Kazuhiro Ohkawa

Characteristics of Cl‐doped ZnSe layers grown by molecular‐beam epitaxy

High‐quality n‐type ZnSe layers have been grown by molecular‐beam epitaxy using chlorine (Cl) as a dopant. The Cl‐doped ZnSe layers showed mirrorlike morphology and good crystallinity, although some degrade in crystallinity is observed at a heavy doping. The carrier concentration of the layer could be widely controlled by a ZnCl2 Knudsen cell temperature. The carrier concentration attained 1×1019 cm−3, where the resistivity was as low as 3×10−3 Ω cm, indicating a remarkable improvement compared to the previous work using group‐III elements as a dopant. Hall mobilities at room temperature were in the range of 200–400 cm2/(V s), depending on the doping level. The Cl‐doped ZnSe layer exhibited strong blue near‐band‐gap photoluminescence (PL) with suppressed deep‐level emission at room temperature. The 4.2‐K PL of the layer was dominated by strong emission of excitons bound to neutral donors originating from substitutional Cl atoms. It was found by a secondary ion‐mass‐spectroscopy analysis that diffusion of ...

Characteristics of p-type ZnSe Layers Grown by Molecular Beam Epitaxy with Radical Doping

p-type ZnSe layers have been grown by molecular beam epitaxy using nitrogen radical doping. We have employed Pt as the electrode material for p-type ZnSe. The Pt electrodes markedly reduced contact resistances. The p-type conduction was confirmed by Hall measurement. Carrier concentration was 4.4×10 15 cm −3 . Hall mobility was a high as 86 cm 2 /V•s because of good crystallinity. The p-type ZnSe layers exhibited the 2.616-eV emission from recombination between free electrons and acceptor holes (FA) in room-temperature photoluminescence measurement. The FA emission provides evidence that the layers are p-type ZnSe

Doping of nitrogen acceptors into ZnSe using a radical beam during MBE growth

A new method of doping for ZnSe was attempted by using a neutral radical beam during the MBE growth. The radical beam dominantly consisted of N2 molecular radicals at A3σ+u state. The sticking coefficient of nitrogen was remarkably enhanced; thus this doping method was able to incorporate N into ZnSe by 1019 cm−3. The existence of shallow N acceptors was confirmed by photoluminescence measurements; recombination of free electrons and acceptor holes (FA) at room temperature and recombination of donor-acceptor pairs at low-temperature were observed. The FA emission was observed only for ZnSe layers with moderate doping level, which shows p-type conduction. The carrier concentration was the order of 1015 cm−3. The activation of N in ZnSe was less than 1%.

Hydrogen Gas Generation by Splitting Aqueous Water Using n-Type GaN Photoelectrode with Anodic Oxidation

Hydrogen gas generation from a counterelectrode was clearly observed for the first time using light-illuminated n-type GaN as a working photoelectrode in an electrolyte. The application of extra bias to a working electrode was required to obtain a sufficient volume of generated gas. The reactions at the GaN photoelectrode were both GaN decomposition and water oxidization, simultaneously.

Molecular-beam epitaxial growth of p- and n-type ZnSe homoepitaxial layers

Undoped, p- and n-type ZnSe homoepitaxial layers have been successfully grown by molecular beam epitaxy on dry-etched substrates. ZnSe substrates were etched with a BCl3 plasma to eliminate polishing damage. The full width at half maximum of an X-ray rocking curve for a homoepitaxial layer has reached 21 arc sec. N-doped ZnSe homoepitaxial layers grown with the active-nitrogen doping technique have shown p-type conduction which is the evidence of p-type conduction in ZnSe. The n-type homoepitaxial layers have been grown with Cl doping. The Cl-doped homoepitaxial layers have shown high Hall mobilities relative to heteroepitaxial layers. These undoped, N-doped and Cl-doped layers have exhibited strain-free photoluminescence properties; free exciton emission, neutral acceptor-bound exciton emission and neutral donor-bound exciton emission showed a single peaks at 2.804, 2.7931 and 2.7980 eV, respectively.

Photoelectrochemical Properties of p-Type GaN in Comparison with n-Type GaN

We studied the photoelectrochemical properties of p-type GaN compared with those of n-type GaN. Band-edge potentials of p-type GaN were determined from the Mott–Schottky plot, and we clarified the potentials to be identical to those of n-type GaN. Photocurrent in p-type GaN shows its potential as an electrode for photoelectrolysis of water.

Photoelectrochemical reaction and H2 generation at zero bias optimized by carrier concentration of n-type GaN.

The authors studied the photoelectrochemical properties dependent on carrier concentration of n-type GaN. The photocurrent at zero bias became the maximum value at the carrier concentration of 1.7x10(17) cm-3. Using the sample optimized carrier concentration, the authors achieved H2 gas generation at a Pt counterelectrode without extra bias for the first time. The authors also discussed the mechanism of the dependence of photocurrent on the carrier concentration of GaN.

CdSe/ZnSe quantum structures grown by migration enhanced epitaxy: Structural and optical investigations

Migration enhanced epitaxy has been applied to induce the change from two-dimensional growth to formation of self-assembling islands in the growth of CdSe on ZnSe. Transmission electron microscopy images from samples with a ZnSe caplayer show a transition from a flat quantum well to an interrupted layer with pronounced thickness fluctuations when the CdSe exceeds its critical thickness. These results are confirmed by high resolution x-ray diffraction measurements, as only for the former sample the 004 rocking curve can be simulated assuming a flat quantum well with abrupt interfaces. Compared to bulk CdSe, the photoluminescence peak is blueshifted by about 0.5 eV. PL excitation experiments indicate that the interrupted layer consists of CdSe islands embedded in Zn1−xCdxSe with a composition gradient. Atomic force microscopy images of uncapped samples show spherical islands with a height of 20 nm and a diameter-to-height ratio of 4:1.

Photoluminescence properties of nitrogen‐doped ZnSe layers grown by molecular beam epitaxy with low‐energy ion doping

Low‐energy ion doping of nitrogen has been employed in the molecular beam epitaxial (MBE) growth of ZnSe on GaAs substrates in an attempt to obtain p‐type crystals. Low‐temperature photoluminescence (PL) measurements of the N‐doped ZnSe indicated a formation of shallow acceptors with an activation energy around 110 meV. The PL spectrum in the excitonic emission region was dominated by an acceptor bound‐exciton emission I1 at 2.790 eV, suggesting a remarkable increase of an acceptor concentration in comparison with the previous work by MBE using a neutral doping source.

CdSe/ZnSe Quantum Dot Structures: Structural and Optical Investigations

Using migration enhanced epitaxy the self-organized formation of CdSe islands on ZnSe in open and overgrown structures has been studied by transmission electron microscopy, high-resolution X-ray diffraction, atomic force microscopy, photoluminescence and excitation spectroscopy. The transition from homogeneous CdSe quantum wells with flat interfaces to fluctuating CdSe films could be observed when exceeding the critical thickness. These interrupted layers contain CdSe quantum dots embedded in Cd1—xZnxSe with a concentration gradient. Islands observed on open structures are unstable in time. Their chemical nature is still unclear due to the fact that similar features are obtained on pure ZnSe. First results on other highly lattice mismatched II–VI systems like CdTe/ZnSe and ZnTe/ZnSe will be presented.