# Presentation: 580632

## Performance Characteristics of a Spin-PolarizedField-Effect Transistor

-        One objective is to understand the structural properties of molecular beam epitaxy prepared Mn-doped GaAs dilute magnetic semiconductor. To the left are elemental concentration profiles determined with Auger. The samples were all prepared using a Mn cell temperature (TMn) set to 900 C.
-        The top figure shows the Mn concentration profile changes approximately linearly with depth for samples grown using a high substrate temperature (Ts=580C). This observation indicates that Mn is floating up during growth and not fully incorporating.  Notice the Ga concentration decreases in concert with the Mn increasing, indicating that Mn is taking Ga sites.
-         The bottom figure shows that at low substrate temperatures (Ts=250C), all the concentration profiles are uniform down to the substrate.  What is surprising, however is that this sample is almost 20% oxygen, indicating the sample is porous.

- Performance Characteristics of a Spin-PolarizedField-Effect Transistor

- Paul M. Thibado

- DMR-0405036
- Intellectual Merit

**Notes:**
Introduction
	Since the discovery of the ferromagnetic semiconductor of Mn doped GaAs (GaMnAs) by Ohno et al., it has been the subject of intense experimental and theoretical work. The combination of ferromagnetism with the versatile semiconducting properties makes it promising for future spintronics applications. GaMnAs can be described qualitatively as a random alloy in which Mn substitutes at Ga sites and takes the dual role of acceptor and local magnetic moment.

Experimental details
	Samples were prepared in an ultrahigh vacuum (~2×10-10 Torr) molecular beam epitaxy (MBE) growth chamber (Riber 32) which includes Ga and Mn effusion cells together with a two-zone As valved-cracker cell.  The MBE chamber is also equipped with a reflection high-energy electron diffraction (RHEED) system operating at 15 keV. Commercially available, “epi-ready” semi-insulating 2" diameter GaAs(001)±0.1o wafers were cleaved into quarters. One quarter was mounted on a 2" diameter standard MBE molybdenum block using indium as solder. The substrate was then loaded into the load-lock chamber without any chemical cleaning.
	After removing the oxide surface, the substrate temperature was set to the desired growth temperature of either 580 or 250 oC. GaMnAs films were then grown for 1 hr. while RHEED was used to monitor the surface reconstruction during and following the growth. After growth, the sample was cooled down at a rate of 1.5 oC/s to 400 oC while simultaneously ramping the As4 BEP to zero. The sample continued cooling at the same rate to 200 oC. It was then transferred to the load-lock chamber and removed from the UHV system. Samples were cleaved into multiple smaller pieces (10 mm × 10 mm) for characterization measurements. 
	The elemental composition profiles in the GaMnAs films were determined by using Auger electron spectroscopy (AES) combined with ion etching. The films were sputtered with an argon ion gun using a focused spot size of about 1 mm in diameter. The depths of the sputtered holes were determined by measuring the crater depth using a Tencor stylus profilometer. In order to accurately calibrate the sputter rate, a standard sample had to be prepared. For this a semi-insulating GaAs substrate was implanted with Mn. The ion implantation was performed with an Extrion 400 at a base pressure of 5 × 10-6 Torr. Using this implanted sample as a calibration standard, the relative sputter rate was determined. This calibration was used to correct the AES depth profiles obtained from the MBE grown samples.

Results 
	The elemental concentration (atomic percent) depth profiles in a GaMnAs thin film grown at a high substrate temperature (580 oC) is shown in the upper figure. Four elements, As, Ga, Mn, and O were found in the film. The percentage of As remains basically unchanged around 50% for all depths measured. The percentage of O drops to a low level near the surface and then stays at zero.  The percentage of Mn is highest at the surface (14%), then decreases to zero at some depth around 800 nm in the sample. This depth is the expected thickness of the thin film deposited on the substrate.  The percentage of Ga is only 31% at beginning and it increases steadily with depth until it reaches 50% around 800 nm. What is the most interesting is that the Mn and Ga concentrations change together with Mn going down, Ga going up, and the sum remaining constant near 50%.
	When the thin film is grown using a low substrate temperature (250 oC), the Auger elemental profile results are shown in the lower figure. All the elemental profiles are flat until a step at about 800 nm, which is the film-substrate interface. For this sample, we also observed four elements: As, Ga, O and Mn in the film, but the elemental percents and depth profiles are significantly different from those in the upper figure. First, the atomic percent of As in the thin film is around 43%, which is lower than that in the upper figure. The Ga concentration keeps a constant value of 34% in the entire epilayer. Once the GaAs substrate is reached, the atomic percents for Ga and As reach 50%, as expected. The Mn concentration is about 5%.  The primary finding is that the percentage of O in the thin film is about 18% and it stays constant all the way to the substrate.  This is surprising because our MBE chamber does not have an intentional source of oxygen inside.

Discussion
	The Mn concentration profile for the sample grown at high substrate temperatures is unusual (see upper figure).  It starts at zero near the substrate interface (about 800 nm) and grows linearly to about 14% at the film’s surface.  This happened even though the Mn atoms were deposited uniformly throughout the thin film.  If we calculate the area under the Mn concentration profile and divide this by the thickness of the sample, we find the average Mn concentration would be about 7%.  This number is close to what we expected to achieve.  It must be that some of the Mn deposited in the earlier planes is not staying in that plane but migrating up as the film grows thicker.  The phenomenon of floating along the growth front has been observed during the growth of GaInAs and MnGe.
	After analyzing all the atomic percentages for the sample grown at a low substrate temperature (see lower figure), the biggest surprise is the large percentage of oxygen in the thin film.  Equally surprising is that the oxygen concentration persists throughout the entire thin film but there is none in the substrate.  Given that there is no oxygen in our MBE growth chamber and the sample grown at 580 oC has no oxygen inside, we believe this oxygen must come from the air after the sample was removed from the vacuum chamber.  Previous studies of low-temperature grown GaAs have shown that a large number of defects exist in these films.  This results in a much lower atomic density in these films (e.g., ~80%-90%), when compared to those grown at the normal high substrate temperature.  We believe, due to a lower density, these films must be more porous and oxygen from the air must be able to easily diffuse into the thin film right down to the substrate.  In addition, since the oxygen does not diffuse out of the sample when reintroduced into the Auger vacuum chamber we believe the oxygen has reacted with the sample.  The most likely candidate for reacting with the oxygen is the Mn.

Conclusions
	GaMnAs films were grown using MBE at high and low substrate temperatures. For the sample grown at a high substrate temperature, Mn atoms seem to float along the growth front giving a graded concentration profile.  Samples grown at low substrate temperature give a uniform distribution for the Mn profile, however high levels of oxygen are present in these films after exposure to air.  The correlated percentage changes between Ga and Mn gives a strong indication that Mn atoms are occupying Ga lattice sites.

## Performance Characteristics of a Spin-PolarizedField-Effect Transistor

- The photograph to the left shows girl scouts and boy scouts visiting the PI’s laboratory.  This event was organized with the local scout leaders and represents a badge earning event.  At this particular time the kids are building atomic structural models and learning the limited yet extensive number of ways that atoms fit together.

- Performance Characteristics of a Spin-PolarizedField-Effect Transistor

- Paul M. Thibado

- DMR-0405036
- Broader Impact

- Scouts Earn a Badge!
- at Full-Day Event