可用于Ⅲ族氮化物生长的50mm 3C-SiC/Si(111)衬底的制备

利用新研制出的垂直式低压CVD(LPCVD)SiC生长系统,获得了高质量的50mm 3C-SiC/Si(111)衬底材料.系统研究了3C-SiC的n型和p型原位掺杂技术,获得了生长速率和表面形貌对反应气体中SiH4流量和C/Si原子比率的依赖关系.利用Hall测试技术、非接触式方块电阻测试方法和SIMS,分别研究了3C-SiC的电学特性、均匀性和故意调制掺杂的N浓度纵向分布.利用MBE方法,在原生长的50mm 3C-SiC/Si(111)衬底上进行了GaN的外延生长,并研究了GaN材料的表面、结构和光学特性.结果表明3C-SiC是一种适合于高质量无裂纹GaN外延生长的衬底或缓冲材料.     

Time-resolved photoluminescence of polycrystalline GaN layers on metal substrates

The low-temperature time-resolved photoluminescence of polycrystalline GaN layers grown by molecular beam epitaxy on metal substrates (Mo and Ta) was investigated. The photoluminescence spectra observed include two emission bands in the ultraviolet spectral region. We assign one of these bands to recombination processes inside cubic nanocrystallites, which are formed in the hexagonal polycrystalline GaN host. The recombination radiation of cubic nanocrystallites is enhanced due to predominant trapping of the nonequilibrium electron-hole pairs in these crystallites.     

MBE growth and characterization of high purity GaAs/AIGaAs on the (110) surface of GaAs

An improved quality of (110) GaAs has been grown by molecular beam epitaxy using As₂ in lieu of As₄. The most pronounced effect of using As₂ is a higher doping efficiency of Si δ-doped GaAs layers, resulting in a mobility of the (110) layers, comparable to the reference (100) samples.

The high quality of the (110) GaAs was confirmed by low temperature photoluminescence. The spectrum of the GaAs layer shows a single dominant free exciton line with a linewidth of 1.0 meV.     

Deep levels,electrical and optical characteristics in SnTe-Doped GaSb Schottky Diodes

The GaSb layers investigated were grown directly on GaAs substrates by molecular beam epitaxy (MBE) using SnTe source as the n-type dopant. By using admittance spectroscopy, a dominant deep level with the activation energy of 0.23-0.26 eV was observed and its concentration was affected by the Sb₄/Ga flux ratio in the MBE growth. A lowest deep-level concentration together with a highest mobility was obtained for GaSb grown at 550°C under a Sb₄/Ga beam equivalent pressure (BEP) ratio around 7, which should correspond to the lowest ratio to maintain a Sb-stabilized surface reconstruction. In the Hall measurement, an analysis of the temperature-dependent mobility shows that the ionized impurity concentration increases proportionally with the sample’s donor concentration, suggesting that the ionized impurity was introduced by an SnTe source. In addition, optical properties of an undoped p-, a lightly and heavily SnTe-doped GaSb layers were studied by comparing their photoluminescence spectra at 4.5K.     

Optical properties of boron modulation-doped SiGe quantum wells and Si thin films

Effects of modulation doping on the radiative recombination in Si thin films and SiGe quantum wells (QWs) grown by molecular beam epitaxy are studied in details by photoluminescence (PL) spectroscopy. It is shown that the formation of a two-dimensional hole gas, either in the SiGe QWs or in the doping-induced notch potential in Si thin films, manifests itself in the appearance of characteristic broad luminescence bands. The PL properties are found to be strongly dependent on structure parameters as well as on the experimental conditions, such as photo-excitation intensity and measurement temperature. In the modulation doped SiGe QWs the recombination process is shown to be strongly affected by correlation phenomena, such as Fermi edge singularity.     

Molecular beam epitaxial growth of BGaAs ternary compounds

B_xGa_1−xAs ternary compounds with boron compositions varying up to x=1% have been grown by molecular beam epitaxy. Reflection high energy electron diffraction and double crystal x-ray diffraction measurements show that grown layers are single crystal with boron composition up to 0.25% and exhibit specular surface morphology. Photoluminescence measurements indicated a monotonic increase in energy bandgap with boron composition up to 0.25%. The layers showed p-type conductivity with hole concentration reaching the low 10¹⁹ cm⁻³ range. Increasing boron concentrations leads to rough surface morphology and reduction in photoluminescence intensity. Initial results indicate that lower growth temperature may be useful for increasing boron incorporation in BGaAs compounds.     

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.     

Effects of heat treatment on the 0.8 eV photoluminescence emission in GaAs grown by molecular beam epitaxy at low temperatures

We report 0.8 eV photoluminescence (PL) emission of GaAs grown at low temperatures between 325 and 400°C by molecular beam epitaxy. Effects of heat treatments of the 0.8 eV emission are compared with those of the 1.467 eV sharp bound exciton lines. This allows us to attribute the 0.8 eV emisson to the As-V_Ga center. We discuss the assigning of the As_i-V_Ga center to the well-known EL6. The PL intensity variation of 0.68 eV EL2 and 0.8 eV As_i-V_Ga seen in substrate materials is explained in terms of dislocation−mediated As_i-V_Ga transformation to EL2 whereas the PL intensity variation of 0.8 eV As_i-V_Ga for molecular beam epitaxy layers can be attributed to the growth condition.     

