Current transport in metal-semiconductor-metal (MSM) structures

The current-voltage characteristics of a metal-semiconductor-metal structure (essentially two metal-semiconductor contacts connected back to back) have been studied based on the thermionic emission theory. When a uniformly doped semiconductor is thin enough that it can be completely depleted before avalanche breakdown occurs, the structure can exhibit many novel transport behaviors. Two outstanding features of the structure are that (1) a wide range of high-level injection of minority carriers can be achieved by varying the barrier heights of the two contacts and (2) the critical voltage at which the minority carrier injection increases rapidly can be varied by varying the semiconductor doping and thickness.

Experimental silicon MSM structures of PtSi-Si-PtSi have been made from n-type silicon with doping of 4×10¹⁴ cm⁻³ and thickness of 12 μm. The critical voltage at room temperature is about 30 V. The current increases over five orders of magnitude with only 10 per cent increase of the voltage. The above results and other measurements over wide temperature range do substantiate the theoretical predictions.     

Dark current characteristics of GaAs metal-semiconductor-metal(MSM) photodetectors

Calculations of the electron and hole components of dark current in a GaAs metal-semiconductor-metal (MSM) photodetector are presented. A quantum-mechanical model for the electron and hole transport behavior in the contact regions which is used to determine dark current as a function of electric field is developed. The model reduces to a conventional thermionic emission model if an ideal barrier transmission coefficient is assumed. In order to assess the accuracy of the model, photodetectors have been fabricated and tested. Theoretical calculations and experimental data are compared, and good agreement is obtained. Possible modifications to enhance the usefulness of the model are discussed     

Structural properties of nitrogen-doped ZnSe epitaxial layers grown by MBE

We have used transmission electron microscopy (TEM) and high-resolution x-ray diffraction (HRXRD) techniques to investigate the structural properties of ZnSe doped with nitrogen, in the concentration range of 1 × 10¹⁸ to 2 × 10¹⁹cm⁻³. The nitrogen-doped layers contain substantial residual compressive strain at layer thicknesses where undoped ZnSe would be completely relaxed. The residual strain is clearly observed both in the inequality of the lattice constants (measured by HRXRD) parallel and perpendicular to the growth direction, and in the reduction of the misfit dislocation density (measured by TEM) relative to undoped ZnSe. In addition to the reduction in dislocation density, the misfit dislocations form a regular rectangular grid, rather than the irregular array seen in undoped ZnSe. The effective relaxed ZnSe lattice constant, as measured by x-ray diffraction, decreases as the nitrogen concentration increases. For the highest nitrogen concentration, this reduction in lattice constant, however, is greater than can be explained by the shorter Zn-N bond distance of theoretical predictions.     

Fast AlGaN metal-semiconductor-metal photodetectors grown onSi(111)

Metal-semiconductor-metal photodetectors have been fabricated on AlGaN grown on Si(111) by molecular beam epitaxy for solar-blind applications. A cutoff wavelength of 290 nm and an ultraviolet/visible contrast of more than three orders of magnitude are achieved. Time response measurements show fast exponential decays. With a minimum decay time of 150 ns. The photodetectors present responsivities that increase with bias, reaching 15 mA/W at 4 V bias     

Performance of a near-infrared GaAs metal-semiconductor-metal (MSM)photodetector with islands

A GaAs metal-semiconductor-metal (MSM) photodetector with ultrasmall gold islands deposited on its photosensitive surface is described. The interdigitated detector is fabricated on a semi-insulating substrate in a MESFET-compatible technology. Responsivity as high as 1.8 A/W is obtained at 0.86 μm and a bias voltage of 8 V. This represents over a sixfold increase with respect to responsivity of a conventional MSM photodetector. The mechanism for dark current is suggested and breakdown characteristics are presented     

