# Presentation: 197877

## Strain Relaxation in  Metamorphic Buffer Layers, Millunchick, DMR-0092602

- Metamorphic buffer layers have a range of lattice constants that relax the stress due to lattice mismatch in semiconductor heterostructures without degrading the crystalline quality of the active region.
- We investigate the strain relaxation in GaAs0.5Sb0.5/GaAs films and show that dislocation interactions play an important role.  As a result of this insight, we are able to predict the linear dislocation density nondestructively.
- Also, we show that the glide velocity has a weak temperature dependence, contrary to conventional wisdom, indicating that other mechanisms, such as roughening, contribute to the strain relaxation.

- GaAs0.5Sb0.5/GaAs

**T=470C**

*Figure 1: Strain relaxation for a GaAs**0.5**Sb**0.5**/GaAs film grown at T=470C (black points) and a fit  (red line) using the Dodson Tsao model taking dislocation interaction into account.  *

- Thickness (A)

- Strain Relaxation

*Figure 2: Arrhenius plot of the fitting parameters for the stress evolution model of GaAs**0.5**Sb**0.5**/GaAs films grown 440<T<505C.  *

**Notes:**
Motivation: A great deal of effort has been recently put into the development of High Electron Mobility Transistors (HEMTs) due to their importance in the wireless communications industry. In order to improve the device characteristics, increasing the In content in the device structure, thus the mobility*,  has been of great interest.  However, InGaAs, which is the most common material used in the channel of these devices, has a lattice mismatch with respect to GaAs leading to strain that can introduce mobility destroying defects.  One strategy is to relax the lattice parameter in a buffer layer between the GaAs substrate and the high In-containing device structure to a value such that mobility eradicating defects are not produced. Several different materials may be used for the buffer layer, such as AlInGaAs or GaAsSb.  The only requirements are that the buffer layer spans the appropriate range in lattice parameter, and have an energy gap sufficiently high to confine carriers to the device region.  The key to the success of this approach is in the ability of the buffer layer to relax stress due to lattice mismatch without degrading the quality of the electronically active regions of the device structure. 
Aim of this work: Determine the kinetics of strain relaxation and defect formation. Our approach utilizes an in situ method called Multi Beam Optical Stress Sensor to study the stress evolution in InGaAs and GaAsSb films.
Research Results: We have investigated the stress relaxation in InGaAs and GaAsSb films, which have lattice mismatches on the order of 3.5%.  Real time strain evolution was obtained using an in situ Multi-Beam Optical Stress Sensor measurement during Molecular Beam Epitaxy of these alloys (black symbols in Fig. 1), and fit using a modified Dodson Tsao (DT) model** for strain relaxation (red line in Fig. 1).  The standard DT model overestimates the real time strain relaxation in the regime where the relaxation rate decreases and reaches saturation.  However, we obtain good agreement when we take dislocation interactions into account. In the case of GaAs0.5Sb0.5/GaSb grown at T=470C, the estimated linear dislocation density is 7.6x104 cm-1.  This compares favorably with 5x104 cm-1, the dislocation density obtained from Transmission Electron Micrographs (TEM).  Similar agreement has been achieved in other compositions of GaAsxSb1-x and InxGa1-xAs. A standard assumption in strain relaxation models is that dislocations glide via an Arrhenius process.  Figure 2 shows an Arrhenius plot of several growths at temperatures between 440 and 505C.  A fit of the data shows that the activation energy (the slope of the line) is 0.13eV, much less than typically quoted values (1-2eV depending on the material system).  This suggests that other mechanisms for strain relaxations, such as roughening, † are in play.
Significance and Impact of these Results: This research provides insight into the physical mechanisms behind strain relaxation and defect formation in compound semiconductors, thus allowing for improved design of metamorphic buffer layers for important microelectronic devices.  Specific to the results presented here, we demonstrate a method to estimate linear defect densities immediately after the growth of the layer, without the need of destructive testing.  In addition, we show that there is a much lower temperature dependence of the defect glide than previously assumed.  In addition to the exciting fact that conventional wisdom in incomplete, this temperature insensitivity opens the processing window for these types of structures. 
Future Work: We are in the process of investigating the stress evolution in graded layers, whereby the lattice parameter varies across the thickness of the film, in order to determine the kinetics in that case.  We are also working with Northrop Grumman Space Technology corporation in order to examine how the performance of devices based on these metamorphic structures are affected by the kinetics of the stress evolution.
*The electron mobility in InAs is higher than that in GaAs.  Therefore, InGaAs alloys have increasing electron mobilities as the In content increases.
**B. W. Dodson and J. Y Tsao, Applied Physics Letters 51 1325 (1987)
†R. Beresford et al, Journal of Crystal Growth 251 106 (2003)