Selective doping of N-type ZnSe layers with chlorine grown by molecular beam epitaxy

N-type ZnSe with electron concentration up to 3 × 10²⁰ cm⁻³ and low resistivity down to 1 × 10⁻⁴ ohm-cm, has been grown using a selective doping technique with chlorine during molecular beam epitaxy. The photoluminescence evaluation shows that the selectively doped ZnSe layers are superior to uniformly doped ones, especially for the case of high-concentration chlorine doping. The in-depth profile of chlorine concentration in a selectively doped sample was measured with secondary-ion mass spectroscopy (SIMS). The SIMS analysis shows only slight diffusion of the incorporated chlorine atoms even in highly doped samples.     

InSb分子束外延原位掺杂技术研究

对InSb分子束外延薄膜的本征掺杂、N型掺杂以及P型掺杂进行了研究,其中分别以Be作P 型以及以Si、Te作N 型的掺杂剂。实验采用半绝缘的GaAs衬底作为InSb分子束外延用衬底,通过采用低温生长缓冲层技术降低大失配应力,获得高质量InSb外延膜。实验样品采用霍尔测试以及SIMS测试掺杂浓度和迁移率分析掺杂规律、掺杂元素偏聚和激活规律的影响因素。     

Influence of crystalline defects on transport properties of GaN grown by ammonia-molecular beam epitaxy and magnetron sputter epitaxy

GaN layers have been grown using an MBE/MSE (molecular beam epitaxy/magnetron sputter epitaxy) dual-mode system. The layers grown by the two techniques exhibited a large difference in crystalline quality and presented a broad spectrum of structural, optical, and transport properties that are useful for an analysis of the role of crystalline defects in GaN epilayers. The model of electron scattering by charged threading dislocations was applied in a theoretical fit of the mobility data. The theoretical fit in combination with x-ray diffraction and photoluminescence studies reveal the correlation between dislocation density, electron mobility, doping characteristics and yellow luminescence.     

Current-voltage characteristics of high current density siliconEsaki diodes grown by molecular beam epitaxy and the influence ofthermal annealing

We present the characteristics of uniformly doped silicon Esaki tunnel diodes grown by low temperature molecular beam epitaxy (T_growth=275°C) using in situ boron and phosphorus doping. The effects of ex situ thermal annealing are presented for temperatures between 640 and 800°C. A maximum peak to valley current ratio (PVCR) of 1.47 was obtained at the optimum annealing temperature of 680°C for 1 min. Peak and valley (excess) currents decreased more than two orders of magnitude as annealing temperatures and times were increased with rates empirically determined to have thermal activation energies of 2.2 and 2.4 eV respectively. The decrease in current density is attributed to widening of the tunneling barrier due to the diffusion of phosphorus and boron. A peak current density of 47 kA/cm² (PVCR=1.3) was achieved and is the highest reported current density for a Si-based Esaki diode (grown by either epitaxy or by alloying). The temperature dependence of the current voltage characteristics of a Si Esaki diode in the range from 4.2 to 325 K indicated that both the peak current and the excess current are dominated by quantum mechanical tunneling rather than by recombination. The temperature dependence of the peak and valley currents is due to the band gap dependence of the tunneling probability     

A study of the transition from high to low resistivity in As-grown GaAs MBE material

This work discusses the transition from high resistivity as-grown GaAs layers to thermally metastable low resistivity as-grown layers by molecular beam epitaxy. This transition occurs at about 430°C and coincides with a reflective high energy electron diffraction reconstruction change from a 2 × 1 to 2 × 4 pattern for an As₄/Ga beam equivalent pressure ratio of 20. For growth temperatures in the range 350 to 430°C, room temperature Hall-effect measurements have shown resistivities of <10₇ ohm-cm and photoluminescence has shown new peaks at 0.747 eV and a band from 0.708 to 0.716 eV at 4.2K, in unannealed material.     

An epitaxial Si/SiO2 superlattice barrier

An epitaxial strain layer Si/SiO₂ superlattice barrier (SLSB) for silicon formed by monolayers of adsorbed oxygen, sandwiched between adjacent thin silicon layers deposited with molecular beam, showed good epitaxy with an effective barrier height of 1.7 eV. Such a barrier should be important for future quantum devices in silicon, as well as new applications in conventional MOS technology.     

Study of photoluminescence spectra of GaMnAs produced by low-temperature molecular beam epitaxy

The luminescence spectra of the GaMnAs layers produced by low-temperature molecular beam epitaxy are studied at temperatures ranging from 4 to 150 K. In the spectra of the GaMnAs layers containing the MnAs clusters, local peaks at the energies of 1.36 and 1.33 eV are observed. It is shown that the red shift of the excitonic luminescence line depends on the Mn content in the semiconductor matrix.     

