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.     

II-VI多量子阱中局域化激子的受激发射

通过对ＺｎｘＣｄ１－ｘＴｅ－ＺｎＴｅ多量子阱样品的光泵受激发射研究讨论了材料中激子局域态对受激发射激子过程以及受激发射特性的影响。两块不同组份和不同局域态密度的ＺｎＣｄＴｅ－ＺｎＴｅ多量子阱样品受激发射过程都是ｎ＝１重空穴激子参与的一系列过程，但在Ｚｎ０．６７Ｃｄ０．３３Ｔｅ－ＺｎＴｅ中发现了激子－激子散射的受激发射过程，而在Ｚｎ０．７８Ｃｄ０．２２Ｔｅ－ＺｎＴｅ中没有发现这一受激发射的激子过程。ｘ＝０．６７的样品中具有较高的局域化激子密度，其受激发射具有较高的阈值。     

Observation of [100] and [010] dark line defects in optically degraded znsse-based leds by transmission electron microscopy

We have used transmission electron microscopy to study the [100] and [010] dark line defects (DLDs) produced after photodegradation of a ZnSSe-based/GaAs heterostructure. Our results show that the DLDs are networks of elongated dislocation loops or half-loops that originate in the quantum well region during device operation. Our results also show that after photodegradation the grownin or pre-existing Frank-type stacking faults become tangles of dislocations. In contrast, the Shockley-type stacking faults remained unchanged for the photodegradation conditions studied indicating that they are more resistant to photodegradation than the Frank-type stacking faults. Our results suggest that the Frank-type stacking faults are the sources of the DLDs. The mechanism for degradation probably starts by the emission of very small clusters of vacancies from the Frank-type faults. Upon further illumination the dislocation loops bounding the vacancies grow by gliding on {111} planes and become hairpin-like dislocation loops.     

On the degradation of InGaAsP/InP-based bulk lasers

Degradation of InGaAsP-InP-based buried-heterostructure bulk (BH-bulk) lasers has been studied by means of electroluminescence (EL), photoluminescence (PL), electron-beam-induced current (EBIC) and transmission electron microscopy (TEM). Lasers with p/n as well as semi-insulating (SI) current-blocking layers were studied. The results show that moderate increases in the threshold current correlate well with formation of dark defects (DD's) (i.e., dark-line defects (DLD's) or dark-spot defects (DSD's), which cannot be distinguished in our case due to the narrowness of the laser stripe.) The DD's were found to be caused by dislocation loops. The dependence of threshold current increase on the number of DD's is explained in terms of a model which includes effects due to the DD's, as well as changes in the regions outside the DD's. The latter is found to be responsible for the major part of the threshold current increase. Values for the ratio between the carrier lifetimes inside and outside the DD's are presented, for the first time. In our lasers, strong degradation differs from moderate degradation in that DD's do not form during aging. The presence of dislocation loops only at the sidewalls of the active stripe in lasers with p/n current-blocking layers points to the sidewalls as being critical. The near absence of dislocation loops and the smaller increase in threshold current in SI lasers which have degraded strongly, compared to the strongly degrading p/n lasers, suggest that strong degradation is a synergistic combination of damage in the sidewalls and Zn indiffusion from the current-blocking layers     

Nanometer fabrication techniques for wide-gap II-VI semiconductors and their optical characterization

We have fabricated dots and wires down to 30 nm diameter and 60 nm width in ZnTe/GaAs and ZnSe/GaAs. The fabrication process relies on electron beam lithography and dry etching using a mixture of CH₄/H₂. We have extensively characterized the flat etched surfaces of both ZnTe and ZnSe using x-ray photoelectron spectroscopy (XPS), Raman scattering, and luminescence spectroscopy. Flat etched samples were also annealed. We found that improvements in the emission spectrum were related probably to defect removal in the asgrown samples and had a secondary impact in the etched and annealed samples. From XPS data, some evidence is found of zinc desorption from the surface, which is later corroborated by Raman scattering in etched wires with the appearance of tellurium modes. The luminescence spectra of flat etched samples show no major changes in the spectral lines, with a hint of a change in relative concentration in donors and acceptors and evidence of ZnTe/GaAs intermixing closer to the interface. No line broadening is observed and the emission intensity is retained. The emission and Raman scattering spectra of etched wires and dots confirms that negligible fabrication damage is incurred as well as the absence of further strain or strain release after etching.     