## Self Assembled Templates in Strained Semiconductor Films, Millunchick, DMR 0104509

- The morphology of strained semiconductor layers plays an important role in the development of the final microstructure of a multilayer film and may act as a template for subsequent growth.
- We examine the morphology of these very thin intermediately strained layers in order to understand how their morphology develops.  For example, GaAs surfaces are rougher, more anisotropic, and correlated along the [110] direction compared to AlAs, despite their having the same lattice mismatch.  We are modeling this behavior in collaboration with groups at UCLA and University of Michigan in order to understand its origin.
- The ultimate goal of this research is to engineer these templates to direct the self assembly of nanostructures.

- a)

- b)

- c)

*In situ Scanning Tunneling Micrographs of 2 monolayer thick films of GaAs (top) and AlAs (bottom) grown on In**0.53**Ga**0.47**As/InP under similar growth conditions.  Also shown to the left of the images are the autocorrelation maps (top), angular spectra (bottom), and average roughness, S**a**, of the films.*

**Notes:**
Motivation: The morphology of strained semiconductor layers plays an important role in the development of the final microstructure of a multilayer film.  Roughening is undesirable in both electronic (due to reductions in carrier mobility) and optical (due to broadening of the emission wavelength) devices.  On the other hand, this roughening may act as a template for subsequent growth.  Our group has shown that growing strain-balanced short period superlattice structures with appropriate growth parameters can result in spontaneous phase separation during growth, creating a film that exhibits carrier confinement.  Furthermore, the characteristic wavelength of the lateral composition modulation is set by the morphology of the thinnest layers.  
Aim of this work: In this work, we have been interested in understanding what physical parameters are involved in establishing the initial template observed for intermediately strained films and in short period superlattices exhibiting lateral composition modulation.
Research Results: The figures show in situ Scanning Tunneling Micrograph (STM) of 2 monolayer thick GaAs (top) and AlAs (bottom) films deposited on In0.53Ga0.47As/InP.  The insets show the autocorrelation (top) and angular spectra (bottom) for the surfaces.  Both of these films have a lattice mismatch of 3.6% in tension, however, the morphologies are quite different despite similar growth conditions.  First, the GaAs film has a higher average roughness Sa.  The angular spectrum, which is a measure of the dominant frequency as a function of angle, shows that this morphology has a higher anisotropy.  This is evident from the fact that the angular spectrum of the GaAs sample is elongated along one direction, while the angular spectrum of the AlAs is uniform in all directions.  The autocorrelation map is a measure of how similar the texture is across the surface.  For GaAs, the autocorrelation map shows that features on the surface are correlated along [110], as indicated by the periodic features in the map.  The autocorrelation map of AlAs does not show any periodicity, indicating that the surface has little regular texture. These image analysis results suggest that parameters other than the elastic strain as dictated by the lattice mismatch, such as the surface diffusivity, edge diffusivity, or intermixing play a role in the evolution of the surface morphology.  
Significance and Impact of these results: This research provides insight into the physical mechanisms behind the morphological evolution of strained layers, thus allowing for the design of templates for self assembly of nanostructures.  Specific to the results presented here, we demonstrate that the morphology of GaAs is much rougher and more highly correlated than AlAs, despite similar growth conditions.  
Future Work: We are in the process of investigating these differences by conducting STM experiments and comparing them to level set calculations in collaboration with Christian Ratsch at UCLA, and kinetic Monte Carlo simulations in collaboration with Peter Smereka and Len Sander at the University of Michigan.

