork-Notre Dame team meeting was held at Notre Dame. During the visit, graduate students gave talks on their research.

Top Row: Yu Jin (UM), Kai Sun (ND), Matt Reason (UM), Cagliyan Kurdak (UM), Tassilo Danneker (UCC); Bottom Row: Alexander Mintairov (ND), Yaya Chu (ND), Yan He (ND), James L. Merz (ND)

Education:7 undergraduates (C.

Budziak, K. Garnett, M. Liu, N. Mangan, D. Mao, C. Proctor, and M. Warren), 9 graduate students (I. Bosa, J. Buckeridge, Y. Chu, T. Danneker, Y. He, J. Herzog, Y. Jin, K. Sun, and L. Webster), and 1 post-doctoral student (M. Reason) contributed to this work. The project is inclusive of under-represented groups, including 7 women students, one of whom is African-American.

Outreach:Activities include lectures to the general public

and involvement of 10 high school students in research during the summer of 2007. The 10 high school students were placed in 8 different laboratories throughout the College of Engineering at UM. The high school program is supported in part by NSF and UM. Professor Kurdak gave 2 lectures in Saturday Morning Physics at U-Michigan, “The Electronics Revolution” and “Future of Electronics,” in 2007.

FRG: Tailoring the Properties ofDilute Nitride Semiconductor Alloys

Rachel S. Goldman, University of Michigan Ann Arbor, DMR 0606406

H.S. Student Andrew Lee presenting poster “Effects of Annealing on the Electrical Properties of GaAsN” at Greenhills School on May 9, 2007.