(GaAl)As/GaAs质子轰击隔离条形DH激光器的退化原因

对研制的(GaAl)As/GaAs质子轰击隔离条形DH激光器的退化原因进行了实验分析。结果表明:快退化主要起因于有源区内的暗点、暗线及暗区等缺陷的增殖;腔面氧化是限制寿命在千小时的原因之一;质子轰击引入的点缺陷移入有源区是器件限制寿命在万小时的原因之一。     

Annihilation of nonradiative recombination centers in GaAs/AlGaAs multiquantum well structures as a result of exposure to plasma

The effect of exposure to a low-energy plasma (CF₄, Ar, Kr) on the photoluminescence properties of GaAs/AlGaAs multiquantum well structures is examined. It is shown that the photoluminescence of the quantum wells in the surface region is quenched after plasma exposure and the depth of this region increases with increase of exposure time. The photoluminescence intensity from the quantum wells located beyond this region increases. We associate these changes in the photoluminescence intensity with the effect of plasma-induced nonequilibrium point defects diffusing with anomalous rapidity into the depth of the structure. Fiz. Tekh. Poluprovodn. 31, 1436–1439 (December 1997)     

Formation of ohmic contacts to p-ZnTe

Formation and temperature stability of sputter deposited gold ohmic contacts to molecular beam epitaxially grown p-type ZnTe (doped with nitrogen to a free hole concentration of ≈3 × 1018 cm³) have been studied using current-voltage (I-V), Auger electron spectroscopy, secondary ion mass spectrometry, and optical and scanning electron microscopy. The I-V characteristics of ≈1500Å Au/p-ZnTe contacts were measured as-deposited and after heat treatments at 150, 200, 250, and 350°C for 15 min intervals up to 90 min. As deposited, the contacts were poor Schottky contacts, but became ohmic after 15 min at all temperatures. There was an increased resistance at t>15 min for T≤250°C, and a very large resistance increase upon heat treatment for all times at 350°C. The interface between the metallization and ZnTe was initially very planar, and remained planar upon formation of the ohmic contact. Upon heating at T>250°C, Au diffused into ZnTe. The ohmic behavior of the Au/p-ZnTe contacts is attributed to this diffusion which created a highly doped near-surface region in the ZnTe. Microscopy showed that Au also migrated across the ZnTe surface forming an extended reaction zone (≈100 μm) around the dot contact at T≥250°C.     

ZnO/ZnSe/ZnTe Heterojunctions for ZnTe-Based Solar Cells

ZnO and ZnSe are proposed as n-type layers in ZnTe heterojunction diodes to overcome problems associated with the n-type doping of ZnTe. The structural properties and electrical characteristics of ZnO/ZnTe and ZnO/ZnSe/ZnTe heterojunctions grown by molecular beam epitaxy on (001) GaAs substrates are presented. ZnO shows a strong preference for c-plane (0001) orientation resulting in a nonepitaxial relationship and high density of rotational domains for growth on ZnTe (001). ZnSe/ZnTe structures demonstrate a (001) epitaxial relationship with high density of {111} stacking faults originating at the heterojunction interface. ZnO/ZnSe/ZnTe heterojunction diodes show excellent diode rectification and clear photovoltaic response with open-circuit voltage of V _OC = 0.8 V.     

Characterization of dark noise in CdZnTe spectrometers

Systematic measurements of dark noise spectra of CdZnTe x- and γ-ray spectrometers, correlated with the dc I-V characteristics and detector technology, are reported. The dark noise of two innovative CdZnTe spectrometer configurations are studied: metal-semiconductor-metal (MSM) resistive detectors with three terminals as well as heterostructure PIN detectors with thermally evaporated n⁺ CdS and p⁺ ZnTe contacts, which are fabricated on high pressure Bridgman CdZnTe (Zn=10%) crystals. The two innovative CdZnTe spectrometer configurations presented here exhibit very low dark (leakage) current. By reducing the dc value of the dark (leakage) current below 1 nA, shot noise becomes the dominant noise mechanism and the contribution of 1/f noise becomes negligible. The use of non-injecting contacts (evaporated gold) for the MSM detectors and the operation of the PIN detector in the reverse bias mode prevent generation-recombination noise which becomes dominant with injecting contacts (for example MSM detectors with evaporated indium and titanium contacts) or when operating the PIN detector in the forward bias mode. Surface leakage is reduced by applying surface passivation but is eliminated only by using the three terminal MSM configuration which exhibits simple shot noise instead of the suppressed shot noise observed in the two terminal MSM spectrometers. The noise measurements are useful for optimizing detector technology.     

