Interfacial reaction and electrical property of Ge/Ni/ZnSe for blue laser diode

Interfacial reaction and electrical property of the Ge/Ni/ZnSe have been investigated as a fundamental study to develop ohmic contacts to p-ZnSe, using x-ray diffraction, Auger electron spectroscopy, cross-sectional transmission electron microscopy, and capacitance-voltage measurement. Ni/ZnSe, Ni/Ge systems have been also studied for comparison with the Ge/Ni/p-ZnSe system and the electrical property could be related to the interfacial reaction. After annealing at 170°C, Ni₃Se₂ and NiGe were formed at Ni/ZnSe and Ge/Ni interface, respectively. The increase of annealing temperature resulted in the decomposition of Ni₃Se₂ through the reaction with Ge. The change of the Schottky barrier height strongly depended on the annealing temperature. According to the result of Ni/ZnSe interface reaction, Ni₃Se₂ in the Ni/ZnSe interface lowered the Schottky barrier height. At higher annealing temperature, the Schottky barrier height increased and it was mainly due to the thermal decomposition of ZnSe.     

Heavy Cr doping of ZnSe by molecular beam epitaxy

Epitaxial ZnSe layers were grown by molecular beam epitaxy (MBE) to study Cr incorporation with the long-term goal of demonstrating an alternate route for achieving transition-metal-doped lasers. Concentrations between 10¹⁵ atoms cm⁻³ and 4×10²⁰ atoms cm⁻³ were achieved. Secondary ion-mass spectroscopy (SIMS) concentration profiles strongly suggest that surface segregation and accumulation of Cr occurs during growth. Photoluminescence (PL) measurements indicate Cr is incorporated in the optically active Cr²⁺ state up to levels of ∼10¹⁹ cm⁻³. Electron paramagnetic resonance (EPR) studies suggest that the Cr atoms exhibit collective magnetic behavior even at these levels. X-ray diffraction (XRD) and reflection high-energy electron diffraction (RHEED) indicate high structural quality is maintained for Cr incorporation for levels up to ∼10¹⁹ atoms cm⁻³.     

掺GeZnSe的稳恒光电导及其局域性效应

在一定的温度以下 ,某些半导体材料的光电导效应在激发光源撤去以后会持久地保持下去 ,当温度升高超过这个温度 (称为淬变温度 )以后 ,这种持续的光电导现象会消除 ,称为稳恒光电导现象 .而且这种光电导效应具有很强的局域性 .采用电学测量方法 ,通过测量激光照射前后电导率随温度的变化研究了掺 Ge的 Zn Se的稳恒光电导效应 ,结果发现淬变温度高达 2 1 0 K的稳恒光电导效应 .并通过研究光电阻随光照位置变化的趋势研究了这种光电导的局域性特性 ,结果发现在淬变温度以上局域性随稳恒光电导消失而消失     

Properties of highly conducting nitrogen-plasma-doped ZnSe:N thin films

Values for the acceptor ionization energy, E_a, and compensating donor ionization energy, E_d, have been obtained from an analysis of variable-temperature photoluminescence data taken for a series of highly conducting nitrogen-plasma doped ZnSe thin films. E_awas found to be highly temperature dependent, with values ranging from E_a ~110 meV at low temperatures to ~60 meV at room temperature. The compensating donor ionization energy ranged from E_d ~31 meV at low temperatures to ~24 meVat room temperature. These results provide clear evidence of thenonhydrogenic nature of the nitrogen acceptor state in heavily doped ZnSe:N thin films and suggest that interstitial bonding of N, at two or more stable sites, may play a central role in the p-type doping and compensation of this material at high doping levels.     

The study of nitrogen doping in ZnSe and ZnSe:Te

A comparative study of nitrogen doping in ZnSe and ZnTe has been performed and the results suggest that dopant solubility seems to be the limiting factor, at least under our growth conditions, in obstructing degenerate p-type doping of ZnSe. In an effort to increase the nitrogen acceptor concentration in ZnSe, we have investigated the effects of Te isoelectronic impurity on the nitrogen doping concentration in ZnSe. It was found that the total nitrogen concentration and the nitrogen acceptor concentration are indeed increased, but the room temperature free hole concentration actually drops slightly. Temperature dependent transport measurements were performed and the results show a large increase in compensation ratio as well as a dramatical reduction in hole mobility. The latter is attributed to the tendency for hole localization at the isoelectronic impurity.     

