Our main research interest is the study and development of electronic and optoelectronic materials and devices. Our research activities are focused on the following areas:
 
interface Monolithic intergration of 6.1 Ĺ material platform for various optoelectronic and electronic device applications. This novel material system, including the combination of (ZnCdBeMg)(TeSe) and (InGaAl)(AsSbBiP) provides very broad bandgap coverage from UV to far infrared (IR).


IR

Related publications:
E. H. Steenbergen Appl. Phys. Lett. 99, 251110 (2011)
D. Ding J. Appl. Phys. 110, 123104 (2011)
J.-J. Li IEEE J. of Photovoltaics 2, 225-230 (2011)
E. H. Steenbergen Appl. Phys. Lett. 071111 – 071114 (2011)
J. Fan J. of Cryst. Growth 323, 127-131 (2010)
E. H. Steenbergen Appl. Phys. Lett. 97 161111 (2010)
S.-N. Wu Progress in Photovoltaics: Research and Applications 18, 328–333 (2010)
S. Wang J. of Cryst. Growth 311, 2116 (2009)

 
MBE growth of various kinds of III-V and II-VI compound semiconductors.  A unique twin-chamber MBE system allows growth of any composition of III-V and II-VI alloys.


The II-VI chamber is capable of growing almost all possible combinations of materials including Zn, Se, Te, Mg, Be, and Cd. The Se and Te cells are valved cell to allow for precise control of composition and graded layers. In addition, it contains five different dopants (Al, Ga, P, Bi, and In) so that various doping profiles in different material systems can be studied.

The III-V chamber contains various group III and V cells including Ga, In, Al, As, Sb and P, which can cover all the typical III-V combinations. The new Bi cell enhances our growth capability to bismide materials. The specially designed doping cell, which contains Be, Te and Si, can satisfy different doping requirements.

 
Ultra-high efficiency multijunction solar cell and very broad wavelength coverage multi-color photodetectors using lattice-matched II/VI and III/V direct bandgap materials grown on GaSb and InAs substrates.

1 . Multijunction solar cell testing
The testing of multijunction solar cells has focused on using various electrical and optical mesurements to characterize the material and device parameters as well as on understanding the fundamental physics of device operation.  We have developed two novel measurement methods–the pulse voltage bias method and pulse light bias method–to eliminate the measurement artifacts in external quantum efficiency (EQE) of multijunction solar cells.  We have also quantitatively studied the underlying physics of the EQE measurement artifacts caused by the effects of shunt and luminescencence coupling.  These convenient and nondestructive characterization approaches are very useful for the design and development of multijunction solar cells.

2. Ultrahigh efficiency single junction solar cells with heterostructures:
GaAs is one of the most promising materials for achieving the ultrahigh efficiency close to the Shockley-Queisser efficiency limit.  The research in GaAs single junction solar cells became very active again after the long stagnancy during 1990-2007.  Efficiency records of GaAs solar cells have been frequently broken by the institutes around world in the past few years.  In our group, we proposed a new type of single junction GaAs solar cell. These would use novel heterostructure concepts to achieve ultrahigh energy conversion efficiency.  This project has involved close collaboration with some of the top industry researchers in the solar field.
 
IR Type-II superlattice semiconductors for infrared photodetector

Antimony-based type-II superlattice (T2SLs) offer advantages for MWIR (Midwave Infrared) and LWIR (Long-Wave Infrared) laser and detector applications due to their broad bandgap tunability and material uniformity. Our group’s recent breakthrough on Ga-free type-II superlattices significantly increased the minority carrier lifetime from ~30 ns to 412ns in LWIR band. Further growth of InAs/InAsSb type-II structures is crucial for the suppression of defect formation and their adverse effects, and will allow us to further understand the defect physics, growth processes, and detector theory.
Related publication: E.H. Steenbergen et.al, APL, vol. 99, p. 251110 (2011)

Our group’s novel idea of optically-addressed multi-color photodetectors allow switching detection bands by applying light onto the detector. This technology allows new spectroscopic applications, also multi-color imager sensors. Multi-color infrared image sensors are further limited by connections per pixel. Optical-addressing allows adding more colors/band to the infrared focal plane arrays keeping the existing simple electronics configuration.
Related publication: (E.H. Steenbergen et.al, APL, vol. 97, p. 161111 (2010))
IR
Figure: Schematic of the optically addressed, two-terminal, multicolor photodetector. The detector structure consists of multiple photo-diodes with different cutoff wavelengths connected in series with tunnel diodes between adjacent photodiodes. The LEDs optically bias the inactive photodiodes in the detector to enable single color detection.

 
LEDs and lasers with wavelegth from visible to MWIR.
 
Materials spectroscopic characterization and device testing. Three labs equipped with two high-resolution spectrometers, two temperature-variable cryostats (1.8 K- 350 K), and a state of the art Nicolet 760 Fourier Transform Infrared spectrometer (FTIR) can allow characterization and testing for wavelengths ranging from UV to 25 microns.
 
 
 
Some of our research highlights are listed below:
 
The bandgap energies are plotted as a function of the lattice constants of major III-V and II-VI materials. The histogram shows the normalized AM 1.5G solar radiation power density as a function of energy. (high resolution image)
   
The group recently added a second dual-chamber MBE system. These MBE systems include many unique features that enable us to grow high-quality materials and, specifically, investigate various dopants and surfactants for a wide range of materials.
   
High-quality MBE growth of ZnTe on GaSb substrate has been demonstrated.  This high-resolution electron micrograph shows the highly coherent nature of the interface.