We report the direct observation of transferred-electron effect in unintentionally doped GaN epilayers grown by metalorganic chemical vapor deposition. The negative differential resistivity (NDR) was observed from the current-electric field characteristics in GaN using a metal-semiconductor-metal (M-S-M) system. The threshold field for the onset of NDR was independent of the spacing of M-S-M fingers, and was measured to be 1.91×105 V/cm for GaN with an n-type carrier concentration of 1014 cm−3. This value is very close to the value obtained from theoretical simulation. This observation is an experimental evidence of transferred-electron effects in GaN, which is important in understanding GaN energy band structure and in the application of Gunn-effect devices using GaN materials.
The observation of optical quenching of photoconductivity in GaN photoconductors at room temperature is reported on. Three prominent quenching bands were found at Ev+1.44, 1.58, and 2.20 eV, respectively. These levels are related to three hole traps in GaN materials based on a hole trap model to interpret the quenching mechanism. The responsivity was reduced about 12% with an additional He–Ne laser shining on the photoconductor.
In 05A10 ~Plattice-matched to GaAs and Ino 5Alo ~As lattice-matched to InP epilayers were grown by atmospheric pressure metalorganic chemical vapor deposition (AP-MOCVD). The effect oftrimethylindium on the purity of the as-grown layers was systematically studied using secondary ion mass spectroscopy (SIMS), deep level transient spectroscopy (DLTS), and capacitance-voltage (C-V) measurements. The SIMS results showed that oxygen is the main impurity in all layers and the oxygen concentration in InA1P was approximately one to four orders of magnitude higher than the oxygen concentration found in InAlAs when the same indium source was used, indicating that more oxygen was introduced by the phosphine source than by the arsine source. Two electron traps in the InAIP epilayers and four electron traps in the InA1As epilayers were observed in this study. When a high-purity indium source was used, the best InAIP epilayer showed only one deep electron trap at 0.50 eV while the best InAlAs epilayer showed no deep levels measured by DLTS. In addition, we also found that a high concentration of oxygen is related to the high resistivity in both material systems; this suggests that semi-insulating (SI) materials can be achieved by oxygen doping and high quality conducting materials can only be obtained through the reduction of oxygen. The oxygen concentration measured by SIMS in the best InA1As epilayer was as low as 3 • 1017 cm -3.
Metal-semiconductor-metal photoconductors made on GaN usually exhibit a slow response time and a low responsivity. We have carried out a systematic study on the performance of the photoconductors made from GaN grown by metalorganic chemical vapor deposition using different growth conditions and have found that both response time and responsivity of the GaN detector are improved when the material is grown using increased ammonia flow rates. The best GaN ultraviolet photoconductive detector shows a response time of 0.3 ms and a responsivity of 3200 A/W at 365 nm under an operation bias of 10 V. We attribute this improvement to the reduction of the point defects in GaN.
Modulation Extrinsic photoconductivity spectra between 1.44eV and 1.75eV of unintentionally n-doped high resistance GaN film grown by MOCVD are measured at room temperature by using wavelength adjustable Ti:Sapphire laser. We find that there are two major deep levels in the GaN material in the used photon energy range. The relaxation time of excess carriers controlled by those levels are in the order of 10-4sec. The concentration of localized states are determined as I.8X108cm-3 and 2.5x10 9 cm-3 , respectively. A physical model is developed to explain the results and process the data. Using a new method we have determined the optical absorption cross section of deep levels are 1.5xl0-1 7 cm 2 and 2.7xl0-1 8 cm 2 , respectively.
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