Well-width dependence of the phase coherent photorefractive effect in ZnSe quantum wells

We report on the well-width dependence of the phase coherent photorefractive (PCP) effect in ZnSe/ZnMgSe single quantum wells (QWs) using 90 fs light pulses. The experiments are performed in a four-wave-mixing configuration at temperatures between 25 and 65 K. The ZnMgSe/ZnSe QWs with 10, 5 and 3 nm well width were grown on GaAs substrate using molecular beam epitaxy. With decreasing QW thickness we observe a reduction of the PCP diffraction efficiency. This is attributed to the higher electron energy in small QWs due to the quantum size effect, which leads to a reduced equilibrium density of captured electrons in the QW. With increasing temperature the PCP signal further decreases which is attributed to thermal activation of the QW electrons back to the GaAs substrate.     

Nitrogen doping of ZnSe and CdTe epilayers: A comparison of two rf sources

Nitrogen is the most promising dopant for p-type ZnSe, and is attractive for CdTe p-type doping, but unwanted compensating centers have limited the maximum achievable carrier densities in both of these material systems. One important factor is the nitrogen source. The characteristics of the nitrogen source (i.e., ratio of charged to neutral species) influence both the nitrogen incorporation rate and the types of centers which may be produced in epilayers. We have performed a study of ZnSe:N and CdTe:N using two different rf plasma sources: an Oxford CARS-25 and an EPI Unibulb. The EPI source has a very high efficiency for producing atomic nitrogen (∼70%) while maintaining a very low total ion content. Both sources gave a high incorporation of nitrogen; however, Hall measurements indicate that compensation is still a problem in p-type doping of these materials. A detailed liquid-helium-temperature photoluminescence study has been performed to monitor changes in the defect structure produced using different source operating parameters.     

Growth of Nanocrystalline ZnSe:N Films by Pulsed Laser Deposition

Nanocrystalline zinc-blende-structured ZnSe:N films have been deposited on GaAs(100) substrates by pulsed laser deposition (PLD). The growth of the nanocrystalline ZnSe:N films is found to be greatly affected by the pressure of ambient N₂. X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM) results show that the morphologies of the as-grown films are sensitive to the ambient pressure at a fixed substrate temperature of 300 °C, and the sizes of the as-grown ZnSe:N nanocrystals increase as the ambient pressure increases from 0.1 Pa to 100 Pa. The average sizes of the as-grown nanocrystals are estimated to be about 19 nm, 29 nm, and 71 nm for 0.1 Pa, 1 Pa, and 100 Pa ambient N₂ pressure, respectively. X-ray photoelectron spectroscopy analyses show that the N-doping concentration in the as-grown film is over 10²¹ cm⁻³. Raman spectra demonstrate the broadening of the longitudinal optical (LO) phonon and transverse optical (TO) phonon modes of the ZnSe nanocrystals. Based on these analyses, the mechanism of the formation of ZnSe:N nanocrystals is discussed.     

Characteristics of deep levels in Al-doped ZnSe grown by molecular beam epitaxy

We investigated the characteristics of deep levels in heavily Al-doped ZnSe layers grown by molecular beam epitaxy, whose electron concentration is saturated. Low-temperature photoluminescence showed deep level emission around 2.25 eV, and its intensity increases with Al concentration. This deep-level is located at 0.55 eV above valence band maximum, implying a point defect such as a self-activated center, Al_ZnV_Zn. Deep-level transient spectroscopy was used to investigate non-radiative trap centers in Al-doped ZnSe layers, and showed the presence of two electron trap centers at depths of 0.16 and 0.80 eV below conduction band minimum, with the electron capture cross-sections of 810⁻¹² and 1×10⁻⁷ cm², respectively. It is suggested the carrier compensation in heavily Al-doped ZnSe layers be ascribed to the deep levels.     

