Electrical properties of molecular beam epitaxial GaAs grown at 300–450°C

We use the Hall effect and a new charge-transfer technique to study molecular beam epitaxial GaAs grown at the low substrate temperatures of 300–450°C. Layers grown from 350–450°C are semi-insulating (resistivity greater than 10⁷ Ω-cm), as grown, because of an As_Ga-related donor (not EL2) at E_C-0.65 eV. The donor concentrations are about 2×10¹⁸ cm⁻³ and 2×10¹⁷ cm⁻³ at growth temperatures of 300 and 400°C, respectively, and acceptor concentrations are about an order of magnitude lower. Relatively high mobilities (∼5000 cm²/V s) along with the high resistivities make this material potentially useful for certain device applications.     

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.     

Defects in molecular beam epitaxial GaAs grown at low temperatures

We have utilized a variable energy positron beam and infrared transmission spectroscopy to study defects in GaAs epilayers grown at low temperatures (LT-GaAs) by molecular beam epitaxy. We have measured the Doppler broadening of the positron-electron annihilation gamma ray spectra as a function of positron implantation energy. From these measurements, we have obtained results for the depth profiles of Ga monovacancies in unannealed LT-GaAs and Ga monovacancies and arsenic cluster related defects in annealed LT-GaAs. We have also studied the effects of the Si impurities in annealed LT-GaAs. The infrared transmission measurements on unannealed LT-GaAs furnish a broad defect band, related to As antisites, centered at 0.370 eV below the conduction band.     

Hall and drift mobilities in molecular beam epitaxial grown GaAs

A series of nominally undoped and Si-doped GaAs samples have been grown by molecular beam epitaxy (MBE) with Hall concentrations ranging from 10¹⁵ to 10¹⁹ cm⁻³ and mobilities measured at 77 and 300K by Hall-van der Pauw methods. Drift mobilities were calculated using the variational principle method and Hall scattering factors obtained from a relaxation-time approximation to permit cross-correlation of experimental data with drift or Hall mobilities and actual or Hall electron concentrations. At 77K, both high purity and heavily doped samples are well represented by either drift or Hall values since piezoelectric acoustic phonon scattering and strongly screened ionized impurity scattering hold the Hall factor close to unity in the respective regimes. Between n≊10¹⁵ and 10¹⁷ cm⁻³, where lightly screened ionized impority scattering predominates, Hall mobility overestimates drift mobility by up to 50 percent and Hall concentration similarly underestimates n. At 300K, polar optical phonons limit mobility and a Hall factor up to 1.4 is found in the lowest doped material, falling close to unity above about 10¹⁶ cm⁻³. Our calculation also agrees remarkably well with the Hall mobility of the highest purity MBE grown sample reported to date.     

Defect state assisted tunneling in intermediate temperature molecular beam epitaxy grown GaAs

Current transport in molecular beam epitaxy (MBE) GaAs grown at low and intermediate growth temperatures is strongly affected by defects. A model is developed here that shows that tunneling assisted by defect states can dominate, at some bias ranges, current transport in Schottky contacts to unannealed GaAs material grown at the intermediate temperature range of about 400°C. The deep defect states are modeled by quantum wells which trap electrons emitted from the cathode before re-emission to semiconductor. Comparison of theory with experimental data shows defect states of energies about 0.5 eVbelow conduction band to provide the best fit to data. This suggests that arsenic interstitials are likely to mediate this conduction. Comparison is also made between as-grown material and GaAs grown at the same temperature but annealed at 600°C. It is suggested that reduction of these defects by thermal annealing can explain lower current conduction at high biases in the annealed device as well as higher current conduction at low biases due to higher lifetime. Quenching of current by light in the as-grown material can also be explained based on occupancy of trap states. Identification of this mechanism can lead to its utilization in making ohmic contacts, or its elimination by growing tunneling barrier layers.     

Infrared studies of be-doped GaAs grown by molecular beam epitaxy at low temperatures

Samples of molecular beam epitaxial GaAs grown at low temperatures doped with Be defects are studied as a function of growth temperature (T_G)-by measuring their localized vibrational modes at 77K using BOMEM Fourier transform infrared spectrometer. Localized vibrational modes of⁹Be_Ga in samples grown at T_G>350°C have been identified at 482 cm⁻¹. Secondary ion mass spectroscopy measurements show that the densities of Be defects remain approximately constant as T_G is lowered, however, additional structure in the⁹Be_Ga localized vibrational mode is observed. Calculations based on Green's function theory suggest that the additional structure in Be-doped LT GaAs can best be explained in terms of a complex center [⁹Be_Ga-As_Ga] involving an intrinsic defect.     