Hg1-xCdxTe metal-semiconductor-metal (MSM)photodetectors

Metal-semiconductor-metal (MSM) detectors with active layers of Hg _1-xCd_xTe (x=0.62-0.74) and electrode spacings of 2, 4, and 6 μm have been fabricated and characterized. Direct-current measurements have shown a low dark current and high responsivity from 0.15 to 1.5 A/W at 10-V bias. The lowest values of dark current (0.16 mA cm²) were obtained for detectors which incorporated an overlayer of CdTe. For detectors without the overlayer, increasing the Cd mole fraction resulted in a decrease in the dark current and a reduction in the 300-nm responsivity. Measurements of frequency response for these detectors show a maximum loss of 8 dB to 20 GHz. These results compare favorably with high-performance MSM detectors based on In_0.53Ga_0.47As with a lattice-matched barrier layer of In_0.52Al_0.48As     

Low dark current GaAs metal-semiconductor-metal (MSM) photodiodes using WSixcontacts

We present the fabrication and characterization of metal-semiconductor-metal (MSM) photodiodes using the same undoped GaAs layer that is used as a buffer layer in the epitaxial structure for GaAs field effect transistors (FET's). To study the dark current mechanism, various metal electrodes used for Schottky contacts are examined. A drastic V-shape relationship between the dark current of MSM photodiode and the Schottky barrier height is found. An extremely low dark current (a few nanoamperes) in the MSM photodiode is obtained by using tungsten silicide as electrode metal. It is concluded that the dark current is a function of a rivalry relation between the electron injection at the cathode and the hole injection at the anode. The internal gain of the MSM photodiode with tungsten silicide contacts is found, and possible mechanisms are discussed. A flat frequency response up to 1.3 GHz is obtained. The results shows the feasibility of MSM photodiodes for use as photodetectors with low minimum detectable power, and their applicability to monolithic integration with FET circuits.     

Analysis of a GaAs metal-semiconductor-metal (MSM) photodetectorwith 0.1-μm finger spacing

The temporal response of a GaAs metal-semiconductor-metal (MSM) photodetector with a finger spacing of 0.1 μm is discussed. The intrinsic detector has a minor effect (25% increase) on the full width at half-maximum (FWHM) of the temporal response of the device and its parasitic circuit elements. The analysis indicates that a long time constant due to the decay of holes is solely responsible for this increase. The smallest FWHM for this detector is estimated to be less than 2.5 ps     

A buried-cap planar stripe (BCP) GaAlAs laser with ZnSe current-confinement region by MBE

A GaAlAs visible laser with a novel structure, called a buried-cap planar (BCP) stripe laser, has been developed. It has an epitaxial ZnSe semiinsulating layer grown by molecular beam epitaxy (MBE) on a cladding layer, leaving a narrow stripe for the current confinement. In this laser a reflective index waveguide is constructed by using a channeled substrate, and the current flow is confined by growing the semi-insulating ZnSe single crystal on the cladding layer after mesa etching the cap layer. There are many advantages in this structure: the diffusion process is not necessary, the current confinement is complete, and the thermal strains and misfit dislocations can be avoided. These advantages assure a highly reliable device performance, and a high-yield fabrication process.     

AlN/GaN/AlN heterostructures grown on Si substrate by plasma-assisted MBE for MSM UV photodetector applications

The AlN/GaN/AlN heterostructures were successfully grown on silicon substrate by plasma-assisted molecular beam epitaxy (MBE). High purity gallium (7N) and aluminum (6N5) were used to grow GaN and AlN, respectively. The structural and optical properties of the samples have been investigated by high-resolution X-ray diffraction (HR-XRD), photoluminescence (PL), Raman spectroscopy, transmission electron microscopy (TEM), selected area electron diffraction (SAED), dark field scanning transmission electron microscopy (DF STEM), and high-angle annular dark field scanning transmission electron microscopy (HAADF STEM). HR-XRD measurement showed that the sample has a typical diffraction pattern of hexagonal AlN/GaN/AlN heterostructures. Raman spectra revealed all four Raman-active modes, i.e., GaN-like E₂ (H), AlN-like A₁ (TO), AlN-like E₂ (H), and AlN-like A₁ (LO) inside the AlN/GaN/AlN heterostructures. Good thickness uniformity of the layers and high-quality hetero-structures without cracking were confirmed by TEM, SAED, DF STEM and HAADF STEM. The fabricated AlN/GaN/AlN heterostructures based metal-semiconductor-metal (MSM) for the UV photodetector shows a rise and fall of photoresponses, suggesting that the AlN/GaN/AlN heterostructures have good carrier transport and crystallinity properties.     