## Girls In Science and Engineering (GISE):Outreach to middle-school-aged girls

- Girls In Science and Engineering (GISE):Outreach to middle-school-aged girls

*Campers measure out ingredients of nail polish*

*Grad student Jenny Lee assists a camper with beadmaking*

*Finished beads*

**MIllunchick**
**DMR 0092602**
**DMR 0104509**

*Campers assemble and test their lava lamps*

**Scientific and Technical concepts** :
- Adhesion and wear
- Phase transitions
- Glass transition temperature
- Viscosity
- Microstructure
- Microscopy
- Miscibility
- Thermal conductivity
- Thermal stress
- Electricity

**Activities:**
- Designed and Tested microfluidic chips with different liquids
- Made glass beads
- Made nail polish
- Built lava lamps

**Notes:**
On June 21-24, 2005, fourteen girls from area middle schools visited the departments of Materials Science and Chemical Engineering (with Prof. Mike Solomon) at the University of Michigan as a part of the Girls in Science and Engineering (GISE, formerly known as Future Science Future Engineering) Program. GISE is a one week day program for girls who have completed the 7th or 8th grade. Each day includes hands-on laboratory experimentation, computer activities and introductions to on-line network systems. Girls meet women scientists and engineers, and obtain information about careers in science and engineering. Participants chose a special project to attend, which consists of several three-hour sessions. 

The fourteen girls who came to our departments participated in several projects that demonstrate various concepts in Materials Science, Chemical Engineering, and in engineering in general. There were several basic projects.  One major project was to examine and manipulate the properties of glass.  First, the girls went to our glass blower’s lab to see a demonstration.  Next, the girls tried their own hand at glass forming by making their own decorative beads.   This project allowed us to introduce the concepts of amorphous versus crystalline structure, the glass transition temperature, viscosity, and thermal stress.  

The second project was to build a lava lamp using standard materials that can be bought from a hardware store.  The lava consists of a mixture of mineral oil and rubbing alcohol.  At room temperature, the mineral oil is heavier than the alcohol, but as it is heated up by a lamp from below, it becomes less dense and floats to the top of the bottle.  As it cools, it becomes more dense and sinks again. This allowed us to discuss the concept of miscibility and thermal conductivity.  This was also the first time many of these girls have had the opportunity to wire a basic electrical device (the light bulb socket).

The third major theme was the engineering of make up.  In this section, the girls made their own nail polish, while discussing the purpose of the the various ingredients (solvent, nitrocellulose, pigment) in the final product.  We also talked about the properties that good nail polish should have (adhesion, wear, nontoxicity).  

The final project involved the design an testing of a microfluidic device. First, the girls worked in small teams to design a device that would fulfill a particular task, namely the detection of a fictitious disease.  The devices were then fabricated in polydimethylsiloxane and tested.  This project introduced the girls to computer aided design, microscopy, diagnostics, and cutting edge concepts in microfluidics.

## Diversity in the Mirecki Millunchick Research Group

- Top Row, left to right: Jessica Bickel (grad), Lee (Trey) Sears (grad), Darius Dixon (grad), Hugh McKay (post doc), Associate Professor Chris Pearson (UM Flint)
- Bottom row, left to right: Benny Perez (grad), Jenny Lee (grad), Associate Professor Joanna Mirecki Millunchick

**Diversity in the Mirecki Millunchick Research Group**

**MIllunchick**
**DMR 0092602**
**DMR 0104509**