Diffusion of zinc into gaas layers grown by molecular beam epitaxy at low substrate temperatures

We report the diffusion of zinc into low temperature (LT) GaAs grown by MBE at 200° C, the problems associated with using a silicon nitride film directly deposited on the LT GaAs as a Zn diffusion mask, and several schemes to avoid the problems. The Zn diffusion coefficient is measured (sealed-ampoule technique) to be about one order of magnitude higher in the LT GaAs than in normal GaAs, attributed to a large quantity of defects including arsenic antisites (As_Ga) in the LT GaAs. The effectiveness of silicon nitride as a Zn diffusion mask depends if the mask is deposited directly on the LT GaAs. The failure of the nitride directly deposited on the LT GaAs to stop the Zn is attributed to arsenic atoms outdiffusing from the As-rich LT GaAs (about 1 at. % excess As) into the nitride. Several structures are introduced including a 100-Å thick GaAs layer on the LT GaAs that are effective in preserving the diffusion mask properties of the silicon nitride.     

Growth condition dependence for GaAs selective epitaxial growth by molecular beam epitaxy

GaAs selective epitaxial growth by conventional molecular beam epitaxy (MBE) was studied while varying its growth conditions, such as substrate temperature. As pressure, growth rate, and Si or Be doping. Selectivity is improved with the increase in substrate temperature, and with the decrease in As pressure or growth rate. Si and Be were doped up to 3 x 10¹⁸ and 3 × 10¹⁹ cm⁻³, respectively. While no Si doping influence was observed, Be doping degraded the surface morphology. Selective epitaxial growth by conventional MBE with appropriate growth conditions will be applicable to device fabrication.     

Strain effects in CdTe (111) epitaxial layers grown on GaAs (100) substrates by molecular beam epitaxy

Spectroscopic ellipsometry and photoreflectance measurements on CdTe/GaAs strained heterostructures grown by moleculclr beam epitaxy were carried out to investigate the effect of the strain and the dependence of the lattice parameter on the CdTe epitaxial layer thicknesses. Compressive strains exist in CdTe layers thinner than 2 μm. As the strain increases, the value of the critical-point energy shift increases linearly. These results indicate that the strains in the CdTe layers grown on GaAs substrates are strongly dependent on the CdTe layer thickness. Author to whom all correspondence should be addressed.     

A crossover from Efros-Shklovskii hopping to activated transport in a GaAs two-dimensional hole system at low temperatures

In this paper,we discuss low-temperature hopping-conductivity behavior in the insulating phase,in the absence of a magnetic field.We conduct a theoretical study of the crossover from hopping to activated transport in a GaAs two-dimensional hole system at low temperatures,finding that a crossover takes place from the Efros-Shklovskii variable-range hopping (VRH) regime to an activated regime in this system.This conductivity behavior in p-GaAs quantum wells is qualitatively consistent with the laws laid down in theories of localized electron interactions.Given sufficiently strong interactions,the holes in the localized states are able to hop collectively.     

Hopping conduction in heavily doped bulk n-type SiC

The electronic properties of heavily doped n-type 4H, 6H, and 15R SiC have been studied with temperature dependent Hall effect, resistivity measurements, and thermal admittance spectroscopy experiments. Hopping conduction was observed in the resistivity experiments for samples with electron concentrations of 10¹⁷ cm⁻³ or higher. Both band and hopping conduction were observed in all three polytypes in resistivity and Hall effect experiments. The hopping conduction activation energy ε₃ obtained from the resistivity measurements varied from 0.003 to 0.013 eV. The ε₃ value obtained from thermal admittance spectroscopy measurements were slightly lower. The nitrogen ionization levels were observed by thermal admittance spectroscopy only in those samples where hopping conduction was not detected by this experiment. Free carrier activation energy E_a for nitrogen was difficult to determine from temperature dependent Hall effect measurements because of the effects of hopping conduction. A new feature in the apparent carrier concentration vs inverse temperature data in the hopping regime was observed.     