Cr2+:ZnSe中红外激光器

Cr2 掺杂Ⅱ-Ⅵ族化合物在中红外波段的输出,在气体检测、遥感、通信、眼科医学、神经外科等领域有着重要的应用前景.目前已经获得了最大1.7 W的连续输出功率,18.5 W的平均脉冲功率,1100 nm的调谐范围和最窄4 ps的脉宽.对Cr2 :ZnSe连续、脉冲、随机纳米激光器以及其它的Cr2 掺杂Ⅱ-Ⅵ族化合物激光器的最新的国内外研究进展进行了综述.     

N-型ZnSe:Cl的分子束外延生长

本文报道利用分子束外延(MBE)技术,采用高纯ZnCl_2作掺杂源,成功地进行了n—型ZnSe:Cl的分子束外延生长。n-ZnSe:Cl/p-GaAs异质结构的伏-安(I—V)特性和热探针测试显示外延层呈n型导电特性。反射高能电子衍射(RHEED)和x射线衍散谱测量表明ZnSe:Cl外延层具有较好的晶体质量。     

室温350~850 nm ZnSe晶体生长及阴极荧光光谱图谱分析

室温下利用阴极荧光光谱技术对ZnSe晶体进行了350~850 nm的无损全光阴极荧光图谱检测,分析了晶体内部缺陷及夹杂情况,室温下测得ZnSe晶体在400~550 nm的阴极荧光光谱,阴极荧光光谱测得462 nm处的ZnSe本征发光峰。缺陷处测得462 nm的本征发光峰和453 nm的缺陷发光峰,结合能谱分析,ZnSe晶体表面缺陷处的Zn：Se比约为6：4。阴极荧光图谱中缺陷处发光峰主要来自Zn夹杂缺陷发光。     

Electrical transport characterizations of nitrogen doped ZnSe and ZnTe films

Temperature-dependent Hall effect measurements are reported on a series of nitrogen doped ZnSe and ZnTe epilayers using a van der Pauw configuration. A Zn(Se,Te) pseudo-graded band gap layer was used to form ohmic contacts to p-type ZnSe. The activation energy of nitrogen in ZnSe at the infinite dilution limit was extrapolated to be 114 meV. For a ZnTe film having a room temperature free hole concentration of p = 4.1 x 10¹⁶ cm⁻³, the activation energy of the nitrogen acceptors was found to be 46 meV.     

Nitrogen doping in ZnSe by photo-assisted metalorgani vapor phase epitaxy

Doping characteristics of nitrogen in ZnSe using photo-assisted vapor phase epitaxy were investigated. The source precursors and the doping source were diethylzinc, dimethylselenide, and tertiarybutylamine. Photoluminescence, electrical properties of Schottky diodes, and electroluminescence from homo and double heterojunction diodes consistently supported p-type behavior of the ZnSe:N layers. At the conditions investigated, higher doping was achieved at lower substrate temperature and irradiation intensity; e.g., 350°C and 45 mW/ cm², respectively. As a guideline, net acceptor concentration was estimated as 2 x 10¹⁷ cm^-3 for the ZnSe:N layer with nitrogen concentration of 5 x 10¹⁷ cm^-3 revealed by secondary ion mass spectroscopy.     

Dislocation nucleation mechanism and doping effect in p-type ZnSe/GaAs

Nitrogen doped ZnSe/GaAs heterostructures grown at 150 and 250°C were studied by transmission électron microscopy (TEM). The density of threading dislocations and the interfacial dislocation structure in ZnSe/GaAs heterostructures are related to the N-doping concentration. In addition, in-situ TEM heating studies show that Frank partial dislocations formed below critical thickness in N-doped ZnSe/GaAs are the sources for nucleation of a regular array of misfit dislocations at the ZnSe/GaAs interface. By the dissociation of the Frank partial dislocations and interaction reactions between the dislocations, the 60° misfit dislocations form. The Frank partial dislocations bound stacking faults which usually form in pairs at the film/substrate interface. The density of stacking faults increases with increasing N-doping concentration. Thus, at high N-doping levels, the dislocation nucleation sources are close together and not all of the Frank partial dislocations dissociate, so that a high density of threading dislocations results in samples with high N-doping concentrations. The high density of threading dislocations in the ZnSe film are found to be associated with a reduction or saturation of the net carrier density.     