Structure of CdTe(111)B grown by MBE on misoriented Si(001)

Single domain CdTe(111)B has been grown on Si(001) substrates tilted 1^o 2^o, and 4^o toward [110]. All the layers started with a double-domain structure, then a transition from a double- to a single-domain was observed by reflection high energy electron diffraction. A microscopic picture of this transition is presented. We also measured the tilt between CdTe(111)B and Si(001). The result does not follow the tilt predicted by the currently existing model. A new model of the microscopic mechanism of CdTe(111)B growth is presented. New evidence indicates that optimizing the tilt of the substrate surface is very crucial in improving the CdTe(111)B crystal quality.     

在Si(211)衬底上分子束外延CdTe的剪切应变

利用高分辨率X射线衍射技术对分子束外延CdTe（211）B/Si（211）材料的CdTe外延薄膜进行了倒易点二维扫描,并通过获得的对称衍射面和非对称衍射面的倒易空间图,对CdTe缓冲层的剪切应变状况进行了分析.研究发现,对于CdTe/Si结构,随着CdTe厚度的增加,[1-1-1]、[01-1]两个方向的剪切角γ[1-1-1]和γ[01-1]都有变小的趋势,且γ[1-1-1]的大小约为γ[01-1]的两倍;对于CdTe/ZnTe/Si结构,ZnTe缓冲层的引入可以有效地降低CdTe层的剪切应变.     

P-Type doping with arsenic in (211)B HgCdTe grown by MBE

Arsenic incorporation and doping in HgCdTe layers grown by molecular beam epitaxy (MBE) were examined in this paper. Arsenic incorporation into MBE-HgCdTe was carried out in two different ways: (1)ex-situ arsenic ion-implantation on indium-doped n-type HgCdTe layers, and (2) through a new approach called arsenic planar doping. We report onex-situ arsenic diffusion on indiumdoped MBE-HgCdTe layers at 450°C. In the investigated layers, arsenic redistribution occurs with a multi-component character. We obtained a diffusion coefficient of D_As = (1-3) × 10⁻¹³ cm²/s at 450°C. Results of differential Hall and fabricated p-n junctions suggest that during high temperature annealing, arsenic preferentially substitutes into Te sublattices and acts as acceptor impurities. In the second case, arsenic has been successfully incorporated during the MBE growth as an acceptor in the planar doping approach. Withoutex-situ annealing, as-grown layers show up to 50% activation of arsenic during the growth. These results are very promising forin-situ fabrication of infrared devices using HgCdTe material.     

AlN-GaN quarter-wave reflector stack grown by gas-source MBE on(100) GaAs

A 15 period, wurtzite-structure AlN-GaN reflector stack has been grown on (100) GaAs by gas-source molecular beam epitaxy. A peak reflectance of over 90% has been realised, in agreement with a matrix-method computer simulation. Our results support the feasibility of vertical Fabry-Perot cavity optoelectronics using nitride materials     

ZnSe单晶薄膜生长控制和掺杂控制

为研制ZnSe基蓝绿色半导体激光器,必须能够对组成激光器的各单元材料实现精确控制生长,包括材料的晶体质量、掺杂浓度和生长速度等。ZnSe是制备蓝绿激光器的基础材料,本文报道利用MBE技术对ZnSe材料进行生长控制和掺杂控制的初步研究结果。     

Characteristics of GdGaO grown by MBE

Ga₂O₃/GdGaO dielectric stacks have been grown on GaAs for MOSFETs. This paper highlights variations in the characteristics of GdGaO as the Gd flux, Ga₂O flux and substrate temperature are changed. The growth rate, composition, crystallinity are discussed and the sheet resistance of final MOSFET structures are presented. The Gd compositional variation with depth is examined using Rutherford back scattering (RBS) and electron energy loss spectroscopy (EELS).     