Comparisons using optoelectronic modulation spectroscopy of n-type GaAs epitaxial layers formed on buffer layers prepared at normal and low temperatures

Optoelectronic modulation spectroscopy (OEMS) has been used to examine defect-related states in GaAs metal-semiconductor, field-effect transistor (MESFET) structures prepared by molecular-beam epitaxy (MBE) on buffer layers formed at normal and low temperatures. The technique was used to simultaneously observe the spectra of defect states near to the interface with the buffer layer and in the active layer. There were few responses that were common to the structures. The most prominent was an electron trap with a depth of 0.92 eV. This was present throughout both structures and may be from a native defect in GaAs. With few exceptions the states seen in a particular structure were present in the active layer as well as near to the interface with the buffer layer suggesting that, in each case, the defects originated from the active/buffer layer interface or from the buffer layer. The most significant difference was that states in the low-temperature (LT)-based structure generally exhibited replicating responses with specific energy periodicities. A model describes this in terms of optical-absorption transitions to the local vibrational states of defects. Five different replica series were observed. The vibrational characteristics of the defects found in the LT material suggest that they are different in character and extent to those defects found in material formed on a normally prepared buffer layer. Their Frank-Condon energies ranged from 9–332 meV.     

Electrical properties of low-temperature GaAs grown by molecular beam epitaxy and migration enhanced epitaxy

Measurements on low-temperature GaAs epitaxial layers (LT-GaAs) grown by molecular beam epitaxy and migration enhanced epitaxy showed that the excess arsenic incorporated during growth played a crucial role in determining their electrical properties. The electrical transport in LT-GaAs grown by a standard molecular beam epitaxy proceeded mainly via a hopping process, which showed a higher activation energy and onset temperature than those usually observed in lightly doped semiconductors. Using migration enhanced epitaxy to grow LT-GaAs, we were able to substantially reduce the density of As-rich defects and to achieve a good Hall mobility in Be-doped LT-GaAs. The study presented here indicates that, with controlled excess arsenic incorporation during growth, LT-GaAs can vary in a range of conduction properties and thus can be engineered for different device applications.     

Raman study of defects in a GaAs buffer layer grown by low-temperature molecular beam epitaxy

Raman scattering measurements on high-resistivity layers of GaAs grown by molecular beam epitaxy at low temperature are presented. Several defect-related features are ob-served, including two peaks attributed to quasi-localized vibrational modes of point de-fects, one with a frequency of 223 cm⁻¹ similar to a mode previously observed in elec-tron-irradiated GaAs, and the other with a frequency of 47 cm⁻¹ similar to a mode observed in ion-implanted GaAs. We suggest that these are due to arsenic interstitials and gallium vacancies, respectively. We also observe peaks at 200 and 258 cm⁻¹, which we believe may be due to vibrational modes in small clusters of arsenic. The 223 cm⁻¹ mode is the only defect-related mode still observed after a 10-min annealing treatment at 600° C, although it is significantly broader and has different symmetry from the 223 cm⁻¹ mode in the unannealed material. This indicates that the 223 cm⁻¹ mode in the annealed material is due, at least in part, to a defect other than the arsenic interstitial.     

Be doped GaAs grown by migration enhanced epitaxy at low substrate temperature

Conductive Be doped GaAs grown by molecular beam epitaxy at low substrate temperatures (300° C) was obtained for the first time by using migration enhanced epitaxy (MEE) without subsequent annealing. The layers were characterized using Hall effect, double crystal x-ray diffraction, and photoluminescence. With low arsenic exposure, the low temperature MEE layers doped with Be had the same carrier density and similar luminescent efficiency as layers grown by conventional MBE at 580° C. Mobility at 77 K was reduced somewhat for layers doped at 2 × 10¹⁷cm⁻³, which also exhibited hopping conductivity below 40 K. Double crystal x-ray diffraction showed that low temperature MEE samples grown at low As exposure had the narrow linewidth associated with conventional MBE material grown at 580° C, unlike layers grown by conventional MBE at low temperatures, which exhibit an expansion in lattice parameter.     

