Tatsuro Beppu

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.

Confirmation of P-Type Conduction in Li-Doped ZnSe Layers Grown on GaAs Substrates

Hall effect measurements were carried out for Li-doped MOCVD ZnSe layers whose GaAs substrates were removed. This made it possible to obtain the original p-type properties for the layers. Hole concentration, Hall mobility and specific resistivity were 4.1×1016 cm-3, 24 cm2/V s and 6.4 Ωcm, respectively. Furthermore, the characteristics for the GaAs inversion layers, which were changed from semi-insulating to p-type by Zn diffusion from the ZnSe layers, were estimated by comparing the results of samples without GaAs substrates with those with GaAs substrates.

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.

High-Efficiency GaP Green LED's with Double n-LPE Layers

High-efficiency GaP green LEDs have been obtained reproducibly by lowering the donor concentration in the light-emitting n-type region in a double n-LPE layer structure by the vapor-doping overcompensation method. The average efficiency for an epoxy-encapsulated diode is about 0.3% at 23 A/cm2. This improvement in average efficiency is mainly due to a large reproducible minority carrier lifetime of about 350 ns, obtained by controlling the donor concentration in the light-emitting n-region to less than 6×1016/cm3 in this structure. The double n-layer structure makes it possible to keep the donor concentration in the light-emitting n-region at a sufficiently low level. The best device demonstrates efficiencies of 0.5% at 23 A/cm2 and 0.56% at 70 A/cm2. A minority carrier lifetime of about 450 ns has been obtained.

GaP Green LED Degradation Associated with P-Type Electrode Formation Process

It has been clearly shown that the p-type electrode formation process has a great influence on the reliability of GaP: N green LEDs. For Lot A, the occurrence of crystal defects during the sintering process was limited only beneath the electrode, and the relative values of η were as high as 91% after 1000 hour operation. For Lot B, however, the defects spread all over the p-type layer surface, and the relative values of were only 55%. The degradation in Lot B was attributed to the decrease in the minority-carrier lifetime in the p-type layer. As a model of the degradation mechanism, it was proposed that the defects associated with Ga vacancies were formed beneath the electrode and that they moved to the vicinity of the p-n junction.

ZnS Blue Light-Emitting Diodes Fabricated by MOCVD

High-quality ZnS epitaxial J-ayers were grown on GaP substrates at a growth temperature higher than 200oC by ]-ow-pressure MOCVD technique. Undoped ZnS electrj_ca1 resistlvity was as high as an insul-ator. Aluminum doping of about 10 ppm l-owers resi-stivity to around 104 CIcm. , Fil-m thickness controll-abil-ity is very good for this low-pressure ZnS growth. A homoepitaxial MIS structure, usihg undoped MOCVD-ZnS for an i-nsulaLor, has been revealed to have higher emission efficiency and reproducibility compared to a heterostructure MIS. This successfultrial has resulted in a promising procedure to obtain MIS ZnS Blue LEDs on GaP substrate by ]-ow-pressure MocvD technique.