Growth direction

1. Growth direction holes AlGaBeAs P GaMnAs p Control of Curie temperature in ferromagneticsemiconductor GaMnAs by modulation doping Margaret Dobrowolska, University of Notre Dame, DMR 0603752 It has long been believed that ferromagnetism of (III,Mn)V semiconductor alloys is described by the Zener model, which predicts that the Curie temperature in these systems scales with increasing hole concentration. Using modulation doped (AlGaBe)As/(GaMn)As structures, we have investigated the properties of (Ga,Mn)As when the hole concentration p increases while the concentration of Mn ions is kept constant. At small values of p the Curie temperature TC of (Ga,Mn)As is seen to increase with increasing p. However, as p continues to increase, this trend is reversed: at some point TC begins to decrease, and eventually the ferromagnetism of (Ga,Mn)As disappears altogether. Our results show unambiguously that the magnetic behavior of (III,Mn)V magnetic semiconductor alloys strongly depends on relative concentrations of holes and Mn ions; and that the Zener model ceases to apply when the hole concentration exceeds that of Mn.

2. Control of Curie temperature in ferromagneticsemiconductor GaMnAs by modulation doping Margaret Dobrowolska, University of Notre Dame,DMR 0603752 Education: Two undergraduate students: Mr. Kevin Miller (St. Bonaventure U.)and Mr. Thomas O’Brien (Notre Dame); and two grad students (Mr. Y.-J. Cho and Mr. Z. Ge) contributed to this work. Mr. Miller was an REU student in the summer of ‘07, and Mr. O’Brien is a Notre Dame undergraduate whom we employ part-time. Mr. Ge has now graduated and is a post-doc at Princeton; and Mr. Cho will graduate in the Fall of ’08. In addition, the Ph.D. research of Mr. D. Y. Shin and Mr. S. J. Chung -- bothgraduate students in Korea University, Seoul, Korea -- has directly benefited from this program. Societal Impact: Our group has been continually active as a resource of materials for other groups. We are currently interacting with at least 25 other institutions by providing them with magnetic semiconductor specimens for their research. The understanding of these materials and their heterostructures, as well as of devices based upon them developed in our laboratory, is thus automatically of benefit to our collaborators (including graduate students in their Ph.D. research), who depend not only on the specimens that we provide, but on the intellectual input from our group in the form of characterization and general understanding of the properties of these materials.