Deep level studies in MBE GaAs grown at low temperature

The photo-induced current transient spectroscopy (PICTS), thermoelectric effect spectroscopy (TEES) and thermally stimulated current (TSC) spectroscopy have been used to characterize the deep levels in the GaAs materials grown at low temperature by molecular beam epitaxy. At least five hole traps and five electron traps have been identified by the TEES measurement employing a simplified sample arrangement. We have studied the behavior of various traps as a function of the growth temperature and the post-growth annealing temperature. Some of the shallower hole traps were annealed out above 650‡ C. Electron traps atE _c- 0.29 eV andE _c- 0.49 eV were present in the material, and have been identified as M3 and M4, respectively. The dominant electron trap, atE _c- 0.57 eV, is believed to be associated with the stoichiometric defect caused by the excess As in the material, and our data show evidence of forming a defect band by this trap. A possible model involving As precipitates is proposed for this trap atE _c-0.57 eV.     

Effect of dopants on arsenic precipitation in GaAs deposited at low temperatures

High resolution x-ray diffraction using synchrotron radiation was used to characterize GaAs grown by MBE at low temperatures (LT-GaAs) LT-GaAs grown at 225°C is nonstoichiometric and exhibits a 0.15% lattice expansion along the growth direction. Annealing LT-GaAs results in arsenic clusters with a well-defined orientation relationship with the GaAs matrix and a relaxation of the LT-GaAs lattice. The arsenic precipitation corresponds to a classical case of diffsion controlled nucleation and growth followed by coarsening. While the rates of growth and coarsening in the n-doped and the p-doped samples are observed to be identical, the effects of the superlattice seem to accelerate the precipitation kinetics in the p-n superlattice sample. The enhanced coarsening in the p-n superlattice sample is consistent with a previously propsed model involving interaction between charged precipitate and arsenic defects.     

Enhancement of Si-donor incorporation by Ga adatoms in Si delta-doped GaAs grown by molecular beam epitaxy

We report on the realization of a modified delta doping technique to obtain doping profiles in MBE grown GaAs, measured by capacitance-voltage (C-V) methods with full-widths at half-maximum (FWHM)s of 25 ± 5Å and peak concentrations of up to 1.1 × 10¹⁹ cm^?3. In this modified delta doping technique, both the Ga and Si shutters were opened for 15 sec during the delta doped layer growth while only the Si shutter is opened during conventional delta doping. Comparison of the two techniques under the same dopant flux and shutter-open-time interval shows that higher sheet-carrier concentrations with narrower FWHMs and higher peak concentrations are obtained with the modified delta doping than with the conventional delta doping method. This suggests that Si donor incorporation is enhanced by the Ga adatoms while broadening of the Si donor distribution is still negligible for this short time interval. The effects of the substrate temperature and the shutter-open time on the Si donor distribution have also been investigated.     

Particulates: A direct origin of oval defects in GaAs layers grown by molecular beam epitaxy

Based on micrographic as well as experimental analyses, we show that particulates which inadvertently adhere to the surface before the wafer is ready for growth are one of the most significant origins for the formation of oval defects on the GaAs layers grown by molecular beam epitaxy. We show that the density of surface particulates is proportional to that of airborne particles surrounding the wafer under preparation, especially during the drying and transferring process. With reduction of airborne particles, we simultaneously reduce the density of oval defects from a few thousand to about 200 cm^-2 for 1-μm thick layers.     

生长温度对于Ge衬底上分子束外延生长的InGaAs/GaAs量子阱结构质量的影响

本文研究了斜切割（100）Ge衬底上InxGa1-xAs/GaAs量子阱结构的分子束外延生长（In组分为0.17或者0.3）。所生长的样品用原子力显微镜、光致发光光谱和高分辨率透射电子显微镜进行了测量和表征。结果发现,为了生长没有反相畴的GaAs缓冲层,必须对Ge衬底进行高温退火。在GaAs外延层和InxGa1-xAs/GaAs量子阱结构的生长过程中,生长温度是一个至关重要的参数。文中讨论了温度对于外延材料质量的影响机理。通过优化生长温度,Ge衬底上的InxGa1-xAs/GaAs量子阱结构的光致发光谱具有很高的强度、很窄的线宽,样品的表面光滑平整。这些研究表面Ge 衬底上的III-V族化合物半导体材料有很大的器件应用前景。