Low-temperature Electron Mobility Research Articles

Low temperature electron mobility in a coupled quantum well system

We study low temperature electron mobility μnin a GaAs/Alx Ga 1−xAs coupled double quantum well structure. Both the extreme barriers are δ -doped with Si so that the electrons diffuse into the adjacent wells forming two sheets of two-dimensional electron gas (2DEG) separated by a thin central barrier. The subband electron wavefunctions and energy levels are numerically obtained as a function of the well width, barrier width and doping concentration. The screening of ionized impurity potential by the 2D-electrons is obtained in terms of the static dielectric response function within the random phase approximation (RPA). μnis calculated by solving the coupled Boltzmann equation in the relaxation time approximation. The coupling of wavefunctions through the barrier, screening of ionized impurities and intersubband scattering effects on μnare investigated.

High-quality GaN grown by gas-source MBE

High-quality GaN epilayers were consistently obtained using a home-made gas-source MBE system on sapphire substrates. Room-temperature electron mobility of the grown GaN film is 300 cm 2/V s with a background electron concentration as low as 2×10 17 cm −3. The full-width at half-maximum of the GaN (0 0 0 2) double-crystal X-ray rocking curve is 6 arcmin. At low temperature (3.5 K), the FWHM of the near-band-edge photoluminescence emission line is 10 meV. Furthermore, using piezoelectric effect alone with the high-quality films, two-dimensional electron gas was formed in a GaN/AlN/GaN/sapphire structure. Its room-temperature and low-temperature (77 K) electron mobility is 680 cm 2/V s and 1700 cm 2/V s, and the corresponding sheet electron density is 3.2×10 13 and 2.6×10 13 cm −2, respectively.

Characterization of high indium content metamorphic InGaAs/InAlAs modulation-doped heterostructures

We have studied electronic and structural characterizations of high indium content metamorphic InGaAs/InAlAs modulation-doped heterostructures. An improved low-temperature electron mobility of μ e =5.45×10 5 cm 2 V s in [ 1 ̄ 1 0] was confirmed for a directed Hall-bar sample when Si-doped InAlAs layer was slightly etched. In addition, in-plane mobility anisotropy of 40% between [1 1 0] and [ 1 ̄ 1 0] directions was confirmed. This anisotropy seems to be originated from the different undulation period between [ 1 ̄ 1 0] and [1 1 0] directions. We theoretically calculated electron mobility taking both alloy disorder scattering and background impurity scattering into account. It is found that the calculated and experimental values are in good agreement.

High electron mobility in AlGaN/GaN heterostructures grown on bulk GaN substrates

Dislocation-free high-quality AlGaN/GaN heterostructures have been grown by molecular-beam epitaxy on semi-insulating bulk GaN substrates. Hall measurements performed in the 300 K–50 mK range show a low-temperature electron mobility exceeding 60 000 cm2/V s for an electron sheet density of 2.4×1012 cm−2. Magnetotransport experiments performed up to 15 T exhibit well-defined quantum Hall-effect features. The structures corresponding to the cyclotron and spin splitting were clearly resolved. From an analysis of the Shubnikov de Hass oscillations and the low-temperature mobility we found the quantum and transport scattering times to be 0.4 and 8.2 ps, respectively. The high ratio of the scattering to quantum relaxation time indicates that the main scattering mechanisms, at low temperatures, are due to long-range potentials, such as Coulomb potentials of ionized impurities.

Polarization-induced charge and electron mobility in AlGaN/GaN heterostructures grown by plasma-assisted molecular-beam epitaxy

The formation of the two-dimensional electron gas (2DEG) in unintentionally doped AlxGa1−xN/GaN (x⩽0.31) heterostructures grown by rf plasma-assisted molecular-beam epitaxy is investigated. Low-temperature electrical-transport measurements revealed that the two-dimensional electron gas density strongly depends on both the thickness of the AlGaN layer and alloy composition. The experimental results agree very well with the theoretical estimates of the polarization-induced 2DEG concentrations. Low-temperature electron mobility was found to be much higher in the structures with lower electron sheet densities. Interface roughness scattering or alloy disorder scattering are proposed to be responsible for this trend. A maximum mobility of 51 700 cm2/V s (T=13 K) was obtained in the Al0.09Ga0.91N/GaN structure with a two-dimensional electron gas density of 2.23×1012 cm−2.

