Emission of terahertz (THz) radiations from interdigitated GaN quantum-wells structures under DC-bias has
been measured at room temperature. This measurements has been performed by a 4K Si-Bolometer associated
with a Fourier Transform Spectrometer. Using an analytical model, we have shown that the observed peak
at approximately 3 THz due to 2D ungated plasma-waves oscillations in the quantum well, is emitted by the
metallic contacts of our device acting as antennas.
We report on measurements of radiation transmission in the 0.220-0.325 THz and 0.75-1.1 THz
frequency ranges through GaN quantum wells grown on sapphire substrates at nitrogen and room
temperatures. Significant enhancement of the transmitted beam intensity with applied voltage is
found at nitrogen temperature. This effect is explained by changes in the mobility of two-dimensional
electrons under electric bias. We have clarified which physical mechanism modifies the electron mobility
and we suggest that the effect of voltage-controlled sub-terahertz transmission can be used for
the development of electro-optic modulators operating in the sub-THz frequency range.
CdTe can be grown directly on silicon substrates by Molecular Beam Epitaxy. On Si(001), CdTe grows in the (111)B orientation. The homo-orientation on Si(001), Si(111) and Si(211) can be obtained if a buffer ZnTe epilayer is grown prior to CdTe growth. A systematic study of the growth of CdTe(111)B on Si(001) surface with different atomic step structures, defined by the miscut tilt angle (theta) and the tilt direction (phi) , has been carried out. Double domain and twin formation is very sensitive to tilt parameters. When growth conditions are optimized, single domain twin free layers are obtained with suitable tilt values. The best films which exhibit double crystal X-ray rocking curve FWHM of 60 arcsec have for tilt parameters (theta) equals 1 degree(s) and (phi) equals 30 degree(s). The heterointerface formation has been studied by photoelectron spectroscopy with synchrotron radiation. It was found that in the very first step of the growth, up to one monolayer of Te is absorbed on Si(001).
We present a review of the recent progress in the doping of HgCdTe grown by molecular beam epitaxy. A detailed analysis of the unintentional/intrinsic, n-type, and p-type doping is presented. Our results show that CdZnTe substrates should be carefully screened to reduce the out-diffusion of impurities from the substrate. N-type HgCdTe layers exhibit excellent Hall characteristics down to indium levels of 2 X 1015 cm-3. Electron mobilities in the range of (2 - 3) X 105 cm2/vs at 23 K were obtained. Measured lifetime data fits very well with the intrinsic band-to-band recombination. However, below 2 X 1015 cm-3 doping levels, minority carrier lifetime is limited by Schockley-Reed recombination. We have implemented planar doping with arsenic as p-type dopant during MBE growth. Our results clearly indicate that arsenic incorporates as an acceptor dopant during the growth of MBE HgCdTe.
Photoconductor array devices were fabricated using molecular beam epitaxially (MBE) grown CdTe. The detectors are stable in the presence of hard x-rays, and they have been tested at room temperature for over a year without any noticeable degradation. The performance of the photoconductor was greatly improved when the detector was cooled using the Peltier effect. The uniformity of the 64 element linear array device was measured at various temperatures. We observed an exponential decrease of the photoconductor dark current with temperatures down to 200 degrees K. The dark current and noise of the array detector decreased by more than 3 orders of magnitude from 300 degrees K to 200 degrees K. As a result, the minimum sensitivity to x-ray photons was increased by nearly 3 orders of magnitude. Finally an x-ray transmission image was obtained using a single element MBE CdTe photoconductor at 230 degrees K.
MBE (molecular beam epitaxy) grown CdTe layers were processed to fabricate a photoconductor array for the diagnosis of short x-ray pulses from synchrotron radiation sources. The MBE (111)B CdTe layers were grown on (100)Si substrates. Photoconductor arrays were fabricated with gaps of 5 - 50 micrometers using conventional photolithography. Electroless Au or sputtered Au/Ni was used as a contact metal. The temporal response of the resulting CdTe photoconductor was measured with mode-locked 100 fsec Ti:Sapphire laser pulses. The FWHM of single crystalline CdTe photoconductor response pulse is as short as 37 psec with a 20 psec risetime. The photoconductor responds linearly to the x-ray tube photon flux with fixed accelerating voltage up to 40 kV. A significant response increase to the x-ray beam is observed for a layer with good crystalline quality. Spatial response of the CdTe photoconductor array was measured using rotating anode and synchrotron x rays for different beam sizes. Excellent spatial resolution was obtained from narrow angular radiation synchrotron x rays. The CdTe photoconductor was exposed to synchrotron x rays for 60 hours without any noticeable degradation.
CdTe(111)B layers have been grown on Si(001) and its vicinal surface. Formation of double domains and twins is found to be related to the surface structure of the Si(001) substrate. The tilt angle and tilt direction of the misoriented Si(001) substrate play an important role in suppressing the formation of double domains and twins. A double-domain and twinned CdTe(111)B layer is always obtained, when it is grown on nominal Si(001) substrate. However, by optimizing the tilt parameters, one can consistently obtain single-domain and twin-free CdTe(111)B layers grown on slightly misoriented Si(001) substrates.
We discuss the influence of the Hg flux on defect formation and we show that under optimized growth parameters the crystal quality of HgCdTe epilayer is similar to that of the CdZnTe substrate. We confirm the MBE growth of HgCdTe requires stringent control in growth conditions and occurs under Te saturated conditions. We show also that diffusion of impurities originating from the substrates is a very serious problem. Indium doped HgCdTe layers have been found to exhibit excellent structural and electrical characteristics.
The current status of molecular beam epitaxy (MBE) of CdTe and HgCdTe on Si(100) is reviewed. CdTe and HgCdTe grow in the (111)B orientation on Si(100); monocrystalline films with two domains are obtained on most nominal Si(100) substrates, single domain films are grown on misoriented substrates and on nominal Si(100) preheated to 900-950 degree(s)C. Double-crystal x-ray rocking curves (DCRCs) with full-width at half-maximum (FWHM) as low as 110 arcsec are reported for HgCdTe on silicon; these layers are n-type, and electron mobilities higher than 5 X 104 cm2V-2s-1 are measured at 23 K for x equals 0.26. Excellent thickness and composition uniformity is obtained: standard deviation of the CdTe thickness 0.4% of the average thickness on 2-in. and 2.3% on 5-in., standard deviation of the Cd concentration in the HgCdTe layers 0.6% of the average concentration on 3-in. and 2.4% on 5-in. First results regarding growth of CdTe on patterned Si substrates are also reported.