Tsutomu Uemoto

Superiority of group VII elements over group III elements as donor dopants in MOCVD ZnSe

Abstract Comparison of group III and group VII elements as donor species in ZnSe MOCVD growth has been studied from the viewpoint of their electrical properties, luminescence qualities and their controllabilities. Low resistivity high quality blue emitting ZnSe has been grown by doping a group VII donor chlorine, in atmospheric MOCVD. The superiority of group VII elements over group III elements with regard to carrier concentration controllability and carrier producing efficiency has been predicted.

The zinc pressure effect in chlorine-doped ZnSe grown by atmospheric metalorganic chemical vapor deposition

An n‐type ZnSe has been grown by atmospheric metalorganic chemical vapor deposition in which hydrogen chloride was used as the chlorine‐donor source. Epitaxial layers were evaluated by photoluminescence measurements and Hall measurements. An excitonic emission intensity at 4.2 K, which is a donor bound excitonic emission, increased with donor doping concentration increase. The electron carrier concentration varied with a quadratic dependence on the dopant concentration. Chlorine‐doped ZnSe characteristics were revealed to be similar in regard to the relation between the electrical and luminescent properties, irrespective of dopant source materials. An increase in zinc partial pressure led to an increase in carrier concentration and the suppression of the self‐activated emission intensity. A zinc excess growth condition was revealed to be effective for quality improvement for n‐type ZnSe and suppression of the compensation effect, which made the carrier producing efficiency high.

p-type carrier concentration control in lithium-doped zinc selenide grown by MOCVD

Abstract Lithium-doped ZnSe epitaxial layers have been grown on GaAs substrates by metalorganic chemical vapor deposition. Tert-butyllithium (TBL) was used as a doping material. Secondary ion mass spectroscopy showed that Li concentrations in ZnSe layers could be altered from 4×10 17 to 4×10 17 cm -3 by variation of the carrier gas flow rate passing through the TBL cylinder. Photoluminescence measured at 4.2 K indicated acceptor concentration increased with TBL carrier gas flow rate. Carrier concentrations N A − N D obtained from C − V measurements increased to about 1×10 15 cm -3 with increasing TBL carrier flow rate.

Investigation of Deep Levels and Residual Impurities in Sublimation-Grown SiC Substrates

Deep levels and residual impurities in sublimation-grown n-type SiC substrates are investigated. Three electron trap centers at 0.26, 0.35 and 0.72 eV below the conduction band (CB) are observed in 4H-SiC for the first time by deep-level transient spectroscopy (DLTS). One electron trap center at 0.68 eV below the CB is also observed in n-type 6H-SiC. The trap concentration and capture cross section are estimated. The concentration of residual impurities V, Cr, Fe and Ti are found to be high. The origin of observed trap centers is discussed with respect to the residual impurities and available information.

Lattice matching effect on the luminescent properties of n-ZnSSe on GaAs grown by MOCVD

Abstract Lattice matching effects on Br-doped ZnSSe/GaAs system were studied. A homogeneous image was observed in the lattice matched system ZnS 0.09 Se 0.91 /GaAs by SEM cathodeluminescence (CL), whereas there were many dark spots on CL images for the lattice mismatched ZnSe/GaAs system. The dislocation density on the (100) surface was estimated as 10 8 cm -2 from TEM cross sectional images and 10 7 cm -2 by etch-pit measurement in the lattice mismatch system. The dislocation density decreased to below 10 6 cm -2 from TEM estimation and 10 4 cm -2 from EPD measurement by lattice matching. The photoluminescence (PL) intensity at room temperature for lattice matched ZnSSe was about 10 times larger than that for lattice mismatched ZnSe with the same doping concentration ( n =10 16 cm -3 ). These studies proved that precise lattice matching is needed to obtain crystals which have a high luminescence efficiency.