Dependencies of low-temperature electronic properties of MBE-grown GaAs/AlGaAs single heterojunctions upon arsenic species

We have grown delta-doped high electron mobility transistor (HEMT) structure by usual solid source molecular beam epitaxy (MBE) with As 4 and As 2 and found highest low-temperature electron mobility of ∼2×10 6 cm 2/V s for the growth temperature, T g=640°C and for the As cracking temperature, T c=800°C, which is rather an As 2-dominant condition. Room-temperature Hall measurements for the undoped GaAs films suggested that a conduction-type transition occurs at certain T c( T c,tr) and it gradually rises as T g increases. PL estimations for the GaAs samples suggested that total impurity (donor+acceptor) density decreases suddenly almost at the T c just before the p/n transition and that donor density slightly increases as T c is increased. It is also presumed from far-infrared photoconductivity (FIRPC) measurements that residual donor density is increased by increasing T c and it is decreased by increasing T g for the fixed T c of 950°C. Those results suggest that the growth condition for the lowest impurity (most intrinsic) GaAs film would give the condition of obtaining highest low-temperature electron mobility in the heterojunction.

Low-temperature electron mobility in a real delta -doped semiconductor.

The low-temperature electron mobility in \ensuremath{\delta}-doped GaAs is calculated by using the Boltzmann equation and the relaxation-time approximation. It is assumed that the electrons are scattered from ionized donors, which are spread uniformly throughout some distance. For donor-ion spreading of 50 to 100 \AA{} the calculated mobilities, although still higher than experimental ones, are for some electron concentrations even 50% lower than those calculated assuming genuine \ensuremath{\delta} doping. The possibility for the observation of such discontinuities which appear in the mobility when the Fermi level passes through the subband bottoms is discussed. \textcopyright{} 1996 The American Physical Society.

Effect of dislocations in strained Si/SiGe on electron mobility

In this article the presence of threading and misfit dislocations in strained modulation-doped Si/SiGe heterostructures and their effect on electron mobility are investigated. It is found that the low-temperature electron mobility is sensitive to threading dislocations when their density exceeds 3×108 cm−2, and decreases by two orders of magnitude when the threading dislocation density is 1×1011 cm−2. The room-temperature mobility is reduced under the same conditions by 10% and 50%, respectively. Misfit segments in the graded Ge-content buffer limit the mobility when the Si channel is 0.4 μm or less away from that buffer. Misfit dislocations at the Si-channel bottom interface very strongly scatter the electrons in the channel once a continuous network of misfit segments is created.

Coexistence of the DX center with nonmetastable states of the donor impurity in Si-doped AlxGa1-xAs: Effects on the low-temperature electron mobility.