Molecular beam epitaxial growth of Cd<Subscript>1−y</Subscript>Zn<Subscript>y</Subscript>Se<Subscript>x</Subscript>Te<Subscript>1−x</Subscript> on Si(211)

We report on the first successful growth of the quaternary alloy Cd_1−yZn_ySe_xTe_1−x(211) on 3-in. Si(211) substrates using molecular beam epitaxy (MBE). The growth of CdZnSeTe was performed using a compound CdTe effusion source, a compound ZnTe source, and an elemental Se effusion source. The alloy compositions (x and y) of the Cd_1−yZn_ySe_xTe_1−x quaternary compound were controlled through the Se/CdTe and ZnTe/CdTe flux ratios, respectively. Our results indicated that the surface morphology of CdZnSeTe improves as the Zn concentration decreases, which fits well with our previous observation that the surface morphology of CdZnTe/Si is poorer than that of CdSeTe/Si. Although the x-ray full-width at half-maximums (FWHMs) of CdZnSeTe/Si with 4% of Zn + Se remain relatively constant regardless of the individual Zn and Se concentrations, etched-pit density (EPD) measurements exhibit a higher dislocation count on CdZnSeTe/Si layers with about 2% Zn and Se incorporated. The enhancement of threading dislocations in these alloys might be due to an alloy disorder effect between ZnSe and CdTe phases. Our results indicate that the CdZnSeTe/Si quaternary material with low Zn or low Se concentration (less than 1.5%) while maintaining 4% total Zn + Se concentration can be used as lattice-matching composite substrates for long-wavelength infrared (LWIR) HgCdTe as an alternative for CdZnTe/Si or CdSeTe/Si.     

Blue-green ZnSe lasers with a new type of active region

We report the results of an experimental study of molecular-beam epitaxy of ZnSe-based laser heterostructures with a new structure of the active region, which contains a fractional-monolayer CdSe recombination region in an expanded ZnSe quantum well and a waveguide based on a variably-strained, short-period superlattice are reported. Growth of a fractional-monolayer CdSe region with a nominal thickness of 2–3 ML, i.e., less than the critical thickness, on a ZnSe surface (Δa/a∼7%) leads to the formation of self-organized, pseudomorphic, CdSe-enriched islands with lateral dimensions ∼10–30 nm and density ∼2×10¹⁰ cm⁻², which serve as efficient centers of carrier localization, giving rise to effective spatial separation of defective regions and regions of radiative recombination and, as a result, a higher quantum efficiency. Laser structures for optical pumping in the (Zn, Mg) (S, Se) system with a record-low threshold power density (less than 4 kW/cm² at 300 K) and continuous-wave laser diodes in the system (Be, Mg, Zn) Se with a 2.5 to 2.8-ML-thick, fractional-monolayer CdSe active region have been obtained. The laser structures and diodes have an improved degradation resistance. Fiz. Tekh. Poluprovodn. 33, 1115–1119 (September 1999)     

Tuning Between Quantum-Dot- and Quantum-Well-Like Behaviors in Type II ZnTe Submonolayer Quantum Dots by Controlling Tellurium Flux During MBE Growth

We report tuning of properties of type II nanostructures between quantum dot (QD)-like and quantum well (QW)-like behaviors in ZnSe layers with ZnTe submonolayer insertions, grown by migration-enhanced epitaxy. The sizes of QDs are estimated from magneto-photoluminescence (PL) measurements, which showed no significant change in the QD lateral size with increasing Te flux, indicating increase in QD density instead. The area density of QDs is estimated from the results of secondary-ion mass spectrometry measurements. It is determined that, in the sample grown using the highest Te flux, the electronic wavefunctions begin to overlap, leading to QW-like behavior before the formation of a full QW layer. This is also confirmed via temperature-dependent time-resolved PL, which showed significant change of excitonic lifetimes and binding energies of type II excitons.     

Growth and characterization of ZnTe:N; p-ZnTe/n-AISb diodes

In recent months the successful p-doping of ZnSe and Zn(S,Se) using a nitrogen plasma source during growth by molecular beam epitaxy was one factor leading to the realization of diode lasers and light emitting diodes. This paper reports the results of the nitrogen doping of ZnTe using similar techniques. Doping levels exceeding the 10¹⁹ cm⁻³ range are reported along with electrical, optical, and microstructural characterization. The nitrogen-doped ZnTe is used to implement p-ZnTe/n-AISb diodes; the growth and characterization of these hetero-junction diodes are described.     