The growth and characterization of cadmium selenide and cadmium zinc selenide epilayers by MOVPE

The growth of cubic CdSe and Cd x Zn 1- x Se on (100) GaAs by MOVPE using dimethylcadmium, dimethylzinc and dimethylse- lenide is reported. It is shown that CdSe growth can be achieved at much lower temperatures than for ZnSe when (CH 3 ) 2 Se is used as the group VI source. Single crystal alloy layers can be grown across the entire composition range from ZnSe to CdSe. However layers are zinc rich compared to the gas phase Cd:Zn ratio. The layer quality - as determined by microscopy, X-ray diffraction and photoluminescence - is good for low values of x , but deteriorates with increasing Cd composition.

Homoepitaxial growth and characterization of ZnSe on different ZnSe substrates by metalorganic vapor phase epitaxy

Abstract The ZnSe crystal quality of the homoepitaxial layers is discussed. The homoepitaxial layers were grown by MOVPE on the three kinds of ZnSe substrate. High quality ZnSe epitaxy was obtained at 500°C and VI/II = 2. The surface morphology was very smooth. Each epilayer showed strong free exciton emission, the FWHM of which was about 1.2 meV. Y emission was not observed. The homoepilayers quality was strongly affected by the presence of defects in the substrate such as strain variation, grain boundaries, etc. Furthermore, epilayer quality was also affected by mechanical damage of the substrate surface. These defects have been observed by X-ray topography using the Berg-Barrett method.

Hg1−xCdxTe epitaxial layers grown by low mercury partial pressure metalorganic chemical vapor deposition and extended defect characterization.

There are two growth methods in Hg1−xCdxTe metalorganic chemical vapor deposition (MOCVD) growth, namely direct alloy growth (DAG) and the interdiffused multilayer process (IMP). HgCdTe layers have been grown by both methods onto CdZnTe (111)B substrates and their structural and electrical properties were investigated. The composition fluctuated significantly along the growth direction for DAG. On the other hand, there were many misfit dislocations in the IMP layers, presumably generated by the lattice mismatch between the HgTe and CdTe layers. Here we have investigated DAG HgCdTe growth under low mercury partial pressure in order to improve depth compositional uniformity, misfit dislocation density and the lamella twin density. This low mercury partial pressure growth gave mercury transport limited growth for HgTe as in the conventional growth of III–V compounds by MOCVD. The Hg0.8Cd0.2Te layers grown by the new method (p-type 3.4x1015 cm-3 for carrier concentration, 520 cm2V·s for mobility in as-grown samples) were found to be improved with respect to depth compositional uniformity and dislocation density.

Observation of Deep Level in p-n Junction Diode of 6H:SiC

We have investigated the deep level trap centers in a p-n junction diode of 6H:SiC by deep level transient spectroscopy (DLTS) measurements. An electron trap center 0.71 eV below the conduction band edge is observed for the first time in 6H:SiC. The estimated trap center concentration and its capture cross section are found to be around 2×1015 cm-3 and 5×10-14 cm2, respectively. The trap center concentration profile is found to be uniform. The origin of trap centers is suggested.

Effects of mercury partial pressure on defects in HgTe epitaxial layers grown by metalorganic chemical vapor deposition

Abstract The effects of mercury partial pressure on the defect density in HgTe grown by metalorganic chemical vapor deposition (MOCVD) have been investigated. A large number of mercury clusters were revealed by transmission electron microscopy in high mercury partial pressure specimens. There were some dislocation loops that seemed to originate on tellurium interstitials in the layers grown under low mercury partial pressure. All the specimens exhibited n-type conduction from Hall effect measurements. The Hall coefficient of HgTe under a mercury partial pressure below 2x10 -2 atm has been found to be controlled by the mercury partial pressure. On the other hand, the Hall coefficient of HgTe above 2x10 -2 atm mercury partial pressure could not be controlled by varying the mercury partial pressure. Therefore, the impurity doping is supposed to be suitable for controlling the hole concentration in this high mercury pressure region.