The coexistence of the DX center with nonmetastable bound states of the ordinary substitutional configuration of the donor impurity is extensively investigated in Si-doped ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As samples having different x AlAs molar fractions in the direct gap region. The occupation of nonmetastable states is evidenced by comparing the ${\mathit{n}}_{\mathit{CV}}$ ``electron density,'' as derived from capacitance-voltage measurements in Schottky diodes, with the ${\mathit{n}}_{\mathit{H}}$ Hall density. In samples of compositions not far from the direct-to-indirect gap transition and with doping levels in the ${10}^{18}$-${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ range, a nonmetastable state SX, degenerate in energy with the conduction band, can reach a significant occupancy when the saturated persistent photoconductivity condition is approached during low-temperature photoionization of DX centers. On the other hand, when the free-electron density is smaller than a critical density of a few ${10}^{16}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$, electrons freeze out into a localized S\ensuremath{\Gamma} state, or into a low mobility impurity band, linked to the \ensuremath{\Gamma} conduction-band edge. A finite occupancy of the SX or S\ensuremath{\Gamma} state gives rise to a significant ${\mathit{N}}_{{\mathit{D}}^{0}}$ density of substitutional donors in the neutral ${\mathit{D}}^{0}$ charge state having a strongly correlated spatial distribution. Electron capture into donor states, DX or not, has a spatially selective character, as can be evidenced by low-temperature mobility data under conditions where impurity scattering dominates. For DX centers, this is demonstrated by the hysteretic behavior of low temperature (${\mathit{T}}_{0}$) ${\mathrm{\ensuremath{\mu}}}_{\mathit{H}}$ vs ${\mathit{n}}_{\mathit{H}}$ data, where any given ${\mathit{n}}_{\mathit{H}}$ value is reached through DX center photoionization steps or through electron capture via a proper thermal cycling. When ${\mathit{N}}_{{\mathit{D}}^{0}}$ is negligible the hysteresis amplitude is maximum. However, whenever ${\mathit{N}}_{{\mathit{D}}^{0}}$ reaches significant values, either at the ${\mathit{T}}_{0}$ temperature or during the thermal cycle, the hysteresis amplitude vanishes. This is systematically observed in all the cases where the occupation of SX or S\ensuremath{\Gamma} states is independently demonstrated through the analysis of ${\mathit{n}}_{\mathit{CV}}$ and ${\mathit{n}}_{\mathit{H}}$ data. Two diverse complementary effects are proposed to explain the observed vanishing of the hysteresis amplitude. \textcopyright{} 1996 The American Physical Society.

The Effect of Strain on the Formation of Dislocations at the SiGe/Si Interface

Recently much interest has been devoted to Si-based heteroepitaxy, and in particular, to the SiGe/Si system. This is mostly for economical reasons: Si-based technology is much more advanced, is widely available, and is cheaper than GaAs-based technology. SiGe opens the door to the exciting (and lucrative) area of Si-based high-performance devices, although optical applications are still limited to GaAs-based technology. Strained SiGe layers form the base of heterojunction bipolar transistors (HBTs), which are currently used in commercial high-speed analogue applications. They promise to be low-cost compared to their GaAs counterparts and give comparable performance in the 2-20-GHz regime. More recently we have started to investigate the use of relaxed SiGe layers, which opens the door to a wider range of application and to the use of SiGe in complementary metal oxide semiconductor (CMOS) devices, which comprise strained Si and SiGe layers. Some recent successes include record-breaking low-temperature electron mobility in modulation-doped layers where the mobility was found to be up to 50 times better than standard Si-based metal-oxide-semiconductor field-effect transistors (MOSFETs). Even more recently, SiGe-basedp-type MOSFETS were built with oscillation frequency of up to 50 GHz, which is a new record, in anyp-type material for the same design rule.

Limit of Electron Mobility in AlGaAs/GaAs Modulation-doped Heterostructures

We have achieved electron mobilities as high as 1.05×107 cm2/Vs at 1.5 K with an electron density of approximately 3×1011/ cm2 for modulation-doped AlGaAs/GaAs by using high purity layers with a residual acceptor concentration of 1×1013/ cm3, and relatively thick spacer layers ∼(75 nm). We found the electron scattering process caused by spatially separated ionized donors to be most important in limiting the observed low-temperature electron mobility, even in these thick-spacer-layer samples. Theoretical calculation predicts that the mobility caused by this scattering mechanism is approximately 1.6×107 cm2/Vs. The observed electron mobility exhibits an anisotropy with respect to the principal axes ([110] and [*BAR*1*BAR*10] directions) on the (001) surface. The anisotropy is such that the mobility in the [*BAR*1*BAR*10] direction is always higher than that in the [110] direction. Theoretical calculations reasonably explained this anisotropy by assuming the existence of islands at the interface which are longer in the [*BAR*1*BAR*10] direction than in the [110] direction, and revealed that the scattering caused by interface roughness was as important as that caused by ionized donors. Thus, these two major components mainly determine the observed low-temperature electron mobility. We also discuss the mobility expected for residual impurity free limit.