Near band-edge optical properties of cubic GaN with and without carbon doping

We report the results of studying the optical properties of cubic GaN thin films with photoluminescence and photoluminescence excitation spectroscopies. The films are deposited by plasma-assisted molecular beam epitaxy on GaAs (001) substrates, with and without intentional doping with carbon atoms (p-type doping). The evolution of the optical spectra of the C-doped samples is consistent with a picture in which carbon enters into N-vacancies at low concentrations, producing a marked improvement in the crystalline properties of the material. At higher concentrations it begins to form complexes, possibly due to interstitial occupation. The temperature dependence on the absorption edge of the doped material is also measured and is analyzed with standard theoretical models.     

气态源分子束外延AlxGa1-xAs(x=0～1)材料中Si的掺杂行为研究

研究了Si在AlxGa1-xAs(0≤x≤1)中的掺杂行为.为比较Al組份对Si掺杂浓度的影响,在用气态源分子束外延生长(GSMBE)掺Si n型AlxGa1-xAs(0≤x≤1)的所有样品时,n型掺杂剂Si炉的温度恒定不变.用Hall效应测量外延层的自由载流子浓度和迁移率,用X射线双晶衍射迴摆曲线测量外延层的组份.测试结果表明,当AlxGa1-xAs中Al组份从0增至0.38时,Si的掺杂浓度从4×1018cm-3降至7.8×1016cm-3,电子迁移率从1900 cm2/Vs降至100 cm2/Vs.这与AlxGa1-xAs材料的Γ-X直接—间接带隙的转换点十分吻合.在AlxGa1-xAs全组份范囲内,自由载流子浓度隨Al组份从0至1呈“V”形变化,在X=0.38处呈最低点.在x>0.4之后,AlxGa1-xAs的电子迁移随Al组分的增加,一直维持较低值且波动幅度很小.     

Influence of AlGaN deep level defects on AlGaN/GaN 2-DEG carrierconfinement

We have used low energy electron-excited nanoscale luminescence spectroscopy (LEEN) to detect the defects in each layer of AlGaN/GaN HEMT device structures and to correlate their effect on two-dimensional electron gas (2-DEG) confinement. We investigated AlGaN/GaN heterostructures with different electrical properties using incident electron beam energies of 0.5 to 15 keV to probe electronic state transitions within each of the heterostructure layers. AlGaN heterostructures of 25 nm thickness and nominal 30% Al concentration grown on GaN buffer layers on sapphire substrates by plasma-assisted molecular beam epitaxy exhibited a range of polarization-induced electron densities and room temperature mobilities. In general, the spectra exhibit AlGaN band edge emission at ~3.8 eV or ~4.0 eV, GaN band edge emission at ~3.4 eV, yellow luminescence (YL) features at 2.18 eV and 2.34 eV, and a large emission in the infrared (<1.6 eV) from the GaN cap layer used to passivate the AlGaN outer surface. These heterostructures also show high strain in the 2 nm-thick GaN layer with evidence for a Franz-Keldysh red shift due to piezoelectric charging. The LEEN depth profiles reveal differences between the structures with and without 2-DEG confinement and highlight the importance of AlGaN defects in the near 2-DEG region     

锗硅双层量子点的光电流特性

在分子束外延 (MBE)系统上用自组织方式生长了硅基双层锗量子点结构 ,并对样品进行光电流谱的测试。通过调节不同外加偏压来改变量子点中的费米能级位置 ,量子点中载流子所处束缚能级将随之发生变化 ,所得到的光电流谱的峰位也将因此而改变。由光电流谱得到的实验结果与常规的光致发光谱的结果相吻合。与单层锗量子点结构相比 ,双层结构的样品在光电特性上有着明显不同 :光电流谱中 ,在 0 .767e V及 0 .869e V处出现了两个峰 ,分别对应于载流子在不同的量子点层中的吸收。用这种结构的样品制成的红外光探测器能够同时对两种不同波长的光进行探测响应     

Microstructural and optical characterization of CdTe(211)B/ZnTe/Si(211) grown by molecular beam epitaxy

CdTe layers have been grown by molecular beam epitaxy on 3 inch nominal Si(211) under various conditions to study the effect of growth parameters on the structural quality. The microstructure of several samples was investigated by high resolution transmission electron microscopy (HRTEM). The orientation of the CdTe layers was affected strongly by the ZnTe buffer deposition temperature. Both single domain CdTe(133)B and CdTe(211)B were obtained by selective growth of ZnTe buffer layers at different temperatures. We demonstrated that thin ZnTe buffer layers (<2 nm) are sufficient to maintain the (211) orientation. CdTe deposited at ∼300°C grows with its normal lattice parameter from the onset of growth, demonstrating the effective strain accommodation of the buffer layer. The low tilt angle (<1°) between CdTe[211] and Si[211] indicates that high miscut Si(211) substrates are unnecessary. From low temperature photoluminescence, it is shown that Cd-substituted Li is the main residual impurity in the CdTe layer. In addition, deep emission bands are attributed to the presence of As_Te and Ag_Cd acceptors. There is no evidence that copper plays a role in the impurity contamination of the samples.