Nov 18, 2015

Processing and characterization of epitaxial GaAs radiation detectors


X-ray detectors made on thick epitaxial GaAs were successfully processed.
CVPE technique was used to grow high purity epi-GaAs with over View the MathML source layer thickness.
Leakage current density of about 10 nA/cm2 indicates high purity of the epi-layer.
DLTS shows a significant concentration of deep level electron traps in the epi-layer.
TCAD simulations with a deep level trap in the epi-layer reproduce the measurements.


GaAs devices have relatively high atomic numbers (Z=31, 33) and thus extend the X-ray absorption edge beyond that of Si (Z  =14) devices. In this study, radiation detectors were processed on GaAs substrates with View the MathML source thick epitaxial absorption volume. Thick undoped and heavily doped p+ epitaxial layers were grown using a custom-made horizontal Chloride Vapor Phase Epitaxy (CVPE) reactor, the growth rate of which was about View the MathML source. The GaAs p+/i/n+ detectors were characterized by Capacitance Voltage (CV), Current Voltage (IV), Transient Current Technique (TCT) and Deep Level Transient Spectroscopy (DLTS) measurements. The full depletion voltage (Vfd) of the detectors with View the MathML source epi-layer thickness is in the range of 8–15 V and the leakage current density is about 10 nA/cm2. The signal transit time determined by TCT is about 5 ns when the bias voltage is well above the value that produces the peak saturation drift velocity of electrons in GaAs at a given thickness. Numerical simulations with an appropriate defect model agree with the experimental results.


  • GaAs
  • Solid state radiation detectors
  • Wafer processing
  • Defect characterization;
  • TCAD simulations

Nov 3, 2015

GaAs photovoltaics and optoelectronics using releasable multilayer epitaxial assemblies

Compound semiconductors like gallium arsenide (GaAs) provide advantages over silicon for many applications, owing to their direct bandgaps and high electron mobilities. Examples range from efficient photovoltaic devices to radio-frequency electronics and most forms of optoelectronics. However, growing large, high quality wafers of these materials, and intimately integrating them on silicon or amorphous substrates (such as glass or plastic) is expensive, which restricts their use. Here we describe materials and fabrication concepts that address many of these challenges, through the use of films of GaAs or AlGaAs grown in thick, multilayer epitaxial assemblies, then separated from each other and distributed on foreign substrates by printing. This method yields large quantities of high quality semiconductor material capable of device integration in large area formats, in a manner that also allows the wafer to be reused for additional growths. We demonstrate some capabilities of this approach with three different applications: GaAs-based metal semiconductor field effect transistors and logic gates on plates of glass, near-infrared imaging devices on wafers of silicon, and photovoltaic modules on sheets of plastic. These results illustrate the implementation of compound semiconductors such as GaAs in applications whose cost structures, formats, area coverages or modes of use are incompatible with conventional growth or integration strategies.

Figure 1Schematic illustration, optical and SEM images, and SIMS profile of GaAs/AlAs multilayers.

Schematic illustration, optical and SEM images, and SIMS profile of GaAs/AlAs multilayers.
a, Schematic illustration of a multilayer stack of GaAs/AlAs and schemes for release through selective etching of the layers of AlAs. b, Corresponding SIMS profile of this stack. c, Cross-sectional SEM image after partial etching of the…

igure 2Multilayer GaAs MESFETs and logic circuits.

Multilayer GaAs MESFETs and logic circuits.
a, Schematic illustration of a GaAs MESFET on a polyimide (PI) coated glass substrate.b, Optical image of arrays of MESFETs on glass substrate. Inset, a single MESFET with source (S), drain (D) and gate (G) metal layers. cVDS (drain–…

Figure 3Multilayer GaAs NIR imagers.

Multilayer GaAs NIR imagers.
a, Schematic illustration of a GaAs metal–semiconductor–metal (MSM) NIR detector on a Si wafer coated with a photocurable polyurethane (PU). Inset, Schottky blocking diode (SD). b, Optical image of a NIR imager consisting of a 16×16 a…

Figure 4Multilayer GaAs single-junction solar cells.

Multilayer GaAs single-junction solar cells.
a, Schematic illustration of GaAs single-junction solar cell on a PET substrate coated with a photodefinable epoxy. b, Optical image of arrays of such devices formed on the source wafer. Inset, magnified view of top (n-type) and bottom…