Low-temperature electron mobility studied by cyclotron resonance in ultrapure GaAs crystals.

We report on a systematic study of the electron scattering processes in ultrapure GaAs crystals over the temperature range 1.6--100 K by using the classical cyclotron resonance (CR) for a low density of photogenerated electrons (${\mathit{n}}_{\mathit{e}}$\ensuremath{\sim}${10}^{10}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$). The highest electron mobility extracted from the CR data is 2.2\ifmmode\times\else\texttimes\fi{}${10}^{6}$ ${\mathrm{cm}}^{2}$/V sec at T=1.6 K. The electron scattering due to the acoustic-phonon piezoelectric potential is found to be the main intrinsic electron-phonon scattering mechanism in the temperature range 1.6T30 K. Electron scattering by neutral impurities is shown to be significant even in these ultrapure GaAs crystals (unlike Ge and Si). The acoustic-phonon piezoelectric and deformation-potential coefficients as well as the impurity concentrations (\ensuremath{\sim}${10}^{13}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$) were obtained from the analysis of the electron-scattering-rate dependence on temperature. A CR line narrowing is observed under increasing microwave power and is interpreted as due to a decrease in the acoustic-phonon piezoelectric scattering efficiency with electron heating. The electron effective mass determined from the CR magnetic field applied along [001] is ${\mathit{m}}^{\mathrm{*}}$=(0.0662\ifmmode\pm\else\textpm\fi{}0.0002)${\mathit{m}}_{0}$.

Mobility degradation in gated Si:SiGe quantum wellswiththermally grown oxides

The authors have grown thermal oxides on the silicon cap layer of modulation doped n-channel Si:SiGe quantum wells. For growth temperatures of 650°C they have observed a degradation of the low temperature electron mobility accompanied by a small increase in the sheet density. Some possible origins for this mobility degradation are discussed.

Characterization of n-channel Si/SiGe modulation doped structures grown by gas source molecular beam epitaxy

We have grown n-channel Si/SiGe modulation doped structures by gas source molecular beam epitaxy using arsine as the n-type dopant source. The structures were characterized by transmission electron microscopy, secondary ion mass spectroscopy, electrochemical capacitance voltage analysis and x-ray diffraction. Arsenic and free electron concentrations in excess of 1019 cm-3 could be obtained with substantial surface segregation. Several different structures have been grown and their transport properties investigated. Low-temperature electron mobilities of up to 60000 cm2 V-1 s-1 in the dark (75800 cm2 V-1 s-1 after illumination) were obtained with a sheet density range (4-7)*1011 cm-2. Parallel conduction is discussed in terms of the effect of illumination.

Identification of a mobility-limiting scattering mechanism in modulation-doped Si/SiGe heterostructures.

High electron mobilities in modulation-doped Si/SiGe in the range of (1.5-1.8) \ifmmode\times\else\texttimes\fi{} ${10}^{5}$ ${\mathrm{cm}}^{2}$/V s at 0.4-1.4 K have been achieved by several groups. Those numbers fall short of the expected theoretical value. We have examined how strain in the Si channel affects the low temperature electron mobility by systematically varying the Ge content in the relaxed buffer underneath, and by changing the Si channel thickness. A clear reduction in mobility is observed at a critical layer thickness, which is identified as the thickness at which remaining threading dislocations glide in the Si channel, resulting in misfit dislocations at the bottom interface. Understanding and control of this mechanism have led to the growth of samples with mobility values in the range of (3-4) \ifmmode\times\else\texttimes\fi{} ${10}^{5}$ ${\mathrm{cm}}^{2}$/V s.

Low-temperature electron mobility in a δ-doped semiconductor

The low-temperature electron mobility in \ensuremath{\delta}-doped GaAs is calculated by using the Boltzmann equation and the relaxation-time approximation. It is assumed that the electrons are scattered from ionized impurities. Screening of charged impurities by electrons occupying several subbands is described with the help of (i) the random-phase approximation, (ii) the Thomas-Fermi method, and (iii) the bulk dielectric constant only. Among those methods mentioned above, the random-phase approximation has proved quite successful in studying the screening while the other two methods are inadequate. The mobility exhibits a drop when the excited subbands become occupied. It is shown, however, that as a consequence of the parity of the subband wave functions, the drop in the mobility when the Fermi level coincides with the bottom of the first excited subband is negligible.