Au Catalyzed Carbon Diffusion in Ni: A Case of Lattice Compatibility Stabilized Metastable Intermediates

Nickel is a crucial catalyst for its excellent performance in active carbon atom–related catalysis such as hydrocarbon steam reforming and 1D carbon nanostructures preparation. The carbon diffusion activity in Ni is of critical importance in understanding the catalytic behavior and thereby the performance optimization. However, the carbonization process is still vague because of the hardly identified intermediates. In this paper, the metastable intermediates of nickel carbonization process are successfully stabilized by taking advantage of the epitaxial growth to elevate the structure transformation energy barrier. X‐ray diffraction, high‐resolution transmission electron microscopy, and synchrotron X‐ray absorption near edge structure data evidence the face‐centered cubic (fcc)? Ni _x C nature of the intermediates and thus an fcc? Ni _x C‐intermediated nickel carbonization process from fcc? Ni to hexagonal close‐packed (hcp)‐Ni₃C is revealed, which is also confirmed by the Vienna ab initio simulation package calculation from the viewpoint of energy evolution. To the best of the knowledge, it is the first time to report the identification of the fcc? Ni _x C. More importantly, the introduction of Au is found promoted the catalytic growth of graphitic carbon using either oleylamine or C₂H₄ as carbon resource. The Au@Ni‐based hybrid catalysts exhibit lower reaction temperature and much higher carbon output than pure Ni.     

Cr2+:ZnSe的激光输出和调谐性能

Cr2 :ZnSe具有很宽的吸收带和发射带,是中红外波段优秀的可调谐激光材料.从吸收光谱、发射光谱以及角度调谐输出对Cr2 :ZnSe晶体的激光输出性能进行了研究.采用真空高温扩散法制备Cr2 :ZnSe晶体,获得了高浓度的Cr2 离子掺杂的厚1.7 mm,直径10 mm的薄片ZnSe晶体.使用中心波长2.05 μm,最大输出功率8 W的Tm离子掺杂的光纤激光器抽运,使用平凹腔结构搭建谐振腔,获得了最大平均功率1.034 W,中心波长2.367 μm,线宽10 nm的连续激光输出.利用角度调谐的方法,对Cr:ZnSe晶体的调谐性能进行了研究,在100 nm范围内获得了调谐输出.     

Gas source molecular beam epitaxy of ZnSe and ZnSe:N

The use of gas source molecular beam epitaxy, using hydrogen selenide and elemental Zn as source materials, has resulted in the growth of high quality ZnSe on closely lattice-matched GaAs and (In,Ga)P. The undoped ZnSe epilayers are comparable in quality to material grown by molecular beam epitaxy, as indicated by narrow double-crystal x-ray diffraction rocking curves and intense photoluminescence dominated by a single donor-bound near-bandedge excitonic feature. Nitrogen species, derived from a radio frequency plasma source, are successfully used as acceptor impurities for ZnSe; photoluminescence spectra confirm the incorporation of nitrogen by the presence of the expected donor-to-acceptor pair recombination. Atomic concentrations of nitrogen as high as 5 x 10¹⁸ cm^-3, are measured by secondary ion mass spectroscopy. Thus far, capacitance-voltage measurements indicate net acceptor concentrations (N_A -N_D) of approximately 10¹⁷ cm^-3.     

Low temperature growth and planar doping of ZnSe in a plasma-stimulated LP-MOVPE system

In a low-pressure metalorganic vapor phase epitaxy process, we used dc-plasma activated nitrogen to dope ZnSe, grown with ditertiarybutylselenide and dimethylzinc-triethylamine. The nitrogen concentration of up to 2 × 10¹⁸ cm⁻³ in the doped layers can be adjusted by the growth temperature, the dc-plasma power, and the N₂ dopant flow. Due to the high n-type background carrier concentration of the order of 10¹⁷ cm⁻³ in undoped samples, the doped layers show n-type conductivity or were semi-insulating because of an additional compensation by hydrogen incorporated with a concentration of the order of 10¹⁸ cm⁻³. A planar doping scheme was applied to reduce this hydrogen incorporation by one order of magnitude, although H₂ was used as carrier gas.