Technical Field
The current disclosure relates generally to a new embodiment of bulk GaAs with an increased carrier lifetime of at least 10 microseconds to be used for optical and electrical devices. The current disclosure also relates to the apparatus, systems, and methods for creating GaAs with very long free carrier lifetimes. More particularly, this very long free carrier lifetime GaAs is expected to be useful as a semiconductor radiation detector material, and also is expected to be useful for applications including, but not limited to, medical imaging, solar cells, diode lasers, and optical limiters.
Background Information
GaAs is a well-known semiconductor and is grown by many methods. It can be produced using a variety of techniques including both bulk melt growth and vapor growth. Commercially available GaAs always has a significant concentration of a defect called EL2 (As on Ga antisite defects) which are known to greatly reduce the free-carrier lifetime of the material. The EL2 antisite defects form deep level traps and are inherent in any melt grown material due to a widening of the solubility curve of As in GaAs as the GaAs crystal temperature is near that of the melting point of the compound.
Crystal growth from the vapor phase can be done at a temperature significantly lower than that of the melting point of the compound, however, most vapor growth techniques such as metal organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE) have growth rates that are much too slow (approximately 1 micron per hour) to make growth of any material approaching that of bulk quantities (>100 micrometers) practical.
GaAs with lower concentrations of EL2 defects or longer free carrier lifetimes would be useful because the electrons generated in a process like the absorption of radiation would be able to travel further distances through the GaAs before being trapped by a defect.
One area where longer lifetime (low EL2 defect) bulk GaAs is expected to be useful is in the manufacture of semiconductor radiation detectors. Improving the energy resolution of gamma radiation detectors is among the most important science and technology objectives for national security applications since it enables the use of high-resolution energy spectroscopy to distinguish between the natural radioactive isotopes, medical or commercially used radioisotopes, and radioisotopes that pose a threat. Gamma radiation detectors must be very thick—on the order of several millimeters—because of the deep penetration of high energy gamma rays which must be fully absorbed for accurate quantitative detection.
Of the two primary classes of radiation detector materials—semiconductors and scintillators—semiconductors are fundamentally capable of much higher energy resolution because the information carriers are the electrons and holes directly produced by the energy cascade. Desirable properties in such a semiconductor are a room temperature operation—which requires a band gap between 1.35-2.55 eV (McGregor and Hermon, Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment Volume 395, Issue 1, Room-temperature compound semiconductor radiation detectors, pages 101-124 (1997), high electron and hole mobilities (μ), and long carrier lifetimes (τ). Historically, however, semiconductor radiation detectors have been limited by high cost, operational complexity, low efficiency due to limited size, and difficulty achieving high-purity, defect-free crystals. Unfortunately, progress in overcoming these obstacles over the years has been slow and incremental.
Progress in the development of high resolution, room temperature, gamma radiation detectors has been severely limited for many years by the size, quality, and transport properties of single crystal compound semiconductors. CdxZn1-xTe (CZT) is the most commercially advanced of these materials, but Te inclusions, twins, and grain boundaries are constant barriers to the size, yield, and cost of these crystals for detector applications.
Gallium arsenide (GaAs) has been studied as a radiation detector since the early 1960's, predating CZT, but its widespread use for gamma ray detection has never been realized due to the presence of native deep level traps (EL2, i.e., AsGa antisites) which reduced its free carrier lifetime at room temperature. Otherwise, GaAs has very attractive intrinsic properties. Its band gap of 1.42 electron voltage (eV) is near optimum. Theory predicts that carrier lifetimes in very pure and well-ordered GaAs should approach 100 microseconds or more, but such long lifetimes have never historically been observed.
A need exists, therefore, for bulk GaAs with free-carrier lifetimes significantly greater than that of the previous state of the art of 1 microsecond.