Properties Of crystalline 3C-SIC Grown From Methyl Trichlorosilane

ABSTRACTCubic silicon carbide grown by thermal decomposition of methyl trichlorosilane in hydrogen has been studied in infrared (IR) transmission, cyclotron resonance (CR), and photoluminescence (PL). Sample characteristics include: The nitrogen concentration, determined from CR and IR can be of the order of 1015 cm-3 or less. Ionized impurity concentrations determined from the linewidths of CR and IR spectra can be less than 7 × 1014 cm-3. Low temperature electron mobilities from CR are near 105 cm2 v-1 s-1. Carrier freeze-out is consistent with either uncompen-sated donors with activation energy near 50 meV or heavily compensated donors with 20 meV activation energy. The latter choice, however, is excluded by the low ionized impurity concentration. These high quality samples show little or no evidence of the apparently shallow (< ∼20 meV) donor which usually dominates the low temperature conductivity of 3C-SiC grown on Si. Nitrogen donor excitation spectra are in excellent agreement with effective mass theory. Only nitrogen donors at 54.2 meV and an unidentified effective-mass donor at 47.8 meV are observed. If material with the characteristics described above can be reliably reproduced then there is substantial expectation that high quality electronic devices based on 3C-SiC can be fabricated.

Systematics of electron mobility in Si/SiGe heterostructures

We have investigated limits to the low temperature electron mobility in modulation-doped Si/SiGe heterostructures grown by ultrahigh vacuum chemical vapor deposition. The temperature dependence and peak value of the mobility suggest that interface roughness is the chief scattering mechanism in samples with a thin buffer layer below the channel. Up to about 2 μm, increasing the thickness of the buffer layer raises the mobility. We have also observed a systematic increase of electron mobility with spacer (or setback) thickness, which is characteristic of remote ion scattering. The influence of background impurities is seen in the decrease of mobility with decreasing two-dimensional gas density.

Spatial correlations of DX charges and electron mobility in AlxGa1-xAs

The low-temperature electron mobility mu in Si-doped AlxGa1-xAs (x=0.25) is not a single-valued function of the free electron density n, mu being larger if a given n value is reached through capture rather than photoionization processes. This difference is reduced by lowering the Si doping level. The result is interpreted as due to spatial correlations amongst charges at the DX centres. Analysis of mobility transients during isothermal capture processes in the temperature range 110-140 K indicates that the fast initial stage of the capture is marked by an increasing order in the spatial distribution of the scattering centres. On the contrary, the slow final stage of the capture process is characterized by a diminishing order in the distribution.

Alloy-scattering and strain-fluctuation-scattering contributions to the low-temperature electron mobility in ternary quantum wells and heterostructures.

We present theory and data for the low-temperature electron mobility in ternary quantum wells (QW's). In our model the electrons are scattered from spatially fluctuating strain-induced potentials and alloy potentials arising from alloy disorder both in lattice-matched and strained QW's. We find that the mobility does not depend sensitively on the amount of the average strain in the ${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As quantum well but on the strain fluctuation. Lower mobilities observed in strained quantum wells are explained in terms of possible clustering effects which may be more severe in strained systems: The theoretical results agree with our data from strained ${\mathrm{In}}_{0.2}$${\mathrm{Ga}}_{0.8}$As/GaAs QW's for a clustered In-atom distribution. On the other hand, the recent data from lattice-matched ${\mathrm{In}}_{0.53}$${\mathrm{Ga}}_{0.47}$As/${\mathrm{In}}_{0.52}$${\mathrm{Al}}_{0.48}$As heterostructures are reasonably explained for a microscopically random In-atom distribution without using the clustering assumption.