用分子束外延技术生长蓝紫激光二极管

日本在欧洲的Sharp实验室(SharpLaboratoriesofEurope雪研制成功了世界上第1个用分子束外延(MBE)生长技术生长的蓝-紫激光二极管。采用氮化物半导体材料制作的这种器件可用于下一代Blu-ray标准数字视频光盘(DVD)存储和视频读放装置。在蓝宝石基质上生长的量子阱脊形波导激光器以脉冲模式在室温下工作,阈值电流密度大约为30kA/cm2。据参加此项研究工作的研究人员介绍,该激光器的性能可与用金属有机化学汽相沉积(MOCVD)法生长的第1个蓝-紫激光器相媲美。很长时间以来,人们采用MOCVD法生长蓝-紫InGaN激光器,而用MBE法生长低功率蓝-…     

ZnSe homoepitaxial MSM photodetectors with transparent ITO contact electrodes

We report the homoepitaxial growth of ZnSe layers on ZnSe substrates by molecular beam epitaxy (MBE). It was found that we can only observe an extremely strong ZnSe (004) x-ray peak with a full-width-at-half-maximum of 21.5 arcsec, which is much smaller than that observed from ZnSe grown on GaAs substrates. Photoluminescence and Hall measurement also indicate that the quality of our homoepitaxial ZnSe layers is good. ZnSe-based homoepitaxial metal-semiconductor-metal photodetectors with transparent indium-tin-oxide (ITO) contact electrodes were also fabricated. It was found that although ITO transparent contact electrodes can result in large photon absorption and large photocurrents, the low Schottky barrier height between ITO and homoepitaxial ZnSe would also result in relatively large dark currents. With an incident wavelength of 450 nm and a 1-V applied bias, it was found that the maximum responsivity is about 0.13 A/W, which corresponds to a quantum efficiency of 35%. Furthermore, it was found that the detector responsivity drops by more than two orders of magnitude across the cutoff region.     

MBE生长高功率半导体激光器

对影响分子束外延（ ＭＢＥ） 材料生长的一些主要因素进行了细致的分析。利用ＭＢＥ生长出ＧａＡｌＡｓ／ＧａＡｓ 梯度折射率分别限制单量子阱激光器（ＧＲＩＮ－ ＳＣＨ－ ＳＱＷ） 材料。利用该材料制作出的列阵半导体激光器的准连续输出功率达到了６０ Ｗ（ｔ＝ ２００ μｓ,ｆ＝ ５０ Ｈｚ） ,峰值波长为８０８ ．４ ｎｍ 。     

Analysis of MBE grown AI(x)Ga(1-x)As-GaAs heteroepitaxial layers by rutherford backscattering

The Rutherford Backscattering (RBS) technique has been optimized for the measurement of thin (>85Å) alternating layers of A1(x)Ga(l-x)As and GaAs, which are at the limit of resolution of standard RBS measurements. RBS analysis of these structures provides both layer thicknesses and Al content. This information is useful for device processing since the layer thicknesses impact the FET threshold voltage and the Al content is important for proper selective chemical etching. Experimental conditions for the beam energy, detection angle, and sample rotation have been determined which allow measurement of the layer thicknesses to a precision of ±20-30Å (for layers >85A) and of the Al content,x, to a precision of ±.02. Comparison of the RBS data with Transmission Electron Microscopy (TEM), Cathodoluminescence (CL), and Reflection High Energy Electron Diffraction (RHEED) measurements on the same material showed very good agreement between the different techniques. The advantage of RBS is that it can measure much thinner layers than CL and it provides thickness and composition information at the same time whereas TEM only provides thickness for this particular material. RBS is currently used tocharacterize a monitor wafer from each day’s run to provide a processing baseline for the MBE growth.