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.     

Oxygen-doped ZnTe phosphors for synchrotron X-ray imaging detectors

ZnTe:O powder phosphors were successfully prepared by a dry synthesis process using gaseous doping and etching media. It was found that dry doping by O₂ through ball-milling was an effective way to synthesize ZnTe:O powder phosphors and produced a red emission centered at 680 nm with a decay time of 1.1 μs. The emission intensity of dry O₂-doped samples was three times more intense than from ZnO-doped samples, possibly due to a more uniform distribution of oxygen substitution on tellurium sites. The samples annealed in a 95% N₂/5% H₂ forming gas atmosphere exhibited a x-ray luminescent efficiency five times higher than did powders annealed in vacuum or N₂ atmosphere. This enhancement was attributed to the removal of surface tellurium oxides. ZnTe:O phosphor screens were prepared with x-ray luminescence efficiencies equivalent to 56% of ZnSe:Cu,Ce,Cl and 76% of Gd₂O₂S:Tb screens under 17-keV radiation. An x-ray imaging resolution of 2.5 lines/mm was resolved, the same as that measured for commercial ZnSe:Cu,Ce,Cl and Gd₂O₂S:Tb screens. These results indicate that ZnTe:O is a promising phosphor candidate for synchrotron x-ray imaging applications.     

The new origin of dark-line defects in planar-stripe DH lasers

The transmission electron microscope (TEM) observations of degraded and undegraded diodes have shown that usually, though not always, small dislocation loops are introduced into the stripe area during selective zinc diffusion for stripe fabrication. These loops grow to giant dislocation loops, which correspond to dark line defects (DLD's) during device operation. The growth velocity of DLD's which originate from the loops with Burgers vectors ofa/2 langle 011 rangleinclined is more rapid than that ofa [001]normal anda/2 langle 110 rangleparallel to the junction plane, and is the same order of magnitude as the growth velocity of common DLD's which originate from the threading dislocations.     

半导体量子点最低导带态的量子限制效应

采用最新计算方法和半导体体材料传统量子计算结果,系统研究了14种半导体(Si,Ge,Sn,AlSb,GaP,GaAs,GaSb,InP,InAs,InSb,ZnS,ZnSe,ZnTe,CdTe)的立方量子点,得到了最低导带态的量子限制效应结果,我们把量子点对尺寸的依赖关系分为三类并详细讨论了它们的差别。     

Theoretical analysis of confined quantum state GaAs/AlGaAssolid-state photomultipliers

A detailed theoretical analysis of the design considerations of a solid-state photomultiplier based on avalanche multiplication of carriers out of confined quantum states is presented. Since these devices are unipolar, much lower noise and higher speed of performance are anticipated as compared with interband avalanche photodiodes. As an example of the design criteria for confined-state photomultipliers, a GaAs/Al_0.32Ga_0.68As multiquantum well structure is analyzed as to impact ionization rate, gain, dark current, and multiplied dark current. It is found that the highest gain is achieved in an asymmetric quantum well structure in which the second barrier height is half as large as the initial barrier height. The gain is further evaluated for a symmetric quantum well device. The effects of the applied electric field, quantum well doping concentration, and layer widths on device performance are examined     

Low pressure MOVPE growth of ZnSe,ZnTe, and ZnSe/ZnTe strained-layer superlattices

ZnSe and ZnTe single-crystal layers have been grown onto (100) GaAs substrates by low-pressure metalorganic vapor-phase epitaxy (LP-MOVPE) using the triethylamine-dimethylzinc adduct [DMZn(NEt₃)] as the zinc precursor. The selenium and tellurium precursors were H₂Se (5% in H₂) and di-isopropyltellurium (DiPTe), respectively. These two semiconductors have been grown with different VI/II molar ratios, at different growth temperatures, and with an overall growth pressure ranging from 40 to 400 Torr. Optimal growth parameters have been determined by optical means for the two materials. This information was then used to grow ZnTe/ZnSe strained-layer superlattices. We have studied structures grown on both ZnSe and ZnTe relaxed buffer layers which display a drastic dependence of the Stokes shift between photoluminescence and the optical bandgap on the nature of the buffer layer. Growth interruptions have been used to optimize the optical properties of the superlattices. Theoretical modeling of superlattice band structures has been performed using results of optical and structural characterizations. Observations of zone center transitions as well as excitons associated with the miniband dispersion of the superlattices are reported, in agreement with the theoretical calculation.